CH stretch/internal rotor dynamics in ethyl radical: High-resolution spectroscopy in the CH3-stretch manifold

Abstract

High-resolution IR absorption spectra of supersonically cooled ethyl radicals (Trot approximately 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=0<--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 (approximately 125 cm(-1)), signaling the presence of interactions between the CH2 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.