Chemical Physics


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Chemical Physics

 

a field of science on the boundary between chemistry and the new branches of physics.

The rise of chemical physics was preceded by many outstanding discoveries in physics in the early 20th century (seeATOMIC PHYSICS and QUANTUM MECHANICS). As a consequence of the rapid progress in physics, new possibilities emerged for the theoretical and experimental solution of chemical problems, which in turn led to the expansion of research using the methods of physics. The current concepts of the structure and electrical properties of atoms and molecules took shape, as well as the current concepts of the nature of the intermolecular forces and the elementary event of chemical interaction. The discovery of unbranched chain reactions by the German scientist M. Bodenstein (1913) and the establishment of the fundamental chemical mechanism of such reactions by W. Nernst heralded a new era in the development of chemical kinetics (seeKINETICS, CHEMICAL). The mechanism of chemical reactions is viewed as a complex set of elementary chemical processes involving molecules, atoms, free radicals, ions, and excited particles. Previously unknown types of chemical reactions were discovered and studied—for example, branched chain reactions (N. N. Semenov, C. Hinshelwood) and the phenomena characteristic of such reactions. The theory of processes of combustion and explosions based on chemical kinetics was created by Semenov.

The term “chemical physics” in the near-modern sense was first used by the German scientist A. Eucken, who in 1930 published A Course in Chemical Physics. This was preceded by the publication, in 1927, of Electronic Chemistry by V. N. Kondrat’-ev, N. N. Semenov, and Iu. B. Khariton, whose title to some extent reveals the meaning of the term “chemical physics.” The Institute of Chemical Physics of the Academy of Sciences of the USSR was founded in 1931. The Journal of Chemical Physics has been published in the United States since 1933.

As early as the 1920’s and 1930’s, chemical physics included research on the structure of the electron shell of the atom, the quantum-mechanical nature of chemical forces, the structure and properties of molecules, crystals, and liquids, and various problems of chemical kinetics, such as the nature of the elementary events of a chemical interaction, the properties of free radicals, the quantum-mechanical theory of the reactivity of compounds, photochemical reactions, reactions in discharges, and the theory of combustion and explosions.

The current stage in the development of chemical physics is characterized by the extensive use of numerous highly effective physical methods that provide extensive data on the structure of atoms and molecules and the mechanisms of chemical reactions. These include spectral and optical methods, mass spectrometry, the molecular beam method, X-ray diffraction analysis, electron microscopy, electromagnetic methods of determining polarizability and magnetic susceptibility, electron diffraction analysis, ionography, neutron diffraction analysis and neutron spectroscopic methods. Other highly effective methods are electron paramagnetic resonance, nuclear magnetic resonance, nuclear quadrupole resonance, double resonances, the spin echo method, the chemical polarization of electrons and nuclei, gamma-resonance spectroscopy, methods of determining the structural and dynamic properties of molecules by means of mesons and positrons, methods of determining the momenta of electrons in molecules, pulsed methods of studying fast processes (pulse radiolysis, and flash photolysis, including laser photolysis), and shock-wave methods.

The importance of quantum chemistry in chemical physics is growing, as is the use of computers for determining the electron structure and properties of chemical compounds and carrying out other calculations related to the theory of chemical reactions (seeQUANTUM CHEMISTRY).

Considerable attention is focused on the study of the mechanisms of elementary events of a chemical transformation in the gaseous and condensed phases. The kinetics of nonequilibrium processes, which are important under conditions of high temperatures and deep vacuum, is being studied intensively with respect to gas-phase reactions, and the role of the vibrational excitation of molecules is being elucidated. The theory of tunneling transitions and the kinetics of chemical reactions is being developed, and criteria that characterize temperatures below which tunneling transitions predominate over barrier transitions are being established. The peculiarities of processes at temperatures close to absolute zero are being studied. Low-temperature chemistry is being developed (low-temperature reactions proceed directionally, with an extremely high yield of the target products, at high, occasionally explosive, rates).

