Umk physical methods for studying the structure of matter. Experimental methods for studying the structure of crystals, determining the structure of substances

82 83 84

Section 4.

Methods and technical means of forensic research of the structure and other properties of substances and materials

It seems appropriate to simultaneously consider methods for conducting phase analysis of substances and studying their structure, since phase composition and structure are interconnected and some methods for their study coincide. At KIWMI, structure and phase composition are mainly studied in metallography and radiography.

Rice. 29. System of methods for studying the phase composition of substances and materials

4.1.

METHODS FOR STUDYING THE PHASE COMPOSITION OF SUBSTANCES AND MATERIALS IN CRIMINOLOGY

Methods for studying the phase composition of substances and materials are designed to establish the qualitative and quantitative content of phases having the same and different chemical compositions (Fig. 29).

Metallographic analysis

The branch of materials science that studies changes in the macro- and microstructure of metals and alloys due to changes in their chemical composition and processing conditions is called metallography. The description of metallographic analysis was given above (in section 3.1. “Methods and technical means of forensic morphoanalysis of substances and materials”).

The study of metallographic sections allows us to determine the structure of the metal and observe different phases in the field of view of the microscope, which can be painted in different colors. This allows you to find out such important circumstances as the features of the product processing technology (forging, heat treatment, etc.), the heating temperature of the sample and the moment of the incident, for example, in a fire, etc. For example, by metallographic analysis it is possible to determine in what atmosphere, oxygen-poor or oxygen-rich, the melting of the wires occurred at the moment of the short circuit. In turn, establishing this circumstance is important for deciding whether the short circuit was the cause of the fire or arose as a result of it.

Metallographic analysis allows one to estimate the quantitative content of inclusions in a thin section and is very clear. However, this research method is destructive and is inferior in accuracy to X-ray phase analysis.

X-ray diffraction phase analysis

X-ray phase analysis is a method for determining the phase composition of solid crystalline and some amorphous substances. Each crystalline substance has a strictly individual geometry of the crystal lattice, which is characterized by a set of interplanar distances. When X-rays pass through a crystal, a diffraction effect occurs. The diffraction pattern is carried out either photographically in special cameras on X-ray film, or using X-ray diffractometers using electronic recording systems.

To resolve the question of the phase present in a sample, it is not necessary to determine its crystal structure. It is enough to calculate the diffraction pattern (x-ray pattern) and compare the resulting series of interplanar distances and relative line intensities with those given in the X-ray data files, the most complete of which is the constantly updated American phase determinant - the Joint Committee on Powder Diffraction Standards (JCPDS) file.

The presence of certain lines in the x-ray diffraction pattern characterizes the qualitative phase composition of the sample. A mixture of several individual chemical compounds produces an x-ray diffraction pattern, which is a superposition of diffraction effects characterizing the individual phases. When comparing interplanar distances of samples and standards, it is often necessary to analyze very large information arrays, so data processing is carried out on a PC using automated systems and databases.

X-ray phase analysis is used to study such KIWMI objects as metals and alloys, medicines, substances of soil origin, paper, perfumes and cosmetics, paints and coatings, etc.

Calorimetric analysis

Calorimetry is a group of methods for measuring thermal effects (amount of heat) accompanying various physical, chemical and biological processes. Calorimetry includes the measurement of heat capacity, heat of phase transitions, thermal effects of magnetization, electrification, dissolution, and chemical reactions (for example, combustion). Instruments used in calometry are called calorimeters.

Thermography methods are used, for example, in the study of polymers. They make it possible to determine the types of polymers, the composition of their mixtures and copolymers, brands of some polymers, the presence and composition of special additives, pigments and fillers, characteristics determined by the technology of synthesis and processing of polymers into products, as well as the operating conditions of the latter. However, combining thermographic and gas chromatographic methods of analysis is more effective.

Thermal methods of analysis

Thermal methods of analysis - methods for studying physico-chemical and chemical processes, based on the registration of thermal effects accompanied by temperature programming conditions. The setup for thermal analysis methods typically includes an oven, sample holders, thermocouples that measure the temperature in the oven, and samples. When a sample is heated or cooled, changes in the temperature of the object over time are recorded. In cases of phase transformations, a plateau or kink appears on the heating (cooling) curve.

Thermogravimetric analysis (TGA) is based on recording changes in the mass of a sample depending on temperature under conditions of programmed changes in the temperature of the environment.

In differential thermal analysis (DTA), the change in temperature difference between the sample under study and a comparison sample, which does not undergo any transformations in a given temperature range, is recorded over time. Effects recorded by DTA can be caused by melting, sublimation, evaporation, boiling, changes in the crystal lattice, and chemical transformations.

4.2. METHODS FOR STUDYING THE STRUCTURE OF SUBSTANCES AND MATERIALS IN CRIMINOLOGY

Depending on the origin, production technology or operating conditions, the same substances or materials may have different structures. For example, hardening or tempering of steel does not change its composition, but changes its structure, as a result of which its mechanical properties(hardness, elasticity, etc.).

As already noted, metallographic and X-ray spectral analyzes are most often used to study the crystal structure of substances and materials. The description of metallographic analysis is given above, so we will focus on X-ray diffraction analysis.

The physical basis of the method is the specific nature of the interaction of X-ray radiation with substances that have an ordered structure. Thermal and mechanical effects on materials and products made from them (especially from metals and alloys) lead to the appearance of residual macrostresses, which, in turn, cause deformation of the crystal lattice. This deformation is recorded during X-ray diffraction studies in the form of line shifts in diffraction patterns and x-ray diffraction patterns. When annealing metals and alloys, there is a release of residual stresses, recrystallization, and grain growth, which leads to a change in the location, shape and width of the X-ray lines. In addition, heating the metal leads to the formation of scale on the surface of the product, the presence of which is recorded on the x-ray diffraction pattern in the form of the appearance of additional lines.

X-ray diffraction analysis: 1) From the diffraction patterns obtained when an X-ray beam passes through the crystal, interatomic distances are determined and the structure of the crystal is determined; 2) Widely applied to determine the structure of proteins and nucleic acid molecules; 3) Bond lengths and angles precisely established for small molecules are used as standard values ​​under the assumption that they remain the same in more complex polymer structures; 4) One of the stages in determining the structure of proteins and nucleic acids is the construction of molecular models of polymers that are consistent with X-ray data and retain standard values ​​of bond lengths and bond angles

Nuclear magnetic resonance: 1) At the core - absorption of electromagnetic waves in the radio frequency range by atomic nuclei having a magnetic moment; 2) Absorption of an energy quantum occurs when the nuclei are in the strong magnetic field of the NMR spectrometer; 3) Nuclei with different chemical environments absorb energy in a magnetic field of slightly different voltage (or, at constant voltage, slightly different frequency radio frequency oscillations); 4) The result is NMR spectrum a substance in which magnetically asymmetric nuclei are characterized by certain signals - “chemical shifts” in relation to any standard ; 5) NMR spectra make it possible to determine the number of atoms of a given element in a compound and the number and nature of other atoms surrounding a given element.

Electron paramagnetic resonance (EPR): 1) Resonant absorption of radiation by electrons is used

Electron microscopy:1) They use an electron microscope that magnifies objects millions of times; 2) The first electron microscopes appeared in 1939; 3) With a resolution of ~0.4 nm, an electron microscope allows you to “see” molecules of proteins and nucleic acids, as well as details of the structure of cellular organelles; 4) In 1950 they were designed microtomes And knives , allowing to make ultrathin (20–200 nm) sections of tissues pre-embedded in plastic

Methods for protein isolation and purification: Once a protein source has been selected, the next step is to extract it from the tissue. Once an extract containing a significant portion of the protein of interest has been obtained and particles and non-protein material have been removed, protein purification can begin. Concentration . It can be carried out by precipitation of the protein followed by dissolution of the precipitate in a smaller volume. Typically, ammonium sulfate or acetone is used. The protein concentration in the initial solution must be at least 1 mg/ml. Thermal denaturation . On initial stage Heat treatment is sometimes used to separate proteins. It is effective if the protein is relatively stable under heating conditions while the accompanying proteins are denatured. In this case, the pH of the solution, the duration of treatment and the temperature are varied. To select optimal conditions, a series of small experiments are first carried out. After the first stages of purification, the proteins are far from a homogeneous state. In the resulting mixture, proteins differ from each other in solubility, molecular weight, total charge of the molecule, relative stability, etc. Precipitation of proteins with organic solvents. This is one of the old methods. It plays an important role in protein purification on an industrial scale. The most commonly used solvents are ethanol and acetone, less often – isopropanol, methanol, and dioxane. The main mechanism of the process: as the concentration of the organic solvent increases, the ability of water to solvate charged hydrophilic enzyme molecules decreases. There is a decrease in protein solubility to a level at which aggregation and precipitation begins. An important parameter affecting precipitation is the size of the protein molecule. The larger the molecule, the lower the concentration of organic solvent causing protein precipitation. Gel filtration Using the gel filtration method, macromolecules can be quickly separated according to their size. The carrier for chromatography is a gel, which consists of a cross-linked three-dimensional molecular network, formed in the form of beads (granules) for easy filling of columns. So Sephadexes- these are cross-linked dextrans (α-1→6-glucans of microbial origin) with specified pore sizes. Dextran chains are cross-linked with three-carbon bridges using epichlorohydrin. The more cross-links, the smaller the hole sizes. The gel thus obtained plays the role of a molecular sieve. When a solution of a mixture of substances is passed through a column filled with swollen Sephadex granules, large particles larger than the pore size of Sephadex will move quickly. Small molecules, such as salts, will move slowly as they move inside the granules. Electrophoresis

Physical principle The electrophoresis method is as follows. A protein molecule in solution at any pH different from its isoelectric point has a certain average charge. This causes the protein to move in an electric field. The driving force is determined by the magnitude of the electric field strength E multiplied by the total charge of the particle z. This force is opposed by the viscous forces of the medium, proportional to the viscosity coefficient η , particle radius r(Stokes radius) and speed v.; E ·z = 6πηrv.

