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

Abstract

In each of the title molecules, torsional and inversion tunneling occurs between six equivalent minima. The coupling between these large-amplitude motions and the CH stretch vibrations stems from the variation of the CH stretching force constants as a function of the torsional and inversion coordinates. First principles 2dimensional maps of the variation of the CH-stretch force constants are reported.Although the torsional barriers differ by more than a factor of 20, the torsioninversion-vibration coupling is similar for two of the species, CH 3 NH 2 and CH 3 CH 2 , but is much weaker and qualitatively different in form for the third species, CH 3 OH 2 + .

The vibrational Jahn–Teller effect in E⊗e systems

“…However, now that we appreciate that JT2 is larger than JT1 along the MEP and that geometric phase is +1, but the torsional tunneling splittings remain inverted [24,28,29]. Likewise the JT2 terms are dominant in a series of G 12 molecules including methylamine and the ethyl radical [48], and in those systems the torsional tunneling patterns are also inverted [49,50]. The fact of inverted tunneling splittings can be derived from symmetry considerations [39,49], but there is evidently no direct connection with geometric phase.…”