CH stretch/internal rotor dynamics in ethyl radical:High-resolution spectroscopy in the CH_3-stretch manifold

摘要

High-resolution IR absorption spectra of supersonically cooled ethyl radicals (T_(rot)approx= 20 K)have been obtained in a slit supersonic jet discharge expansion,revealing first rotationally resolved data for CH-stretch excitation of the methyl group.Three different vibrational bands are observed,one parallel (k=o<- 0)and two perpendicular (|k| = 1 <- 0),which for a nearly decoupled methyl rotor framework would correspond to symmetric and (nearly degenerate)asymmetric CH-stretch excitations.However,the splitting between the two asymmetric CH-stretch excitations is anomalously large (approx=125 cm~(-1)),signaling the presence of interactions between the CH_2 radical moiety and the opposing CH bond on the methyl group.This suggests an improved zeroth-order vibrational description as an isolated CH stretch,strongly redshifted by hyperconjugation,with localized vibrations in the remaining CH bonds split into symmetric and asymmetric stretches.Such a dynamical picture highlights a remarkably strong coupling between methyl CH-stretch vibrations and C-C torsional geometry and begins to elucidate discrepancies with previous matrix observations.