Determination of protein molecular weight. Mass spectrometry (mass spectroscopy, mass spectrography, mass spectral analysis, mass spectrometric analysis) is a method for studying a substance by determining the mass-to-charge ratio. Proteins are capable of acquiring multiple positive and negative charges. Atoms chemical elements have a specific mass. Thus, an accurate determination of the mass of the analyzed molecule allows one to determine its elemental composition (see: elemental analysis). Mass spectrometry also provides important information about the isotopic composition of the molecules being analyzed.

Methods for isolating and purifying enzymes Isolation of enzymes from biological material is the only real way to obtain enzymes . Enzyme sources: fabrics; bacteria grown on a medium containing an appropriate substrate; cellular structures (mitochondria, etc.). It is necessary to first select the necessary objects from biological material.

Methods for isolating enzymes: 1) Extraction(translation into solution): buffer solution (prevents acidification); drying with acetone ; processing the material with a mixture of butanol and water ; extraction with various organic solvents, aqueous solutions of detergents ; processing of material with perchlorates, hydrolytic enzymes (lipases, nucleases, proteolytic enzymes)

Butanol destroys the lipoprotein complex, and the enzyme passes into the aqueous phase.

Treatment with detergent results in true dissolution of the enzyme.

Fractionation. Factors influencing the results: pH, electrolyte concentration. It is necessary to constantly measure enzyme activity.

Fractional precipitation with pH changes

Fractional denaturation by heating

Fractional precipitation with organic solvents

· fractionation with salts – salting out

fractional adsorption (A. Ya. Danilevsky): the adsorbent is added to the enzyme solution, then each portion is separated by centrifugation

§ if the enzyme is adsorbed, it is separated and then eluted from the adsorbent

§ if the enzyme is not adsorbed, then treatment with an adsorbent is used to separate ballast substances

the enzyme solution is passed through a column with an adsorbent and fractions are collected

Enzymes are adsorbed selectively: column chromatography; electrophoresis; crystallization – obtaining highly purified enzymes.

The cell as the minimum unit of life.

Modern cell theory includes the following basic provisions: The cell is the basic unit of structure and development of all living organisms, the smallest unit of the living. The cells of all unicellular and multicellular organisms are similar (homologous) in structure, chemical composition, and basic manifestations of vital functions. and metabolism. Cell reproduction occurs by dividing them, i.e. every new cell. In complex multicellular organisms, cells are specialized in the function they perform and form tissues; Organs are made up of tissues. Kl is elementary living system, capable of self-renewal, self-regulation and self-production.

Cell structure. the sizes of prokaryotic cells average 0.5-5 microns, the sizes of eukaryotic cells average from 10 to 50 microns.

There are two types of cellular organization: prokaryotic and eukaryotic. Prokaryotic cells have a relatively simple structure. They do not have a morphologically separate nucleus; the only chromosome is formed by circular DNA and is located in the cytoplasm. The cytoplasm contains numerous small ribosomes; There are no microtubules, so the cytoplasm is motionless, and cilia and flagella have a special structure. Bacteria are classified as prokaryotes. Most modern living organisms belong to one of three kingdoms - plants, fungi or animals, united in the superkingdom of eukaryotes. Organisms are divided into unicellular and multicellular. Unicellular organisms consist of one single cell that performs all functions. All prokaryotes are unicellular.

Eukaryotes- organisms that, unlike prokaryotes, have a formed cell nucleus, delimited from the cytoplasm by a nuclear envelope. The genetic material is contained in several linear double-stranded DNA molecules (depending on the type of organism, their number per nucleus can range from two to several hundred), attached from the inside to the membrane of the cell nucleus and forming a complex with histone proteins in the vast majority, called chromatin. Eukaryotic cells have a system of internal membranes that, in addition to the nucleus, form a number of other organelles (endoplasmic reticulum, Golgi apparatus, etc.). In addition, the vast majority have permanent intracellular prokaryotic symbionts - mitochondria, and algae and plants also have plastids.

Biological membranes, their properties and functions One of the main features of all eukaryotic cells is the abundance and complexity of the structure of internal membranes. Membranes separate the cytoplasm from environment, and also form the shells of nuclei, mitochondria and plastids. They form a labyrinth of endoplasmic reticulum and stacked flattened vesicles that make up the Golgi complex. Membranes form lysosomes, large and small vacuoles of plant and fungal cells, and pulsating vacuoles of protozoa. All these structures are compartments (compartments) intended for certain specialized processes and cycles. Therefore, without membranes the existence of a cell is impossible. plasma membrane, or plasmalemma,- the most permanent, basic, universal membrane for all cells. It is a thin (about 10 nm) film covering the entire cell. The plasmalemma consists of protein molecules and phospholipids. Phospholipid molecules are arranged in two rows - with hydrophobic ends inward, hydrophilic heads towards the inner and outer aquatic environment. In some places, the bilayer (double layer) of phospholipids is penetrated through and through by protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules penetrate the lipid bilayer halfway on one side or the other (semi-integral proteins). There are peripheral proteins on the surface of the membranes of eukaryotic cells. Lipid and protein molecules are held together due to hydrophilic-hydrophobic interactions. Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not interconnected by covalent bonds and are able to move quite quickly in the plane of the membrane. Thanks to this, membranes can change their configuration, i.e. they have fluidity. Membranes are very dynamic structures. They quickly recover from damage and also stretch and contract with cellular movements. Membranes different types cells differ significantly both in their chemical composition and in the relative content of proteins, glycoproteins, lipids in them, and, consequently, in the nature of the receptors they contain. Each cell type is therefore characterized by an individuality, which is determined mainly glycoproteins. Branched chain glycoproteins protruding from the cell membrane are involved in recognition of factors external environment, as well as in mutual recognition of related cells. For example, an egg and a sperm recognize each other by cell surface glycoproteins that fit together as separate elements of a whole structure. Such mutual recognition is a necessary stage preceding fertilization. Associated with recognition transport regulation molecules and ions through the membrane, as well as an immunological response in which glycoproteins play the role of antigens. Sugars can thus function as information molecules (like proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, and enzyme proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, provide a structural connection between the cytoskeleton and cell membranes, or serve as receptors for receiving and converting chemical signals from the environment. selective permeability. This means that molecules and ions pass through it at different speeds, and the larger the size of the molecules, the slower the speed at which they pass through the membrane. This property defines the plasma membrane as osmotic barrier . Water and gases dissolved in it have the maximum penetrating ability; Ions pass through the membrane much more slowly. The diffusion of water through a membrane is called by osmosis.There are several mechanisms for transporting substances across the membrane.

Diffusion- penetration of substances through a membrane along a concentration gradient (from an area where their concentration is higher to an area where their concentration is lower). With facilitated diffusion special membrane transport proteins selectively bind to one or another ion or molecule and transport them across the membrane along a concentration gradient.

Active transport involves energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins that form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, which actively pumps Na + ions out while absorbing K - ions. Due to this, a higher concentration of K - and a lower concentration of Na + is maintained in the cell compared to the environment. This process requires ATP energy. As a result of active transport using a membrane pump in the cell, the concentration of Mg 2- and Ca 2+ is also regulated.

At endocytosis (endo...- inward) a certain area of ​​the plasmalemma captures and, as it were, envelops extracellular material, enclosing it in a membrane vacuole that arises as a result of invagination of the membrane. Subsequently, such a vacuole connects with a lysosome, the enzymes of which break down macromolecules into monomers.

The reverse process of endocytosis is exocytosis (exo...- out). Thanks to it, the cell removes intracellular products or undigested residues enclosed in vacuoles or vesicles. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. This is how digestive enzymes, hormones, hemicellulose, etc. are removed.

