MOLECULE

Molecule (novolat. molecula, a diminutive from Lat. Moles - mass), the smallest particle of matter, with its chemical properties. AM is composed of atoms, rather - of atomic nuclei, electrons surrounding their internal and external valence electrons forming chemical bonds (see valence). Internal e-atoms do not normally participate in the formation of chemical bonds. The composition and structure of the molecules of the substance does not depend on the way to retrieve it. In the case of monoatomic molecules (eg, inert gases) the concept of AM and the same atom. 

For the first time the notion of Moscow was introduced in chemistry because of the need to distinguish between M. as the least number of substances entering into chemical reactions, from the atom as the smallest amount of this element, which is a part of M. (The International Congress in Karlsruhe, 1860). The main characteristics of the structure of AM were identified by the study of chemical reactions, analysis and synthesis of chemical compounds, as well as through the use of several physical methods. 

The atoms are combined in Moscow, in most cases, the chemical bonds. Typically, such a relationship as one, two or three pairs of electrons, which hold together the two atoms. M. may contain positively and negatively charged atoms, ie, ions in this case, electrostatic interactions are implemented. In addition to these, in Moscow, there are weaker interactions between the atoms. In the non-valence atoms act repulsive force. 

Composition M. chemical formulas express. Empirical formula (for example, S2N6O for ethanol) on the basis of atomic ratios of elements in a substance as defined by chemical analysis, and molecular weight. 

The development of teaching about the structure of molecules is inextricably linked to the success first of all organic chemistry. The theory of the structure of organic compounds, created in the 60's. 19. production of AM Butlerova, FA Kekule, A. Cooper and others, helped provide the structure of molecules of structural formulas or structure formulas representing the sequence of valence chemical bonds in M. In the same empirical formula can exist M . a different structure with different properties (the phenomenon of isomerism). These are, for example, ethanol and dimethyl ether S2N5ON (SN3) 2O. The structural formulas of these compounds differ:

 In some cases isomeric M. fast becoming one to another and between them is established a dynamic equilibrium (see tautomerism). In the future, Y. H. Goff guys, and whatever the French chemist A. J. Le Belle came to understand the spatial arrangement of atoms in a molecule and to explain the phenomenon of stereoisomers. A. Werner (1893) circulated a general idea of the theory of the structure at the complex inorganic compounds. Top 20. Chemistry has detailed the theory of the structure of M. coming from the study only in their chemical properties. Remarkably, that the direct physical methods of study, developed later, in the vast majority of cases, fully confirmed the structural formulas of chemistry, established through the study of macroscopic amounts of matter, rather than individual M. 

In physics the concept of AM was necessary to explain the properties of gases, liquids and solids. Direct experimental proof of the existence of M. was first obtained in the study of Brownian motion (French physicist Jean Perrin, 1906). 

In a solid AM can not maintain or preserve their identities. Thus, most M. organic compounds form molecular crystals, the lattice sites which are M. associated with one another by relatively weak intermolecular interactions. On the contrary, the ion (for example, in the case of NaCI) and atomic (diamond) crystals no separate AM and the crystal is similar to a PM (see Crystal chemistry). M. The structure may change during the transition from crystal to gas. Thus, N2O5 in the gas consists of a single AM, in the crystal - from ion NO2 + and NO3-; gas PCI5 - from Moscow to the configuration of trigonal bipiramidy, solid - from oktaedricheskogo PCl6-ion and the tetrahedral ion PCl4 +. 

Equilibrium mezhyadernye distance r0 and the dissociation energy D (at 25 ° C) of some diatomic molecules
Van der Waals radii significantly greater than covalent. Knowing the values of van der Waals, covalent, and ionic radii, it is possible to construct a visual model of PM, reflecting the form and size of their electronic shells (Fig. 2). 

Covalent chemical bonds are located in Moscow, under certain angles depending on the state of hybridization of atomic orbitals (see valence). Thus, for AM-saturated organic compounds characterized by tetrahedral arrangement of linkages formed by carbon atoms, for meters with a double bond (C = C) - flat location links carbon atoms in compounds M. triple bond (C º C) - the linear arrangement of the links:

