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

Author Institution: Department of Chemistry, The University of Akron, OH 44325-3601In methylamine (CH$_3$NH$_2$), there are six equivalent mimina that are connected by torsion and inversion tunneling. In the G$_1$$_2$ molecular symmetric group, there are four species, $A = \{A$_1$, A$_2$\}$, $~B = \{B$_1$, B$_2$\}$, $~E$$_1$ and $E$$_2$ that combine with distinct nuclear states. The ground vibrational state of CH$_3$NH$_2$ is split by torsion and inversion tunneling into a multiplet pattern of four distinct energy levels. The experimental tunneling pattern for CH$_3$NH$_2$ in the $\nu_1$$_1$ asymmetric CH-stretch fundamental has been previously reported at this meeting. In the experimental pattern, the degenerate species ($E$$_1$ and $E$$_2$) are at the top and bottom of the multiplet and the non-degenerate species ($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 $\nu_2$ and $\nu_1$$_1$ fundamentals. This model gives similar patterns for $\nu_2$ and $\nu_1$$_1$, in which $E$$_1$ and $E$$_2$ 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 $\nu_2$, $\nu_3$ and $\nu_1$$_1$ fundamentals. For this model, we found that, for $\nu_2$ and $\nu_1$$_1$, both $E$$_1$ and $E$$_2$ are at the bottom of the multiplet, in contrast to $\nu_3$ and the ground state where they are at the top. The fact that neither model reproduces the observed tunneling pattern for $\nu_1$$_1$, suggests that additional isolated perturbations or systematic interactions are present in the experimental spectra