Thus, biological membranes, as the main structural elements of a cell, serve not just as physical boundaries, but are dynamic functional surfaces. Numerous biochemical processes take place on the membranes of organelles, such as active absorption of substances, energy conversion, ATP synthesis, etc.

Functions of biological membranes the following: They delimit the contents of the cell from the external environment and the contents of organelles from the cytoplasm. They ensure the transport of substances into and out of the cell, from the cytoplasm to organelles and vice versa. They act as receptors (receipt and transformation of chemical substances from the environment, recognition of cell substances, etc.). They are catalysts (providing for near-membrane chemical processes). Participate in energy conversion.

“Wherever we find life we ​​find it associated with some proteinaceous body, and wherever we find any proteinaceous body which is in the process of decomposition, we find without exception the phenomenon of life.”

Proteins – high molecular weight nitrogen-containing organic compounds, characterized by a strictly defined elemental composition and decomposing into amino acids during hydrolysis.

Features that distinguish them from other organic compounds

1. Inexhaustible variety of structure and at the same time its high specific uniqueness

2. Huge range of physical and chemical transformations

3. The ability to reversibly and quite naturally change the configuration of the molecule in response to external influences

4. Tendency to form supramolecular structures and complexes with other chemical compounds

Polypeptide theory of protein structure

only E. Fisher (1902) formulated the polypeptide theory buildings. According to this theory, proteins are complex polypeptides in which individual amino acids are linked to each other by peptide bonds that arise from the interaction of α-carboxyl COOH and α-NH 2 groups of amino acids. Using the example of the interaction of alanine and glycine, the formation of a peptide bond and a dipeptide (with the release of a water molecule) can be represented by the following equation:

The name of the peptides consists of the name of the first N-terminal amino acid with a free NH 2 group (with the ending -yl, typical for acyls), the names of subsequent amino acids (also with endings -yl) and the full name of the C-terminal amino acid with a free COOH group. For example, a pentapeptide of 5 amino acids can be designated by its full name: glycyl-alanyl-seryl-cysteinyl-alanine, or abbreviated Gly-Ala-Ser-Cys-Ala.

experimental evidence of the polypeptide theory protein structure.

1. Natural proteins contain relatively few titratable free COOH and NH 2 groups, since the absolute majority of them are in a bound state, participating in the formation of peptide bonds; Mainly free COOH and NH 2 groups at the N- and C-terminal amino acids of the peptide are available for titration.

2. In the process of acid or alkaline hydrolysis squirrel Stoichiometric amounts of titratable COOH and NH 2 groups are formed, which indicates the disintegration of a certain number of peptide bonds.

3. Under the action of proteolytic enzymes (proteinases), proteins are split into strictly defined fragments, called peptides, with terminal amino acids corresponding to the selectivity of the action of proteinases. The structure of some of these fragments of incomplete hydrolysis was proven by their subsequent chemical synthesis.

4. The biuret reaction (blue-violet coloring in the presence of a solution of copper sulfate in an alkaline medium) is given by both biuret containing a peptide bond and proteins, which is also evidence of the presence of similar bonds in proteins.

5. Analysis of X-ray diffraction patterns of protein crystals confirms the polypeptide structure of proteins. Thus, X-ray diffraction analysis with a resolution of 0.15–0.2 nm allows not only to calculate the interatomic distances and sizes of bond angles between the C, H, O and N atoms, but also to “see” the picture of the general arrangement of amino acid residues in the polypeptide chain and the spatial its orientation (conformation).

6. Significant confirmation of the polypeptide theory protein structure is the possibility of synthesizing by purely chemical methods polypeptides and proteins with an already known structure: insulin - 51 amino acid residues, lysozyme - 129 amino acid residues, ribonuclease - 124 amino acid residues. The synthesized proteins were similar to natural proteins physical and chemical properties and biological activity.

Introduction

Experimental methods

1 X-ray electron spectroscopy

1.2 Infrared spectroscopy

1.3 Diffraction methods

Theoretical methods

1 Semi-empirical methods

2 Nonempirical methods

3 Quantum mechanical methods

4 Hückel method

Conclusion

List of sources used

INTRODUCTION

In modern organic chemistry, various physical research methods are of great importance. They can be divided into two groups. The first group includes methods that make it possible to obtain various information about the structure and physical properties of a substance without making any chemical changes in it. Of the methods in this group, perhaps the most widely used is spectroscopy in a wide range of spectral regions - from not too hard X-rays to radio waves of not very long wavelengths. The second group includes methods that use physical influences that cause chemical changes in molecules. IN last years New ones have been added to the previously used well-known physical means of influencing the reactivity of a molecule. Among them, the effects of hard X-rays and high-energy particle flows produced in nuclear reactors are of particular importance.

The purpose of this course work is - learn about methods for studying the structure of molecules.

Coursework objective:

find out the types of methods and study them.

1. EXPERIMENTAL METHODS

1.1 X-ray electron spectroscopy

Figure 1—Electronic spectrometer diagram: 1—radiation source; 2-sample; 3- analyzer; 4-detector; 5-screen for protection against magnetic field

Figure 2 - X-ray electron spectrum of Cls ethyl trifluoroacetate

XPS makes it possible to study all elements, except H, when their content in the sample is ~ 10 -5 g (the detection limit of an element using XPS is 10 -7 -10 -9 g). The relative content of an element can be a fraction of a percent. Samples can be solid, liquid or gas. The value of Eb electron<#"606051.files/image003.gif">

The same formula is used to calculate the atomic factor, which describes the distribution of scattering density inside the atom. The atomic factor values ​​are specific for each type of radiation. X-rays are scattered by the electron shells of atoms. The corresponding atomic factor is numerically equal to the number of electrons in an atom if expressed in the name of electronic units, i.e. in relative units of the amplitude of X-ray scattering by one free electron. Electron scattering is determined by the electrostatic potential of the atom. The atomic factor for an electron is related by the relation:

research molecule spectroscopy diffraction quantum

Figure 2 - Dependence of the absolute values ​​of the atomic factors of X-rays (1), electrons (2) and neutrons (3) on the scattering angle

Figure 3 - Relative dependence of angle-averaged atomic factors of X-rays (solid line), electrons (dashed line) and neutrons on atomic number Z

Accurate calculations consider deviations of the distribution of electron density or potential of atoms from spherical symmetry and the name atomic temperature factor, which takes into account the influence of thermal vibrations of atoms on scattering. For radiation, in addition to scattering on the electron shells of atoms, resonant scattering on nuclei can play a role. The scattering factor f m depends on the wave vectors and polarization vectors of the incident and scattered waves. The intensity I(s) of scattering by an object is proportional to the square of the amplitude: I(s)~|F(s)| 2. Only the modules |F(s)| can be determined experimentally, and to construct the scattering density function (r) it is also necessary to know the phases (s) for each s. Nevertheless, the theory of diffraction methods makes it possible to obtain the function (r) from the measured I(s), i.e., to determine the structure of substances. In this case, the best results are obtained when studying crystals. Structural analysis . A single crystal is a strictly ordered system; therefore, during diffraction, only discrete scattered beams are formed, for which the scattering vector is equal to the reciprocal lattice vector.

To construct the function (x, y, z) from experimentally determined values, the trial and error method, construction and analysis of the function of interatomic distances, the method of isomorphic substitutions, and direct methods for determining phases are used. Processing experimental data on a computer makes it possible to reconstruct the structure in the form of scattering density distribution maps. Crystal structures are studied using X-ray structural analysis. This method has determined more than 100 thousand crystal structures.

For inorganic crystals using various methods refinements (taking into account corrections for absorption, anisotropy of the atomic temperature factor, etc.) it is possible to restore the function with a resolution of up to 0.05

Figure 4 - Projection of nuclear density of crystal structure

This makes it possible to determine the anisotherapy of thermal vibrations of atoms, features of the distribution of electrons caused by chemical bonds, etc. Using X-ray diffraction analysis, it is possible to decipher the atomic structures of protein crystals, the molecules of which contain thousands of atoms. X-ray diffraction is also used to study defects in crystals (in X-ray topography), study surface layers (in X-ray spectrometry), and qualitatively and quantitatively determine the phase composition of polycrystalline materials. Electron diffraction as a method for studying the structure of crystals has a following. features: 1) the interaction of matter with electrons is much stronger than with x-rays, therefore diffraction occurs in thin layers of matter with a thickness of 1-100 nm; 2) f e depends on the atomic nucleus less strongly than f p, which makes it easier to determine the position of light atoms in the presence of heavy ones; Structural electron diffraction is widely used to study finely dispersed objects, as well as to study various types of textures (clay minerals, semiconductor films, etc.). Low energy electron diffraction (10 -300 eV, 0.1-0.4 nm) - effective method studies of crystal surfaces: the arrangement of atoms, the nature of their thermal vibrations, etc. Electron microscopy reconstructs the image of an object from the diffraction pattern and allows you to study the structure of crystals with a resolution of 0.2-0.5 nm. Neutron sources for structural analysis are nuclear reactors with fast neutrons, as well as pulsed reactors. The spectrum of the neutron beam emerging from the reactor channel is continuous due to the Maxwellian velocity distribution of neutrons (its maximum at 100°C corresponds to a wavelength of 0.13 nm).

