15min:
TWO MODEL HAMILTONIANS FOR TORSION-INVERSION TUNNELING IN THE CH-STRETCH VIBRATIONALLY EXCITED STATES OF METHYLAMINE .

MAHESH B DAWADI AND DAVID S PERRY, Department of Chemistry, The University of Akron, OH 44325-3601.

In methylamine (CH3NH2), there are six equivalent mimina that are connected by torsion and inversion tunneling. In the G12 molecular symmetric group, there are four species, A = $1A1, A2$1, ~B = $1B1, B2$1, ~E1 and E2 that combine with distinct nuclear states. The ground vibrational state of CH3NH2 is split by torsion and inversion tunneling into a multiplet pattern of four distinct energy levels. The experimental tunneling pattern for CH3NH2 in the nu11 asymmetric CH-stretch fundamental has been previously reported at this meeting. In the experimental pattern, the degenerate species (E1 and E2) are at the top and bottom of the multiplet and the non-degenerate species (B and A) are between them. In this work, we present two models for the torsion-inversion tunneling behavior in the CH-stretch excited states. Each model includes the lowest order torsion-inversion-vibration interactions available in the context of the model. The first model, which extends Hougen's treatment of methanol, couples the two vibrational angular momentum components of the asymmetric CH-stretches to the large-amplitude motion to yield predicted tunneling patterns for the nu2 and nu11 fundamentals. This model gives similar patterns for nu2 and nu11, in which E1 and E2 are in the middle of the multiplet and the non-degenerate species are at the top and bottom. The second model, which follows conceptually Wang and Perry's local mode treatment of methanol, couples the three local CH-stretches to each other and to the large-amplitude motion to yield the tunneling patterns for the nu2, nu3 and nu11 fundamentals. For this model, we found that, for nu2 and nu11, both E1 and E2 are at the bottom of the multiplet, in contrast to nu3 and the ground state where they are at the top. The fact that neither model reproduces the observed tunneling pattern for nu11, suggests that additional isolated perturbations or systematic interactions are present in the experimental spectra.