Vibrational symmetry classification and torsional tunneling splitting patterns in G 6 (EM), G 12 , and G 36 (EM) molecules

Authors: Lattanzi F.; Di Lauro C.

Source: Molecular Physics, Volume 103, Number 5, March 10, 2005 , pp. 697-708(12)

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Abstract:

It is shown that the torsional splitting patterns in methanol-like molecules, with the excitation of small amplitude vibrational modes in the methyl group, are determined by mechanisms that can be formulated in an almost identical fashion to that for ethane-like molecules. This is achieved by treating ethane-like molecules by the internal axis method (IAM) and methanol-like molecules by the principal axis method (PAM) or rho-axis method (RAM). Using the extended molecular groups G 6 (EM) or C 6v (M) for methanol and G 36 (EM) for ethane, vibrations perpendicular to the internal rotation axis are conveniently described by modes of higher degeneracy (E for methanol and G s for ethane) in the absence of coupling of top and frame. Head-tail coupling operators, except the cos-type barrier terms, lower the degeneracy, causing vibrational splittings. Coupled vibrational pairs with torsional splitting patterns that we call 'regular' (pure A 1 , A 2 pairs for methanol and pure E 1d , E 2d pairs for ethane) or 'inverted' (pure B 1 , B 2 pairs for methanol and pure E 1s , E 2s pairs for ethane) can be formed as limit cases. Actual splitting patterns occur between the above limits, and are basically determined by torsional Coriolis coupling, which can tune more or less to resonance pairs of uncoupled basis levels linked by specific head-tail coupling operators. The inversion of torsional splitting patterns, observed in perpendicular vibrational modes of the methyl group of methanol, can be predicted by these theoretical considerations. Similar considerations apply to molecules of G 12 symmetry.

Document Type: Research article

DOI: http://dx.doi.org/10.1080/00268970412331333410

Affiliations: 1: Gruppo di Chimica Fisica Dipartimento di Chimica Farmaceutica Università di Napoli Federico II Via D. Montesano 49 I-80131 Napoli Italy

Publication date: 2005-03-01

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Vibrational symmetry classification and torsional tunneling splitting patterns in G 6 (EM), G 12 , and G 36 (EM) molecules

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