Beam monochromatization is carried out in different ways - with the help of monochromator crystals, etc. Neutron diffraction is used, as a rule, to clarify and supplement X-ray structural data. The absence of a monotonic dependence of f and on the atomic number allows one to determine the position of light atoms quite accurately. In addition, isotopes of the same element can have very different values ​​of f and (for example, f and hydrocarbons are 3.74.10 13 cm, for deuterium 6.67.10 13 cm). This makes it possible to study the arrangement of isotopes and obtain complementary information. structural information by isotope substitution. Study of magnetic interaction. neutrons with magnetic moments of atoms provides information about the spins of magnetic atoms. Mössbauer radiation is distinguished by an extremely small linewidth - 10 8 eV (while the linewidth of the characteristic radiation of X-ray tubes is 1 eV). This results in a high level of time and space. consistency of resonant nuclear scattering, which allows, in particular, to study the magnetic field and electric field gradient on nuclei. The limitations of the method are the weak power of Mössbauer sources and the obligatory presence in the crystal under study of nuclei for which the Mössbauer effect is observed. Structural analysis non-crystalline substances. Individual molecules in gases, liquids and amorphous solids are differently oriented in space, so it is usually impossible to determine the phases of scattered waves. In these cases, the scattering intensity is usually represented using the so-called. interatomic vectors r jk, which connect pairs of different atoms (j and k) in molecules: r jk = r j - r k. The scattering pattern is averaged over all orientations:

.1 Semi-empirical methods

Semi-empirical methods of quantum chemistry, methods of calculating mol. characteristics or properties of a substance using experimental data. At their core, semi-empirical methods are similar to non-empirical methods for solving the Schrödinger equation for polyatomic systems, however, to facilitate calculations in semi-empirical methods, additional additions are introduced. simplification. As a rule, these simplifications are associated with the valence approximation, that is, they are based on the description of only valence electrons, as well as with the neglect of certain classes of molecular integrals in the exact equations of the non-empirical method within which the semi-empirical calculation is carried out.

The choice of empirical parameters is based on a generalization of the experience of ab initio calculations, taking into account chemical concepts about the structure of molecules and phenomenological patterns. In particular, these parameters are necessary to approximate the influence of internal electrons on valence electrons, to set effective potentials created by core electrons, etc. The use of experimental data to calibrate empirical parameters allows us to eliminate errors caused by the simplifications mentioned above, but only for those classes of molecules whose representatives serve as reference molecules, and only for those properties from which the parameters were determined.

The most common are semi-empirical methods based on ideas about mol. orbitals (see Molecular orbital methods, Orbital). In combination with the LCAO approximation, this makes it possible to express the Hamiltonian of a molecule in terms of integrals on atomic orbitals. When constructing semi-empirical methods in mol. In integrals, products of orbitals depending on the coordinates of the same electron (differential overlap) are distinguished and certain classes of integrals are neglected. For example, if all integrals containing the differential overlap cacb for a are considered zero. b, it turns out the so-called. method of completely neglecting the differential. overlap (PPDP, in English transcription CNDO-complete neglect of differential overlap). Partial or modified partial neglect of differential overlap is also used (corresponding to ChPDP or MChPDP, in English transcription INDO - intermediate neglect of differential overlap and MINDO-modified INDO), neglect of diatomic differential overlap - PDDP, or neglect of diatomic differential overlap (NDDO), - modified neglect of diatomic overlap (MNDO). As a rule, each of the semi-empirical methods has several options, which are usually indicated in the name of the method with a number or letter after a slash. For example, the PPDP/2, MCDP/3, MPDP/2 methods are parameterized for calculating the equilibrium configuration of molecular nuclei in the ground electronic state, charge distribution, ionization potentials, enthalpies of formation of chemical compounds, the PPDP method is used to calculate spin densities. To calculate electronic excitation energies, spectroscopic parameterization is used (PPDP/S method). It is also common to use corresponding computer programs in the names of semi-empirical methods. For example, one of the extended versions of the MPDP method is called the Austin model, as is the corresponding program (Austin model, AM). There are several hundred different variants of semi-empirical methods; in particular, semi-empirical methods have been developed that are similar to the configuration interaction method. Given the external similarity of different versions of semi-empirical methods, each of them can be used to calculate only those properties for which the empirical parameters were calibrated. In max. simple semi-empirical calculations, each mol. the orbital for valence electrons is defined as the solution of the one-electron Schrödinger equation with the Hamilton operator containing the model potential (pseudopotential) for an electron located in the field of nuclei and the averaged field of all other electrons in the system. Such a potential is specified directly using elementary functions or integral operators based on them. In combination with the LCAO approximation, this approach allows for many conjugated and aromatic mol. systems, limit yourself to the analysis of p-electrons (see Hückel's method); for coordination compounds, use calculation methods of ligand field theory and crystal field theory, etc. When studying macromolecules, e.g. proteins or crystalline formations are often used semi-empirical methods, in which the electronic structure is not analyzed, but the potential energy surface is determined directly. The energy of the system is approximately considered the sum of pairwise interaction potentials of atoms, for example. Morse (Morse) or Lennard-Jones potentials (see Intermolecular interactions). Such semi-empirical methods make it possible to calculate equilibrium geometry, conformational effects, isomerization energy, etc. Often, pair potentials are supplemented with multiparticle corrections specific for individual fragments of the molecule. Semi-empirical methods of this type are usually referred to as molecular mechanics. In a broader sense, semi-empirical methods include any methods in which the parameters are determined by solving inverse problems. systems are used to predict new experimental data and build correlation relationships. In this sense, semi-empirical methods are methods for assessing reactivity, effective charges on atoms, etc. The combination of semi-empirical calculation of the electronic structure with correlation. relationships allows you to evaluate the biological activity of various substances, rates of chemical reactions, parameters technological processes. Semi-empirical methods also include some additive schemes, for example. methods used in chemical thermodynamics for estimating the energy of formation as the sum of the contributions of individual fragments of the molecule. The intensive development of semi-empirical methods and non-empirical methods of quantum chemistry makes them important tools for modern research into chemical mechanisms. transformations, dynamics of an elementary chemical act. reactions, modeling of biochemical and technological processes. When used correctly (taking into account the principles of construction and methods for calibrating parameters), semi-empirical methods make it possible to obtain reliable information about the structure and properties of molecules and their transformations.

2.2Non-empirical methods

A fundamentally different direction of computational quantum chemistry, which played a huge role in modern development chemistry in general, consists in a complete or partial refusal to calculate the one-electron (3.18) and two-electron (3.19)-(3.20) integrals appearing in the HF method. Instead of the exact Fock operator, an approximate one is used, the elements of which are obtained empirically. The parameters of the Fock operator are selected for each atom (sometimes taking into account a specific environment) or for pairs of atoms: they are either fixed or depend on the distance between the atoms. In this case, it is often (but not necessarily - see below) assumed that the many-electron wave function is single-determinant, the basis is minimal, and the atomic orbitals are X; - symmetric orthogonal combinations of OST Xg Such combinations can be easily obtained by approximating the original AO with Slater functions "Xj(2.41) using the transformation Semi-empirical methods are much faster than ab initio ones. They are applicable to large (often very large, for example, biological) systems and for some classes of compounds they give more accurate results. However, it should be understood that this is achieved through specially selected parameters that are valid only within a narrow class of compounds. When transferred to other compounds, the same methods can give completely incorrect results. In addition, parameters are often selected to reproduce only certain molecular properties, so it is not necessary to assign physical meaning to individual parameters used in the calculation scheme. Let us list the main approximations used in semi-empirical methods.

Only valence electrons are considered. It is believed that electrons belonging to atomic cores only screen the nuclei. Therefore, the influence of these electrons is taken into account by considering the interaction of valence electrons with atomic cores, rather than with nuclei, and by introducing the core repulsion energy instead of the internuclear repulsion energy. The polarization of the cores is neglected.

In MO, only AOs with a principal quantum number corresponding to the highest electron-occupied orbitals of isolated atoms (minimum basis) are taken into account. It is assumed that the basis functions form a set of orthonormal atomic orbitals - OCT, orthogonalized according to Löwdin.

For two-electron Coulomb and exchange integrals, the zero differential overlap (NDO) approximation is introduced.