 Thus, the polyatomic AM has a configuration in space, ie, a certain geometry of relationships, which can not be changed without the gap. Moscow has a particular symmetry of the atoms. If Moscow does not have a plane and a center of symmetry, it can exist in two configurations, representing a mirror of one another (mirror antipodes, or stereoisomers, see isomer). All the major biologically functional substances in nature appear in the form of one particular stersoizomera. M., containing a single connection, or sigma-connection may exist in different conformation, resulting in the bends of atomic groups around single bonds. Important features of biological macromolecules and synthetic polymers is determined by their conformational properties. The interaction of atoms in a molecule. The nature of chemical bonds in Moscow remained enigmatic until the creation of quantum mechanics - classical physics could not explain the saturability and orientation of valence bonds. Fundamentals of the theory of chemical bond have been established and the B. Geytlerom German scientist F. London in 1927 for example, the simplest molecule H2. In the future, theory and methods of calculation have been significantly improved, particularly through the widespread use of molecular orbital method and quantum chemistry allows to calculate the interatomic distances, energy, Moscow, energy of chemical bonds and the distribution of electron density for the complex M., with the calculated data agree well with experiment. Chemical communication in M. overwhelming number of organic compounds are covalent. On the contrary, there are a number of inorganic ion and the donor-acceptor context (see chemical bond), realized as a result of collectivization of lone pair electrons of the atom. Energy Education M. atoms from many series of similar compounds approximately additive. In other words, in these cases, you can take that energy M. amount of energy it has links with a constant value in the row. It should be feasible to attribute the chemical bonds nearly autonomous electronic envelope. Additivity of the energy is not always fulfilled M.. A striking example of a violation of additivity are planar organic compounds from M. Sc. conjugative addition, ie, with multiple links, alternating with a single. In these cases, valence electrons, determine the multiplicity of relationships, so called. p-electrons, are common to the whole system involving relationships, delocalized. This Delocalization of electrons leads to further stabilization of M. For example, the energy of formation of 1,3-butadiene M. N2S = CH-CH = CH2 longer expected to additivity at 16.8 kJ / mol (4 kcal / mol). Aligning the electron density due to collectivization of p-electrons relations is reflected in lengthening and shortening of the double bonds isolated. In the right hexagon mezhuglerodnyh links and benzene (see formula), all links are equal and have the length, intermediate between the length of the unit and double bond. Pairing relations evident in the molecular spectra (see below). 

The modern quantum-mechanical theory of chemical bond takes into account not only the partial delocalization of p-, and s-electrons are observed in all molecules. Generally speaking, it does not violate the additivity of the energies of molecules. 

The vast majority of the total spin of valence electrons in the AM is zero, ie, full of spins of electrons in pairs. M. containing unpaired electrons - free radicals (eg, atomic hydrogen H, methyl CH 3), are usually unstable, because when they connect with one another a substantial lowering of energy due to education valence bonds. The most effective method for studying the structure of free radicals is electron spin resonance (ESR). 

Electrical and optical properties of molecules. Behavior of matter in an electric field is determined by the basic electrical characteristics of AM - permanent dipole moment and polarizability. Dipole moment is divergent centers of gravity of the positive and negative charges in Moscow, ie, the electrical asymmetry M. M. Accordingly, a center of symmetry, such as H2, no permanent dipole moment, on the contrary, the electrons are removed in HCl to Cl atom and the dipole moment equal to 1,03 D (1,03 × 10-18 units. JLG). Polarizability is characterized by the ability of the electronic shell of any move by the action of AM electrical field, resulting in the AM creates induced dipole moment. The values of dipole moment and polarizability are experimentally through measurements of permittivity. In the case of additivity properties M. M. dipole moment can be represented as a sum of dipole moments (with regard to their direction), the same applies to the polarizability M. 

Optical properties of matter describe its behavior in an alternating electric field of light wave - thus they are determined polarizability M. substances. Since polarizability is directly related refraction and scattering of light, optical activity and other phenomena studied molecular optics - Physical Optics section devoted to the study of optical properties of matter. 

Magnetic properties of molecules. M. macromolecule and the vast majority of himyicheskih diamagnetic compounds (see diamagnetism). Magnetic susceptibility of M. (c) in some organic compounds can be expressed as the sum of the values of c for the individual links, but the additivity c performed worse than additivity polarizability a. And c, and a determined by the properties of electrons outside Moscow, the two values are connected with one another. 

Paramagnetic M. possessing a permanent magnetic moment (see paramagnetism). Those with an odd number of M. electrons in the outer shell (eg, NO, and any free radicals), Moscow, containing atoms with unconfined (blank) the inner lining (transition metals, etc.). Magnetic susceptibility of paramagnetic substances is dependent on temperature, because thermal motion impedes orientation of magnetic moments in a magnetic field. The structure of paramagnetic M. efficiently studied by ESR. 

Atomic nuclei of elements whose atomic number or odd mass number of nuclear-spin paramagnetism. For such nuclei the nature of the nuclear magnetic resonance (NMR) spectrum of which depends on the electronic environment of nuclei in M. Therefore, the NMR spectra provide a source of very detailed information about the structure of AM, including the highly complex, such as proteins (see also the nuclear quadrupole resonance , Magnetism, Magnetohimiya). 

