Physical Principles of Resonance Methods in Chemistry

At the National Institute of Chemieal Physies and Biophysics (NICPB), Laboratory of Chemical Physics the research activities range from studying fundamental aspects to applications. Strong emphasis is given to the deployment of modem physical methods in chemistry and biochemistry, which include principles of mass-spectroscopy and ion-cyclotron resonance environmental analyses and theoretical and experimental particle physics. 

Nuclear magnetic resonance has become such an importnat technique in organic chemistry that contemporary textbooks in the subject discuss its principles quite thoroughly, as do texts in physical and analytical chemistry. We note only a few pertinent highlights of the method ... 

Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-57]. These applications are reviewed in several excellent sources [1, 54,58], and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-61], and the interested reader is referred to these for details. 

This last sentence makes clear Slater s attitude toward quantum chemistry. Contrary to Pauling, who was definitely in favor of the valence bond method as the mathematical expression of a chemical theory such as resonance. Slater considered both methods as mathematical approximations used in order to gain access to more fundamental physical principles. What really attracted Slater was the search for a unified view of the problem of molecule formation, in which both approximate methods would be treated so that one would manage to get at the essential physical features of the problem situation, forgetting, in the process, the particular method of approximation used. 

In principle the deviation <5 can be determined by the use of usual analytical chemistry or a highly sensitive thermo-balance. These methods, however, are not suitable for very small deviations. In these cases the following methods are often applied to detect the deviation physico-chemical methods (ionic conductivity, diffusion constant, etc.), electro-chemical methods (coulometric titration, etc.), and physical methods (electric conductivity, nuclear magnetic resonance, electron spin resonance, Mossbauer effect, etc.), some of which will be described in detail. 

It is doubtful that a distinction can be made between the various charge distributions proposed by purely chemical means, although this might be possible in principle. We thus rely on physical methods such as optical studies, x-ray analysis, polarography, magnetic susceptibility, and electron paramagnetic resonance (EPR) studies for a determination of the stmcture of these complex cations. This is not to say that the chemistry of these compounds is no longer of... 

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