Spectral theory of chemical bonding

These speculations about the ionic, polar, or electronic nature of chemical bonding, which arose largely from solution theory, resulted mostly in static models of the chemical bond or atom structure. In contrast is another tradition, which is more closely identified with ether theory and electrodynamics. This tradition, too, may be associated with Helmholtz, especially by way of his contributions to nineteenth-century theories of a "vortex atom" that would explain chemical affinities as well as the origin of electromagnetism, radiation, and spectral lines. 

Nonrelativistic quantum mechanics, extended by the theory of electron spin and by the Pauli exclusion principle, provides a reliable theory for the computation of atomic spectral frequencies and intensities, of cross sections for scattering or capture of electrons by atomic systems, of chemical bonds and many properties of solids, including magnetic properties, although with much more complicated systems it has not always proved possible to develop with adequate accuracy the consequences of the theory. Quantum mechanics has also had a limited success in nuclear theory although m this field it is possible that a more fundamental system of mechanics is required. 

Others have defined physical chemistry as that field of science that applies the laws of physics to elucidate the properties of chemical substances and clarify the characteristics of chemical phenomena. The term physical chemistry is usually applied to the study of the physical properties of substances, such as vapor pressure, surface tension, viscosity, refractive index, density, and crystallography, as well as to the study of the so-called classical aspects of the behavior of chemical systems, such as thermal properties, equilibria, rates of reactions, mechanisms of reactions, and ionization phenomena. In its more theoretical aspects, physical chemistry attempts to explain spectral properties of substances in terms of fundamental quantum theory, the interaction of energy with matter, the nature of chemical bonding, the relationships correlating the number of energy states of electrons in atoms and molecules with the observable properties shown by these systems, and the electrical, thermal, and mechanical effects of individual electrons and protons on solids and liquids. ... 

Chapter 10, in relation to classical bond orders and bond energies. In the meantime, Chapter 9 deals with the molecular orbital separation, conjugated systems, non-localizable tt molecular orbitals and resonance. In Chapter 11 a brief extension of molecular orbital theory is made to include three categories of systems fullerenes, transition metal complexes, solid aggregates (and band theory). Finally, Chapter 12 mainly illustrates the direct relations between orbitals and chemical reactivity and between orbitals and spectroscopy, with emphasis on electronic transitions and on spectral parameters in NMR spectroscopy. 

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