Spectra and structure of molecules. Electrical, optical, magnetic and other properties of PM is ultimately linked to the wave functions and energies of various states M.; through them and form an electric dipole moment and magnetic moment, and polarizability, and magnetic susceptibility. Direct information about the state of M and the transition probability between them provide a molecular spectra. 

The frequencies in the spectra corresponding to rotational transitions depend on the moments of inertia of Moscow, where the definition of the spectroscopic data gives the most accurate values of interatomic distances in M. 

The total number of lines or bands in the oscillatory spectrum of AM depends on its symmetry. The frequencies of oscillations observed in the spectra are determined, on the one hand, the masses of atoms and their arrangement on the other - the dynamics of atomic interactions. The theory of vibrations of polyatomic M. respectively based on the theory of chemical structure and classical mechanics-related fluctuations. Study of vibrational spectra allows to draw some conclusions about the structure of AM, the interatomic and intermolecular interactions, to study the phenomenon of tautomerism, isomerism turning. 

Electronic transitions in M. characterize the structure of their electronic shells, the chemical bonds. Spectra of Moscow, with a large number of related links are characterized by long-wavelength absorption bands falling in the visible region. Substances that were built from Moscow, have color, they are all organic dyes. The study of electron-vibrational spectra of AM need to understand the natural and magnetic optical activity. 

The molecules in chemistry, physics and biology. The concept of Moscow - for the chemistry major, and most of the details of the structure and function, science is obliged to M. Chemical Research. When a chemical reaction takes place turning one M. other. Such transformations typically need some excess energy M. - activation energy (see Chemical Kinetics). The act of chemical interaction taking place through the configuration of M. Sc. activated complex or transition state M. The nature and rate of chemical reaction is determined by this condition, in turn, dependent on the structure of interacting M. Chemistry solves two main tasks related to M. - sets the structure of M. by chemical reactions and, conversely, based on the structure of M. determines the course of reactions. Wide range of important problems of modern chemistry, including those outstanding, is the theory of chemical reactivity. The study of these problems requires the application of theoretical methods of quantum chemistry, and experimental data from chemical and physical methods. 

Physical phenomena, determined the structure and properties of M., studied molecular physics. Thermodynamic properties of any substance, which was built from Moscow, ultimately expressed in terms of values of energies of all possible states of Moscow, found from the spectroscopic data. M. The structure and intermolecular interactions are responsible for the equilibrium properties of matter. The same applies to the nonequilibrium, kinetic, properties. Establishment of equilibrium requires a certain time - a time of relaxation. With rapid changes in the equilibrium state of matter may not have time to install. These phenomena are observed, for example, by passing ultrasound through the material and affect the absorption and dispersion of sound waves (see Molecular Acoustics). Equilibrium is established by the interaction of AM with their collisions in the gas and liquid, as a result of absorption and emission of light, etc. The time of relaxation of Moscow in a condensed medium depends on the temperature, which increases with the increasing mobility of M. In some cases, M. in a fluid almost lose their motility prior to crystallization: vitrification occurs substance. M. Motility determined the ability of substances to the diffusion, the viscosity, thermal conductivity, etc. The direct study of the mobility of Moscow, the definition of relaxation times are the methods of absorption and dispersion of electromagnetic waves, NMR, EPR and other methods. 

Equilibrium and kinetic properties of large chain M., forming polymers (see the macromolecule), specific. Features of macromolecules are determined first of all their flexibility - the ability to stay in a large number of different conformations resulting from rotation around single bonds. 

The development of biology, chemistry and molecular physics has led to the construction of molecular biology investigating basic phenomena of life, based on the structure and properties of biologically functional M. The organism exists in a finely balanced chemical and non-chemical interactions between M. Thus, the study of structure and properties of PM is fundamental to the natural sciences in general. 

Lit.: Syrkin Ya K, Dyatkina ME, Chemical bonding and structure of molecules, M. - L., 1946, L. Pauling, Nature of chemical bond, trans. England., M. - L., 1947; Волькенштейн MV, Structure and physical properties of molecules, M. - L., 1955, its same, Molecules and Life, Moscow, 1965; his own, Intersections of Science, M., 1972, VN Kondrat'ev, Structure of atoms and molecules, 2 ed., Moscow, 1959; Kozman W., Introduction to quantum chemistry, trans. England., M., 1960; Sleter J., Electronic structure of molecules, trans. England., M., 1965.

MOLECULE