The molecular structure within the structural region may correspond to a set of modifications of the molecule that retain the same system of valence chemical bonds with different spatial organization of the nuclei. In this case, the deep minimum of the PES additionally has several shallow (equivalent or nonequivalent in energy) minima, separated by small potential barriers. Various spatial forms of a molecule, transforming into each other within a given structural region by continuously changing the coordinates of atoms and functional groups without breaking or forming chemical bonds, constitute the many conformations of the molecule. A set of conformations whose energies are less than the lowest barrier adjacent to a given structural region of the PES is called a conformational isomer, or conformer. Conformers corresponding to local minima of the PES are called stable or stable. Thus, molecular structure can be defined as the set of conformations of a molecule in a certain structural region. A type of conformational transition often found in molecules is the rotation of individual groups of atoms about bonds: internal rotation is said to occur, and the various conformers are called rotational isomers, or rotamers. During rotation, the electronic energy also changes, and its value during such movement can pass through a maximum; in this case we speak of an internal rotation barrier. The latter are largely due to the ability of these molecules to easily adapt the structure when interacting with different systems. Each energy minimum of the PES corresponds to a pair of enantiomers with the same energy - right (R) and left (S). These pairs have energies that differ by only 3.8 kcal/mol, but they are separated by a barrier with a height of 25.9 kcal/mol and, therefore, are very stable in the absence of external influences. Results of quantum chemical calculations of internal rotation barrier energies for some molecules and corresponding experimental values. Theoretical and experimental values ​​of rotation barriers for C-C connections, C-P, C-S differ by only 0.1 kcal/mol; for the C-0, C-N, C-Si bonds, despite the use of a basis set with the inclusion of polarization functions (see below), the difference is noticeably higher. 1 However, we can state a satisfactory accuracy in calculating the energies of internal rotation barriers using the HF method.

In addition to spectroscopic applications, such calculations of internal rotation barrier energies for simple molecules are important as a criterion for the quality of a particular calculation method. Internal rotation deserves great attention in complex molecular systems, for example, in polypeptides and proteins, where this effect determines many biologically important functions of these compounds. Calculating potential energy surfaces for such objects is a complex task, both theoretically and practically. A common type of conformational transition is inversion, such as occurs in pyramidal molecules of the AX3 type (A = N, Si, P, As, Sb; X = H, Li, F, etc.). In these molecules, the A atom can occupy positions both above and below the plane formed by three X atoms. For example, in the ammonia molecule NH3, the CP method gives an energy barrier value of 23.4 kcal/mol; this is in good agreement with the experimental value of the inversion barrier - 24.3 kcal/mol. If the barriers between the PES minima are comparable to the thermal energy of the molecule, this leads to the effect of structural non-rigidity of the molecule; Conformational transitions in such molecules occur constantly. To solve the HF equations, the self-consistent field method is used. In the solution process, only the orbitals occupied by electrons are optimized; therefore, the energies of only these orbitals are found physically justifiably. However, the method. HF also gives the characteristics of free orbitals: such molecular spin orbitals are called virtual. Unfortunately, they describe the excited energy levels of a molecule with an error of about 100%, and they should be used with caution to interpret spectroscopic data - there are other methods for this. As well as for atoms, the HF method for molecules has different versions, depending on whether the one-determinant wave function is an eigenfunction of the operator of the square of the total spin of the system S2 or not. If the wave function is constructed from spatial orbitals occupied by a pair of electrons with opposite spins (closed-shell molecules), this condition is satisfied, and the method is called the restricted Hartree-Fock (HRF) method. If the requirement to be an eigenfunction of the operator is not imposed on the wave function, then each molecular spin-orbital corresponds to a specific spin state (a or 13), that is, electrons with opposite spins occupy different spin-orbitals. This method is usually used for molecules with open shells and is called the unrestricted HF method (UHF), or the method of different orbitals for different spins. Sometimes low-lying energy states are described by orbitals doubly occupied by electrons, and valence states are described by singly occupied molecular spin orbitals; This method is called the restricted Hartree-Fock method for open shells (OHF-00). As in atoms, the wave function of molecules with open shells does not correspond to a pure spin state, and solutions may arise in which the spin symmetry of the wave function is reduced. They are called NHF-unstable solutions.

2.3 Quantum mechanical methods

Advances in theoretical chemistry and the development of quantum mechanics have created the possibility of approximate quantitative calculations of molecules. There are two important calculation methods: the electron pair method, also called the valence bond method, and the molecular orbital method. The first of these methods, developed by Heitler and London for the hydrogen molecule, became widespread in the 30s of this century. In recent years, the molecular orbit method has become increasingly important (Gund, E. Hückel, Mulliken, Herzberg, Lenard-Jones).

In this approximate calculation method, the state of the molecule is described by the so-called wave function ψ, which is composed according to a certain rule from a number of terms:

The sum of these terms must take into account all possible combinations resulting from the pairwise bonding of carbon atoms due to π electrons.

In order to facilitate the calculation of the wave function ψ, the individual terms (C1ψ1, C2ψ2, etc.) are conventionally depicted graphically in the form of corresponding valence schemes, which are used as auxiliaries in mathematical calculations. For example, when in the specified way calculate the benzene molecule and take into account only π-electrons, then there are five such terms. These terms correspond to the following valence schemes:

The given valence schemes are often depicted taking into account σ bonds, for example for benzene

Such valence patterns are called "unperturbed structures" or "limit structures"

The functions ψ1, ψ2, ψ3, etc. of various limiting structures are included in the wave function ψ with larger coefficients (with greater weight) the lower the energy calculated for the corresponding structure. The electronic state corresponding to the wave function ψ is the most stable compared to the electronic states represented by the functions ψ1, ψ2, ψ3, etc.; the energy of the state represented by the function ψ (of a real molecule) is naturally the smallest compared to the energies of limiting structures.

When calculating the benzene molecule using the electron pair method, five limiting structures (I-V) are taken into account. Two of them are identical to the classical Kekule structural formula and the Dewar tri-formula. Since the energy of the electronic states corresponding to the limiting structures III, IV and V is higher than for structures I and II, the contribution structures III, IV and V to the mixed wave function of the benzene molecule ψ is less than the contribution of structures I and II. Therefore, to a first approximation, two equivalent Kekulé structures are sufficient to depict the electron density distribution in a benzene molecule.

Limit structures do not correspond to any real electronic states in unexcited molecules, but it is possible that they can occur in an excited state or at the moment of a reaction.

The above qualitative side of the theory of resonance coincides with the concept of mesomerism, somewhat earlier developed by Ingold and independently by Arndt.

According to this concept, the true state of a molecule is intermediate ("mesomeric") between the states depicted by two or more "limit structures" that can be written for a given molecule using the rules of valency.

In addition to this basic position of the theory of mesomerism, its apparatus includes well-developed ideas about electronic displacements, in the justification, interpretation and experimental verification of which Ingold plays an important role. According to Ingold, the mechanisms of electronic displacements (electronic effects) are different depending on whether the mutual influence of atoms is carried out through a chain of simple or conjugated double bonds. In the first case, this is the induction effect I (or also the static induction effect Is), in the second case, the mesomeric effect M (static conjugation effect).

In a reacting molecule, the electron cloud can be polarized by an inductive mechanism; this electronic displacement is called the inductomeric effect Id. In molecules with conjugated double bonds (and in aromatic molecules), the polarizability of the electron cloud at the time of reaction is due to the electromer effect E (dynamic conjugation effect).

The resonance theory does not raise any fundamental objections as long as we are talking about ways to image molecules, but it also has great claims. Similar to how in the electron-pair method the wave function is described by a linear combination of other wave functions ψ1, ψ2, ψ3, etc., the resonance theory proposes to describe the true wave function of a molecule as a linear combination of the wave functions of limiting structures.

However, mathematics does not provide criteria for choosing certain “resonance structures”: after all, in the electron pair method, the wave function can be represented not only as a linear combination of wave functions ψ1, ψ2, ψ3, etc., but also as a linear combination of any other functions , selected with certain coefficients. The choice of limiting structures can only be made on the basis of chemical considerations and analogies, i.e. here the concept of resonance essentially does not provide anything new in comparison with the concept of mesomerism.

When describing the distribution of electron density in molecules using limiting structures, it is necessary to constantly keep in mind that individual limiting structures do not correspond to any real physical state and that no physical phenomenon of “electronic resonance” exists.

Numerous cases are known from the literature when supporters of the concept of resonance attributed the meaning of a physical phenomenon to resonance and believed that certain individual limiting structures were responsible for certain properties of substances. The possibility of such misconceptions is inherent in many points of the concept of resonance. Thus, when they talk about “various contributions of limiting structures” to the real state of the molecule, the idea of ​​the real existence of these relationships can easily arise. A real molecule in the concept of resonance is considered a "resonant hybrid"; this term may suggest the supposedly real interaction of limiting structures, like the hybridization of atomic orbits.

The term “stabilization due to resonance” is also unsuccessful, since the stabilization of a molecule cannot be caused by a non-existent resonance, but is a physical phenomenon of delocalization of the electron density, characteristic of conjugated systems. It is therefore appropriate to call this phenomenon stabilization due to conjugation. The conjugation energy (delocalization energy, or mesomerism energy) can be determined experimentally, independently of the “resonance energy” resulting from quantum mechanical calculations. This is the difference between the energy calculated for a hypothetical molecule with a formula corresponding to one of the limiting structures, and the energy found experimentally for a real molecule.

