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.

Interacting Rydberg and valence states in radicals and molecules: experimental and theoretical studies

“…In addition, the lowest energy Rydberg states, which interact most strongly with the valence states, often have quantum defects larger than predicted by the Rydberg formula, lowering their energy even further. Most hydrocarbon free radicals have IE 5 9 eV and, because of hyperconjugation [11,[15][16][17][18][19][20], the IEs of many organic closed-shell molecules decrease rapidly with increasing size of the hydrocarbon chain and can be as low as those of radicals. Therefore, the effects described in this review are manifest in the excited state behaviour of a large number of molecules.…”

Jet-Cooled Spectroscopy of the α-Methylbenzyl Radical: Probing the State-Dependent Effects of Methyl Rocking Against a Radical Site

“…These spectra provide complementary insight into the methyl group orientation and hyper-conjugation-induced effects on the methyl CH stretch absorptions. Extending a model first introduced by Haber and co-workers for the ethyl radical, 32 we show that the pattern of methyl CH stretch transitions and their frequency shifts with electronic excitation reflect a change in the balance of effects due to hyperconjugation and steric interactions.…”

“…83 32 where hyperconjugation acts as a stabilizing force between the partially filled radical site and the methyl group that has implications on the methyl CH stretch modes. For α-MeBz, the methyl CH bond force constants k n oscillate periodically as …”

Contrasting patterns of coupling between the CH stretches and the large-amplitude motions in the molecules, CH3NH2, CH3OH2+ and CH3CH2

“…Methylamine, ethyl radical and protonated methanol (CH 3 XH 2 with X=N, C, O) belong to the G 12 molecular symmetry group [1]. Each of them has two largeamplitude degrees of freedom, the methyl torsion and inversion at X, which together connects six equivalent potential wells [2][3][4][5][6][7][8]. CH 3 NH 2 has a high barrier to inversion (~1950 cm -1 ) [3,[9][10][11], whereas in protonated methanol, CH 3 OH 2 + , the barrier is lower (~875 cm -1 ) [3].…”

“…CH 3 NH 2 has a high barrier to inversion (~1950 cm -1 ) [3,[9][10][11], whereas in protonated methanol, CH 3 OH 2 + , the barrier is lower (~875 cm -1 ) [3]. In the ethyl radical, CH 3 CH 2 , there is no barrier to inversion, only a low-barrier internal rotation path [3,[5][6][7][8]12]. Methylamine, protonated methanol and ethyl radical have torsional barriers of 703.8 cm -1 , 399.6 cm -1 and 8.0 cm -1 , respectively [3].…”