A hybrid program for fitting rotationally resolved spectra of floppy molecules with one large-amplitude rotatory motion and one large-amplitude oscillatory motion

Published

October 6, 2015

Author(s)

Isabelle Kleiner, Jon T. Hougen

Abstract

A new hybrid-model fitting program for methylamine-like molecules has been developed, based on an effective Hamiltonian in which the ammonia-like inversion motion is treated using a tunneling formalism, while the internal-rotation motion is treated using an explicit kinetic energy operator and potential energy function. The Hamiltonian in the computer program is set up as a 22 partitioned matrix, where each diagonal block contains a traditional torsion-rotation Hamiltonian (as in the earlier program BELGI), and the two off-diagonal blocks contain tunneling terms. This hybrid formulation permits the use of the permutation-inversion group G6 (isomorphic to C3v) for terms in the two diagonal blocks, but requires G12 for terms in the off-diagonal blocks. The first application of the new program is to 2-methylmalonaldehyde. Microwave data for this molecule were previously fit using an all-tunneling Hamiltonian formalism to treat both large-amplitude-motions. For 2-methylmalonaldehyde, the hybrid program achieves the same quality of fit as was obtained with the all-tunneling program, but fits with the hybrid program eliminate a large discrepancy between internal rotation barriers in the OH and OD isotopologues of 2-methylmalonaldehyde that arose in fits with the all-tunneling program. This large isotopic shift in internal rotation barrier is thus almost certainly an artifact of the all-tunneling model. Other molecules for application of the hybrid program will be mentioned.

Citation

Journal of Physical Chemistry A

Volume

119

Pub Type

Journals

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DOI Link

Keywords

2-methylmalonaldehyde, Group theory, Hybrid fitting Hamiltonian, Hydrogen-transfer, Internal rotation, Tunneling formalism

Citation

Kleiner, I. and Hougen, J. (2015), A hybrid program for fitting rotationally resolved spectra of floppy molecules with one large-amplitude rotatory motion and one large-amplitude oscillatory motion, Journal of Physical Chemistry A, [online], https://doi.org/10.1021/acs.jpca.5b08437 (Accessed October 17, 2025)