BROADENING AND SHIFTS OF VIBRATIONAL BANDS DUE TO THE EFFECT OF THERMAL CHEMICAL-REACTIONS

The effect of thermal chemical reactions on vibrational spectra is discussed qualitatively by contrasting vibrational spectra with magnetic resonance spectra. The difference in time scale characteristic of each type of spectra leads to very different effects of reaction. Ordinary thermal reactions do not have a fast enough "transit time" - time spent in the transition state - to lead to a simple effect on vibrational spectra. As a consequence, obtaining reaction rates from the broadening or shift of vibrational bands requires detailed knowledge of the other broadening/shift mechanisms involved, and this makes the analysis difficult. The qualitative effect of fast reactions is illustrated by means of "Kubo diagrams" in which the band behavior is plotted as a function of the Kubo parameters. It is pointed out that one important case of vibrational spectra for which exchange coupling does lead to the magnetic resonance/Bloch equation result occurs at very low temperatures where only four energy levels are involved. Here the exchange involved is not due to a chemical reaction. The process of pseudocollapse is identified as the progressive broadening of bands arising from an anharmonic potential, which leads to shifts having no direct relation to reaction rates. References are provided to correct treatments of the effects of reaction rates. © 1990 American Chemical Society.