With the above reservations, the method of describing the distribution of electron density in molecules using several limiting structures can undoubtedly be used along with two other also very common methods.

2.4 Hückel method

Hückel method, quantum chemical method for approximate calculation of energy levels and mol. orbitals of unsaturated org. connections. It is based on the assumption that the movement of an electron near an atomic nucleus in a molecule does not depend on the states or number of other electrons. This makes it possible to simplify the task of determining the mol. orbitals (MO) represented by a linear combination of atomic orbitals. The method was proposed by E. Hückel in 1931 for calculating the electronic structure of hydrocarbons with conjugated bonds. It is believed that the carbon atoms of a conjugated system lie in the same plane, relative to which the highest occupied and lowest virtual (free) MOs (frontier molecular orbitals) are antisymmetric, i.e., they are orbitals formed by atomic 2pz orbitals (AO) of the corresponding C atoms. The influence of other atoms, for example. N, or mol. fragments with saturated connections are neglected. It is assumed that each of the M carbon atoms of the conjugated system contributes one electron to the system and is described by one atomic 2pz orbital (k = 1, 2, ..., M). A simple model of the electronic structure of a molecule, given by the Hückel method, allows us to understand many chemical reactions. phenomena. For example, the nonpolarity of alternant hydrocarbons is due to the fact that the effective charges on all carbon atoms are equal to zero. In contrast, the nonalternant fused system of 5- and 7-membered rings (azulene) has a dipole moment of ca. 1D (3.3 x 10 -30 C x m). In odd alternant hydrocarbons the main energy source is. the state corresponds to an electronic system in which there is at least one singly occupied orbital. It can be shown that the energy of this orbital is the same as in a free atom, and therefore it is called. non-binding MO. Removing or adding an electron changes the population of only the nonbonding orbital, which entails the appearance of a charge on some atoms, which is proportional to the square of the corresponding coefficient in the expansion of the nonbonding MO in the AO. To determine such a MO, a simple rule is used: the sum of the coefficient Ck for all atoms adjacent to any given one must be equal to zero. In addition, the coefficient values ​​must correspond to the additional normalization condition: This leads to a characteristic alternation (alternation) of charges on atoms in mol. ions of alternant hydrocarbons. In particular, this rule explains the separation by chemical. properties of the ortho and para positions in the benzene ring compared to the meta position. The regularities established within the framework of the simple Hückel method are distorted when all interactions in the molecule are more fully taken into account. However, usually the influence of many heterogeneous complementary factors (for example, core electrons, substituents, interelectron repulsion, etc.) does not qualitatively change the orbital picture of the electron distribution. Therefore, the Hückel method is often used to model complex reaction mechanisms involving org. connections. When heteroatoms (N, O, S, ...) are introduced into the molecule, the parameters of the matrix H taken for the heteroatom and for carbon atoms become significant. Unlike the case of polyenes, different types of atoms or bonds are described by different parameters or, and their ratio significantly affects the type of MO; The quality of predictions obtained within the framework of the simple Hückel method, as a rule, ultimately deteriorates. Simple in concept, visual and not requiring complex calculations, the Hückel method is one of the most common means of creating a quantum chemical model of the electronic structure of complex molecules. systems Naib. Its use is effective in cases where the properties of the molecule are determined by the basic topological structure of the chemical. bonds, in particular the symmetry of the molecule. Attempts to construct improved versions of the Hückel method within the framework of simple molecular orbital methods make little sense, since they lead to calculation methods comparable in complexity to the more accurate methods of quantum chemistry.

Conclusion

Currently, “an entire branch of science has been created - quantum chemistry, which deals with the application of quantum mechanical methods to chemical problems. However, it would be fundamentally mistaken to think that all questions of the structure and reactivity of organic compounds can be reduced to problems of quantum mechanics. Quantum mechanics studies the laws of motion of electrons and nuclei, i.e., the laws of the lowest form of motion, in comparison with the one studied by chemistry (the movement of atoms and molecules), and the highest form of motion can never be reduced to the lowest. Even for very simple molecules, issues such as the reactivity of substances, the mechanism and kinetics of their transformations cannot be studied only by the methods of quantum mechanics. The basis for studying the chemical form of the movement of matter is chemical research methods, and the leading role in the development of chemistry belongs to the theory of chemical structure.

The vast majority of information about substances, their properties and chemical transformations was obtained through chemical or physicochemical experiments. Therefore, the main method used by chemists should be considered a chemical experiment.

The traditions of experimental chemistry have evolved over centuries. Even when chemistry was not an exact science, in ancient times and in the Middle Ages, scientists and artisans sometimes accidentally, and sometimes purposefully discovered methods for obtaining and purifying many substances that were used in economic activity: metals, acids, alkalis, dyes, etc. Alchemists contributed greatly to the accumulation of such information (see Alchemy).

Thanks to this, by the beginning of the 19th century. chemists were well versed in the basics of experimental art, especially methods for purifying all kinds of liquids and solids, which allowed them to make many important discoveries. And yet, chemistry began to become a science in the modern sense of the word, an exact science, only in the 19th century, when the law of multiple ratios was discovered and atomic-molecular science was developed. Since that time, chemical experiment began to include not only the study of the transformations of substances and methods of their isolation, but also the measurement of various quantitative characteristics.

A modern chemical experiment involves many different measurements. Both the equipment for conducting experiments and chemical glassware have changed. In a modern laboratory you will not find homemade retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure. Working methods have also become standard, which in our time no longer has to be reinvented by every chemist. A description of the best of them, proven by many years of experience, can be found in textbooks and manuals.

Methods for studying matter have become not only more universal, but also much more diverse. An increasingly important role in the work of a chemist is played by physical and physicochemical research methods designed to isolate and purify compounds, as well as to establish their composition and structure.

The classical technique of purifying substances was extremely labor intensive. There are cases where chemists spent years of work isolating an individual compound from a mixture. Thus, salts of rare earth elements were isolated in pure form only after thousands of fractional crystallizations. But even after this, the purity of the substance could not always be guaranteed.

Modern chromatography methods make it possible to quickly separate a substance from impurities (preparative chromatography) and check its chemical identity (analytical chromatography). In addition, classical but highly improved methods of distillation, extraction and crystallization are widely used to purify substances, as well as such effective modern methods, such as electrophoresis, zone melting, etc.

The task that confronts a synthetic chemist after isolating a pure substance - to establish the composition and structure of its molecules - relates to a large extent to analytical chemistry. With the traditional working technique, it was also very labor-intensive. Almost the only measurement method previously used was elemental analysis, which allows one to establish the simplest formula of a compound.

To determine the true molecular as well as structural formula, it was often necessary to study the reactions of a substance with various reagents; isolate the products of these reactions in individual form, in turn establishing their structure. And so on - until, based on these transformations, the structure of the unknown substance became obvious. Therefore, establishing the structural formula of a complex organic compound often took a lot of time, and such work was considered complete if it ended with a counter synthesis - the production of a new substance in accordance with the formula established for it.

This classical method was extremely useful for the development of chemistry in general. Nowadays it is rarely used. As a rule, an isolated unknown substance, after elemental analysis, is studied using mass spectrometry, spectral analysis in the visible, ultraviolet and infrared ranges, as well as nuclear magnetic resonance. For a reasonable derivation of a structural formula, the use of a whole complex of methods is required, and their data usually complement each other. But in a number of cases, conventional methods do not give an unambiguous result, and one has to resort to direct methods of determining the structure, for example, X-ray diffraction analysis.

Physicochemical methods are used not only in synthetic chemistry. They are no less important when studying the kinetics of chemical reactions, as well as their mechanisms. The main task of any experiment to study the rate of a reaction is to accurately measure the time-varying, and usually very small, concentration of the reactant. To solve this problem, depending on the nature of the substance, you can use both chromatographic methods and different kinds spectral analysis, and methods of electrochemistry (see Analytical chemistry).

The perfection of technology has reached such high level, that it became possible to accurately determine the rate of even “instantaneous”, as previously believed, reactions, for example, the formation of water molecules from hydrogen cations and anions. With an initial concentration of both ions equal to 1 mol/l, the time of this reaction is several hundred billionths of a second.

Physicochemical research methods are specially adapted for the detection of short-lived intermediate particles formed during chemical reactions. To do this, the devices are equipped with either high-speed recording devices or attachments that ensure operation at very low temperatures. These methods successfully record the spectra of particles whose lifespan under normal conditions is measured in thousandths of a second, for example, free radicals.

In addition to experimental methods in modern chemistry calculations are widely used. Thus, the thermodynamic calculation of a reacting mixture of substances makes it possible to accurately predict its equilibrium composition (see Chemical equilibrium).

Calculations of molecules based on quantum mechanics and quantum chemistry have become generally accepted and in many cases indispensable. These methods are based on a very complex mathematical apparatus and require the use of the most advanced electronic computers - computers. They make it possible to create models of the electronic structure of molecules that explain the observable, measurable properties of unstable molecules or intermediate particles formed during reactions.

