Is there chemically significant trapping of vibrational energy in localized and chemically identifiable moieties?

Abstract

Excitation of high CH and OH overtones is generally followed by quite rapid initial delocalization of energy. For example, in the excitation of CH overtones in benzene substantial energy is transferred into lower-frequency frame modes [via doorways opened by Fermi-resonance(s)] in 50–100 fs. There is a similar mixing of OH stretching overtones and OOH bends in hydrogen peroxide, on a similar ultrafast timescale. Recent theoretical studies indicate that relaxation of CH stretches on (non-rotating) terminal methyl groups of alkane chains results in rapid and considerable energy scrambling of stretching and wagging modes localized on the H3C— group. In these latter two cases subsequent relaxation, e.g. that leading to rupture of the O—O bond in H2O2 or energy flow from the (non-rotating) methyl group into the hydrocarbon chain, takes place on a timescale on to two orders of magnitude slower. Thus in these two cases it may be useful to speak not of localization of energy in a single ‘local mode’ in the form of a CH or OH high overtone, but temporary localization in a somewhat larger moiety. These theoretical perceptions are discussed in the light of very preliminary theoretical work on non-RRKM behaviour in overtone-induced dissociation of the chloroethyl radical and its homologues. Also discussed is the possibility that specific isotopic substitution can break the most important non-linear resonances, leading to initial relaxation, and thus lengthen the timescale of energy localization.