Intensive work is being carried out on the chemistry of high energies. This area of chemical physics is related to research on the kinetics, mechanism, and practical applications of processes in which the energies of individual atoms, molecules, and radicals exceed the energy of thermal motion and often the energy of the chemical bonds as well.

An important branch of chemicophysical research is photochemistry, which is of great importance to the theory of chemical processes and the solution of problems of photosynthesis, photo-reception, photography, and the photostabilization of polymeric materials (seePHOTOCHEMISTRY). Modern pulsed methods are used to investigate extremely fast photoprocesses, which is important for establishing the mechanism of elementary reactions. Also being studied is the mechanism of photochromism, the understanding of which is essential because of the extensive use of photochromic glass in engineering.

Theoretical and applied research is being conducted in the field of low-temperature plasma, and the general principles of the nonequilibrium kinetics of chemical reactions in plasma and the scientific foundations for plasmochemical technology are being developed (seePLASMA CHEMISTRY).

The study of the chemical transformations of condensed substances as a result of their compression under the action of shock waves is a comparatively new area of research in chemical physics. The kinetics of fast nonisothermal reactions is being studied under conditions of adiabatic expansion and compression of gases.

The role and importance of research in nuclear chemistry are growing. Nuclear chemistry studies the chemical consequences of nuclear processes (nuclear reactions, radioactive decay) and conducts research in the chemistry of new transuranium elements and the distinctive systems (particularly mesonic atoms) that arise when positrons and mesons act on matter. The methods of radiation chemistry are also being developed (seeRADIATION CHEMISTRY).

One of the fundamental consequences of the theory of chain processes is the conclusion that high concentrations of free atoms and radicals are formed in the course of branched chain reactions. This conclusion underlies numerous theoretical and experimental efforts that are of great practical importance. Research on chain processes involving energetic branchings of chains is being developed, and chemical lasers are being created on the basis of these processes. The study of the influence of magnetic fields on the mechanism of reactions involving free radicals has emerged as a new scientific field. The study of thermal explosion, combustion, and detonation retains its great theoretical and practical importance.

Considerable attention is being focused on the study of the kinetics and mechanism of chemical reactions in solids (see alsoTOPOCHEMICAL REACTION) and the chemicophysical aspects of catalysis (seeCATALYSIS). In the field of heterogeneous catalysis, chemical physics focuses on the study of the properties of particles adsorbed on the surface of the catalyst, the determination of the structure and distribution of active centers on the surface of solids, and the clarification of the elementary event of heterogeneous catalysis. Metallocomplex catalysis, which is nearly as effective as enzymatic catalysis, is becoming a promising area of chemicophysical study.

In the area of electrochemistry, chemical physics is seeking to find a quantum-chemical substantiation of the unique features of electrochemical reactions and is conducting experimental research on the mechanism of the elementary event of electrode reactions and processes within a solution that are accompanied by electron transfer, the investigation of solvated electrons, and the theoretical analysis of dark emission and photoemission of electrons from a metal into a solution.

Chemicophysical methods and approaches are becoming an effective tool of scientific research in all branches of chemical science. Modern physical chemistry also makes increasing use of the latest achievements of physics and physical methods of investigation in solving chemical problems (seePHYSICAL CHEMISTRY).

REFERENCES

Kondrat’ev, V. N., N. N. Semenov, and Iu. B. Khariton. Elektronnaia khimiia. Moscow-Leningrad, 1927.
Eucken, A. Kurs khimicheskoifiziki, fases. 1–3. Moscow-Leningrad, 1933–35. (Translated from German.)
Semenov, N. N., V. N. Kondrat’ev, and N. M. Emanuel’. “Khimicheskaia fizika v Akademii nauk SSSR.” Vestnik Akademii nauk SSSR, 1974, no. 2, p. 49.
Semenov, N. N. Khimicheskaia fizika (Fizicheskie osnovy khimicheskoi kinetiki). Chernogolovka, 1975.

N. M. EMANUEL

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