Methods for studying substances developed by chemists and physical chemists are useful not only in chemistry, but also in related sciences: physics, biology, geology. Neither industry nor Agriculture, neither medicine nor criminology. Physicochemical instruments occupy a place of honor on spacecraft, with the help of which the near-Earth space and neighboring planets are explored.

Therefore, knowledge of the basics of chemistry is necessary for every person, regardless of his profession, and the further development of its methods is one of the most important directions of the scientific and technological revolution.

Substance analysis methods

X-ray diffraction analysis

X-ray diffraction analysis is a method for studying the structure of bodies, using the phenomenon of X-ray diffraction, a method for studying the structure of matter by the spatial distribution and intensity of X-ray radiation scattered on the analyzed object. The diffraction pattern depends on the wavelength of the x-rays used and the structure of the object. To study atomic structure, radiation with a wavelength on the order of the size of the atom is used.

X-ray diffraction analysis methods are used to study metals, alloys, minerals, inorganic and organic compounds, polymers, amorphous materials, liquids and gases, protein molecules, nucleic acids, etc. X-ray diffraction analysis is the main method for determining the structure of crystals.

When studying crystals, it provides the most information. This is due to the fact that crystals have a strictly periodic structure and represent a diffraction grating for x-rays created by nature itself. However, it also provides valuable information when studying bodies with a less ordered structure, such as liquids, amorphous bodies, liquid crystals, polymers and others. Based on numerous already deciphered atomic structures, the inverse problem can also be solved: from the X-ray diffraction pattern of a polycrystalline substance, for example, alloy steel, alloy, ore, lunar soil, the crystalline composition of this substance can be established, that is, a phase analysis can be performed.

X-ray diffraction analysis makes it possible to objectively determine the structure of crystalline substances, including complex substances such as vitamins, antibiotics, coordination compounds, etc. A complete structural study of a crystal often allows one to solve purely chemical problems, for example, establishing or clarifying the chemical formula, type of bond, molecular weight at a known density or density at a known molecular weight, symmetry and configuration of molecules and molecular ions.

X-ray diffraction analysis is successfully used to study the crystalline state of polymers. X-ray diffraction analysis also provides valuable information in the study of amorphous and liquid bodies. X-ray patterns of such bodies contain several blurred diffraction rings, the intensity of which quickly decreases with increasing intensity. Based on the width, shape and intensity of these rings, one can draw conclusions about the features of short-range order in a particular liquid or amorphous structure.

X-ray diffractometers "DRON"

X-ray fluorescence analysis (XRF)

One of the modern spectroscopic methods for studying a substance in order to obtain its elemental composition, i.e. its elemental analysis. The XRF method is based on the collection and subsequent analysis of a spectrum obtained by exposing the material under study to X-ray radiation. When irradiated, the atom goes into an excited state, accompanied by the transition of electrons to higher quantum levels. The atom remains in an excited state for an extremely short time, on the order of one microsecond, after which it returns to a quiet position (ground state). In this case, electrons from the outer shells either fill the resulting vacancies, and the excess energy is emitted in the form of a photon, or the energy is transferred to another electron from the outer shells (Auger electron). In this case, each atom emits a photoelectron with an energy of a strictly defined value, for example, iron, when irradiated with X-rays, emits photons K? = 6.4 keV. Then, according to the energy and number of quanta, the structure of the substance is judged.

In X-ray fluorescence spectrometry, it is possible to conduct a detailed comparison of samples not only in terms of the characteristic spectra of elements, but also in terms of the intensity of background (bremsstrahlung) radiation and the shape of Compton scattering bands. It's gaining special meaning in the case when the chemical composition of two samples is the same according to the results of quantitative analysis, but the samples differ in other properties, such as grain size, crystallite size, surface roughness, porosity, humidity, the presence of water of crystallization, quality of polishing, spray thickness, etc. Identification is performed on based on a detailed comparison of spectra. There is no need to know the chemical composition of the sample. Any difference in the compared spectra irrefutably indicates that the sample under study differs from the standard.

This type of analysis is carried out when it is necessary to identify the composition and some physical properties of two samples, one of which is a reference. This type of analysis is important when looking for any differences in the composition of two samples. Scope of application: determination of heavy metals in soils, sediments, water, aerosols, qualitative and quantitative analysis of soils, minerals, rocks, quality control of raw materials, production process and finished products, analysis of lead paints, measurement of concentrations of valuable metals, determination of oil and fuel contamination , determination of toxic metals in food ingredients, analysis of trace elements in soils and agricultural products, elemental analysis, dating of archaeological finds, study of paintings, sculptures, for analysis and examination.

Typically, preparing samples for all types of X-ray fluorescence analysis is not difficult. To conduct a highly reliable quantitative analysis, the sample must be homogeneous and representative, have a mass and size not less than that required by the analysis technique. Metals are ground, powders are crushed to particles of a given size and pressed into tablets. Rocks are fused to a glassy state (this reliably eliminates errors associated with sample heterogeneity). Liquids and solids are simply placed in special cups.

Spectral analysis

Spectral analysis- a physical method for the qualitative and quantitative determination of the atomic and molecular composition of a substance, based on the study of its spectra. Physical basis of S. a. - spectroscopy of atoms and molecules, it is classified according to the purposes of analysis and types of spectra (see Optical spectra). Atomic S. a. (ACA) determines the elemental composition of a sample from the atomic (ion) emission and absorption spectra; molecular S. a. (MSA) - molecular composition of substances based on molecular spectra of absorption, luminescence and Raman scattering of light. Emission S. a. produced by the emission spectra of atoms, ions and molecules excited by various sources of electromagnetic radiation in the range from?-radiation to microwave. Absorption S. a. carried out using the absorption spectra of electromagnetic radiation by the analyzed objects (atoms, molecules, ions of matter in various states of aggregation). Atomic spectral analysis (ASA) Emission ASA consists of the following main processes:

1. selection of a representative sample reflecting the average composition of the analyzed material or the local distribution of the determined elements in the material;
2. introducing a sample into a radiation source, in which evaporation of solid and liquid samples, dissociation of compounds and excitation of atoms and ions occur;
3. converting their glow into a spectrum and recording it (or visual observation) using a spectral device;
4. interpretation of the obtained spectra using tables and atlases of spectral lines of elements.

This stage ends qualitative ASA. The most effective is the use of sensitive (so-called “last”) lines that remain in the spectrum at a minimum concentration of the element being determined. Spectrograms are viewed on measuring microscopes, comparators, and spectroprojectors. For qualitative analysis, it is enough to establish the presence or absence of analytical lines of the elements being determined. Based on the brightness of the lines during visual inspection, one can give a rough estimate of the content of certain elements in the sample.

Quantitative ASA is carried out by comparing the intensities of two spectral lines in the spectrum of the sample, one of which belongs to the element being determined, and the other (comparison line) to the main element of the sample, the concentration of which is known, or an element specially introduced at a known concentration (“internal standard”).

Atomic absorption S. a.(AAA) and atomic fluorescent S. a. (AFA). In these methods, the sample is converted into vapor in an atomizer (flame, graphite tube, stabilized RF or microwave discharge plasma). In AAA, light from a source of discrete radiation, passing through this vapor, is attenuated, and by the degree of attenuation of the intensities of the lines of the element being determined, its concentration in the sample is judged. AAA is carried out using special spectrophotometers. The AAA technique is much simpler compared to other methods; it is characterized by high accuracy in determining not only small, but also large concentrations of elements in samples. AAA successfully replaces labor-intensive and time-consuming chemical analysis methods without being inferior to them in accuracy.

In AFA, atomic pairs of the sample are irradiated with light from a resonant radiation source and the fluorescence of the element being determined is recorded. For some elements (Zn, Cd, Hg, etc.), the relative limits of their detection by this method are very small (10-5-10-6%).

ASA allows measurements of isotopic composition. Some elements have spectral lines with a well-resolved structure (for example, H, He, U). The isotopic composition of these elements can be measured on conventional spectral instruments using light sources that produce thin spectral lines (hollow cathode, electrodeless HF and microwave lamps). To carry out isotopic spectral analysis of most elements, high-resolution instruments are required (for example, the Fabry-Perot standard). Isotopic spectral analysis can also be carried out using the electronic vibrational spectra of molecules, measuring isotopic shifts of bands, which in some cases reach significant values.

ASA plays a significant role in nuclear technology, the production of pure semiconductor materials, superconductors, etc. More than 3/4 of all analyzes in metallurgy are performed using ASA methods. Quantometers are used to carry out operational (within 2-3 minutes) control during melting in open-hearth and converter production. In geology and geological exploration, about 8 million analyzes are performed per year to evaluate deposits. ASA is used for environmental protection and soil analysis, in forensics and medicine, seabed geology and the study of the composition of the upper atmosphere, in isotope separation and determining the age and composition of geological and archaeological objects, etc.

