Anomalous torsional tripling in the ν 9 and ν 10 CH3-deformation modes of ethane 12CH3 13CH3
Abstract We have investigated the anomalous torsional behavior in the coupled ν 9 and ν 10 vibrational fundamentals of 12 CH 3 13 CH 3 , both states exhibiting a splitting into three components, instead of two, only in those rotational levels which are very close to resonance. We conclude that the intrinsic additional splitting, which occurs in the E-torsional components, for these two vibrational states is too small to be detected in the high resolution infrared spectrum, but it is substantively enhanced by their coupling. It is shown that this effect requires the simultaneous action of torsion independent operators, such as Fermi-type and z-Coriolis, not allowed in the more symmetric isotopologue 12 CH 3 12 CH 3 , and torsion dependent operators, such as torsional-Coriolis, connecting the two vibrational states. Our conclusions lead to a simple model for the coupling of ν 9 and ν 10 , with effective Fermi-type matrix elements W for the A-torsional components, and W ± w for the two pairs of E-torsional components. This causes the additional splitting in the E-pairs. This model is consistent with the mechanism causing the Coriolis-dependent decrease of the A-E torsional splitting in degenerate vibrational states. Exploratory calculations were performed making use of results from a normal mode analysis, showing that the effects predictable by the proposed model are of the correct order of magnitude compared to the observed features, with coupling parameter values reasonably consistent with those determined by the least squares fit of the observed transition wavenumbers. Highlights The deformations of the two molecular halves in n 9 and n 10 are almost identical. The n 9 and n 10 modes exhibit in general a tunnelling splitting into two components. The anomalous torsional tripling in n 9 and n 10 of ethane(12,13) has been clarified. Different types of interaction operators cause together the anomalous tripling. Parameter values from a normal mode analysis support the present model. Graphical abstract [DISPLAY OMISSION]
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