Spectroscopy in Correlated Chemical Dynamics

In principle, knowledge of an atom or molecule s electronic structure i.e., the quantum mechanical wavefunction) would enable one to predict both its physical properties and its chemical behavior, including the outcome of reactions with other atoms or molecules whose electronic structure are equally well known (cf Daudel, 1973 Daudel et al., 1982). But because the Schrddinger equation cannot be solved exactly for any system more complicated than the hydrogen atom, the wavefunction of atoms and molecules must be approximated. Spectroscopy provides us with an observational link between the macroscopic and microscopic realms of matter, and it has been both a guide to our conceptual understanding of matter and a means to approximate parameters that are used in semiempirical computational chemistry (cf. Segal, 1977). 

This same tactic was applied to the analysis of chemical reaction dynamics. Lacking a complete quantum mechanical description of molecules, chemists recognized that they could alter the electronic structure of a molecule by functionalizing it with nonparticipating substituents. The degree to which the substituents perturbed a molecule s electronic structure could be assessed via spectroscopy and made quantitative in the form of a so-called substituent parameter (cf. Hammett, 1970). The prineipal requirement of the chosen spectroscopic parameter is that it be 

For small molecules, structure-reactivity analysis proceeds by synthesizing a series of substituted variants of the parent moleeule (e.g., substituted benzenes, cf. Brown Goldman, 1962), which are then made to react in some identical manner. The information that one obtains eonsists of reaction rates for each of the molecular variants and some physical parameter, selected according to the criterion cited in the previous paragraph, measured for each variant. The interrelationship between chemical reaction rate and electronic structure is then parametrically written as 

But even more ambitious probes of the strueture-funetion correlation are afforded by moleeular biology. There exist techniques that enable one to introduce 

One needs, however, to ensure that the spectroscopic probe of our system provides information (in the form of spectroscopic peaks) that is energetically commensurate to the perturbations one expects to see as a result of inter- or intramolecular interactions. And here it is worthwhile to examine the energetics of 

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