Infrared spectroscopy

The IR method includes obtaining, studying and applying emission, absorption and reflection spectra in the infrared region of the spectrum (0.76-1000 microns). ICS is mainly concerned with the study of molecular spectra, because The majority of vibrational and rotational spectra of molecules are located in the IR region. The most widespread study is the study of IR absorption spectra that arise when IR radiation passes through a substance. In this case, energy is selectively absorbed at those frequencies that coincide with the rotation frequencies of the molecule as a whole, and in the case of a crystalline compound, with the vibration frequencies of the crystal lattice.

IR absorption spectrum - probably unique of its kind physical property. There are no two compounds, with the exception of optical isomers, with different structures but the same IR spectra. In some cases, such as polymers with similar molecular weights, the differences may be almost imperceptible, but they are always there. In most cases, the IR spectrum is a “fingerprint” of a molecule, which is easily distinguishable from the spectra of other molecules.

In addition to the fact that absorption is characteristic of individual groups of atoms, its intensity is directly proportional to their concentration. That. measuring the absorption intensity gives, after simple calculations, the amount of a given component in the sample.

IR spectroscopy is used in studying the structure of semiconductor materials, polymers, biological objects and living cells directly. In the dairy industry, the infrared spectroscopy method is used to determine the mass fraction of fat, protein, lactose, solids, freezing point, etc.

The liquid substance is most often removed as a thin film between caps of NaCl or KBr salts. The solid is most often removed as a paste in petroleum jelly. Solutions are removed in collapsible cuvettes.

spectral range from 185 to 900 nm, double-beam, recording, wavelength accuracy 0.03 nm at 54000 cm-1, 0.25 at 11000 cm-1, wavelength reproducibility 0.02 nm and 0.1 nm, respectively	The device is designed for recording IR spectra of solid and liquid samples. Spectral range – 4000…200 cm-1; photometric accuracy ± 0.2%.

Absorption analysis of visible and near ultraviolet region

The principle of operation of the most common photometric instruments for medical laboratory research - spectrophotometers and photocolorimeters (visible light) - is based on the absorption method of analysis or the property of solutions to absorb visible light and electromagnetic radiation in the ultraviolet range close to it.

Each substance absorbs only such radiation, the energy of which is capable of causing certain changes in the molecule of this substance. In other words, a substance absorbs radiation of only a certain wavelength, while light of a different wavelength passes through the solution. Therefore, in the visible region of light, the color of a solution perceived by the human eye is determined by the wavelength of radiation not absorbed by this solution. That is, the color observed by the researcher is complementary to the color of the absorbed rays.

The absorption method of analysis is based on the generalized Bouguer-Lambert-Beer law, which is often simply called Beer's law. It is based on two laws:

1. The relative amount of energy of the light flux absorbed by the medium does not depend on the intensity of the radiation. Each absorbing layer of the same thickness absorbs an equal proportion of the monochromatic light flux passing through these layers.
2. The absorption of a monochromatic flux of light energy is directly proportional to the number of molecules of the absorbing substance.

Thermal analysis

Research method physical-chemical. and chem. processes based on recording thermal effects accompanying the transformation of substances under temperature programming conditions. Since the change in enthalpy?H occurs as a result of most physical-chemical. processes and chemistry reactions, theoretically the method is applicable to a very large number of systems.

In T. a. it is possible to record the so-called heating (or cooling) curves of the sample under study, i.e. change in temperature of the latter over time. In the case of k.-l. phase transformation in a substance (or mixture of substances), a plateau or kinks appear on the curve. The method of differential thermal analysis (DTA) is more sensitive, in which the change in temperature difference DT is recorded over time between the sample under study and a comparison sample (most often Al2O3), which does not undergo this no transformations within the temperature range.

In T. a. it is possible to record the so-called heating (or cooling) curves of the sample under study, i.e. change in temperature of the latter over time. In the case of k.-l. phase transformation in a substance (or mixture of substances), plateaus or kinks appear on the curve.

Differential thermal analysis(DTA) has greater sensitivity. It records the change in time of the temperature difference DT between the sample under study and a comparison sample (most often Al2O3), which does not undergo any transformations in a given temperature range. The minima on the DTA curve (see, for example, Fig.) correspond to endothermic processes, and the maxima to exothermic processes. Effects recorded in DTA, m.b. caused by melting, changes in the crystal structure, destruction of the crystal lattice, evaporation, boiling, sublimation, as well as chemical. processes (dissociation, decomposition, dehydration, oxidation-reduction, etc.). Most transformations are accompanied by endothermic effects; Only some processes of oxidation-reduction and structural transformation are exothermic.

In T. a. it is possible to record the so-called heating (or cooling) curves of the sample under study, i.e. change in temperature of the latter over time. In the case of k.-l. phase transformation in a substance (or mixture of substances), plateaus or kinks appear on the curve.

Mat. The relationships between the peak area on the DTA curve and the parameters of the device and the sample make it possible to determine the heat of transformation, the activation energy of the phase transition, some kinetic constants, and conduct a semi-quantitative analysis of mixtures (if the DH of the corresponding reactions is known). Using DTA, the decomposition of metal carboxylates, various organometallic compounds, and oxide high-temperature superconductors is studied. This method was used to determine the temperature range for the conversion of CO into CO2 (during the afterburning of automobile exhaust gases, emissions from thermal power plant pipes, etc.). DTA is used to construct phase diagrams of the state of systems with different numbers of components (physical-chemical analysis), for quality. evaluation of samples, e.g. when comparing different batches of raw materials.

Derivatography- a comprehensive method of chemical research. and physical-chemical processes occurring in a substance under conditions of programmed temperature changes.

Based on a combination of differential thermal analysis (DTA) with one or more physical. or physical-chemical methods such as thermogravimetry, thermomechanical analysis (dilatometry), mass spectrometry and emanation thermal analysis. In all cases, along with transformations in the substance that occur with a thermal effect, the change in the mass of the sample (liquid or solid) is recorded. This makes it possible to immediately unambiguously determine the nature of processes in a substance, which cannot be done using data from DTA alone or other thermal methods. In particular, an indicator of phase transformation is the thermal effect, which is not accompanied by a change in the mass of the sample. A device that simultaneously records thermal and thermogravimetric changes is called a derivatograph. In a derivatograph, the operation of which is based on a combination of DTA with thermogravimetry, the holder with the test substance is placed on a thermocouple freely suspended on the balance beam. This design allows you to record 4 dependences at once (see, for example, Fig.): the temperature difference between the sample under study and the standard, which does not undergo transformations, on time t (DTA curve), changes in mass Dm on temperature (thermogravimetric curve), rate of change mass, i.e. derivative dm/dt, from temperature (differential thermogravimetric curve) and temperature from time. In this case, it is possible to establish the sequence of transformations of the substance and determine the number and composition of intermediate products.

Chemical methods of analysis

Gravimetric analysis based on determining the mass of a substance.
During gravimetric analysis, the analyte is either distilled off in the form of some volatile compound (distillation method), or precipitated from solution in the form of a poorly soluble compound (precipitation method). The distillation method is used to determine, for example, the content of water of crystallization in crystalline hydrates.
Gravimetric analysis is one of the most universal methods. It is used to define almost any element. Most gravimetric techniques use direct determination, whereby the component of interest is isolated from the mixture being analyzed and weighed as an individual compound. Some elements of the periodic table (for example, compounds of alkali metals and some others) are often analyzed using indirect methods. In this case, two specific components are first isolated, converted into gravimetric form and weighed. One or both of the compounds are then transferred to another gravimetric form and weighed again. The content of each component is determined by simple calculations.

The most significant advantage of the gravimetric method is the high accuracy of the analysis. The usual error of gravimetric determination is 0.1-0.2%. When analyzing a sample of complex composition, the error increases to several percent due to imperfect methods of separation and isolation of the analyzed component. The advantages of the gravimetric method also include the absence of any standardization or calibration using standard samples, which are necessary in almost any other analytical method. To calculate the results of gravimetric analysis, knowledge only of molar masses and stoichiometric ratios is required.

The titrimetric or volumetric method of analysis is one of the methods of quantitative analysis. Titration is the gradual addition of a titrated solution of a reagent (titrant) to the solution being analyzed to determine the equivalence point. The titrimetric method of analysis is based on measuring the volume of a reagent of a precisely known concentration spent on the reaction of interaction with the substance being determined. This method is based on the accurate measurement of the volumes of solutions of two substances that react with each other. Quantitative determination using the titrimetric method of analysis is performed quite quickly, which makes it possible to carry out several parallel determinations and obtain a more accurate arithmetic average. All calculations of the titrimetric method of analysis are based on the law of equivalents. According to the nature of the chemical reaction underlying the determination of the substance, titrimetric analysis methods are divided into the following groups: neutralization or acid-base titration method; oxidation-reduction method; precipitation method and complexation method.