title: A Hybrid Program for Fitting Rotationally Resolved Spectra of Floppy Molecules with One Large-Amplitude Rotatory Motion and One Large-Amplitude Oscillatory Motion.
abstract: A new hybrid-model fitting program for methylamine-like molecules has been developed, on the basis of an effective Hamiltonian in which the ammonia-like inversion motion is treated using a tunneling formalism, whereas 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 C(3v)) 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 isotopologs 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 are mentioned.
authors.affiliation: Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR 7583 CNRS/IPSL, Universités Paris Est & Paris Diderot , 61 avenue du Général de Gaulle, F-94010 Créteil cedex, France.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Kleiner
authors.affiliation: Sensor Science Division, NIST , Gaithersburg, Maryland 20899-8441, United States.
authors.forename: Jon T
authors.initials: JT
authors.lastname: Hougen