NOVEL PATTERNS OF TORSION - INVERSION TUNNELING AND TORSION - ROTATION COUPLING IN THE $\nu_{11}$ CH - STRETCH REGION OF CH$_3$NH$_2$

Author Institution: Department of Chemistry, The University of Akron, OH 44325-3601; Department of Physics, Centre for Laser, Atomic and Molecular Studies (CLAMS) University of New Brunswick, Saint John, New Brunswick, Canada E2L 4L5The high-resolution infrared spectrum of CH$_3$NH$_2$ hasbeen recorded using slit-jet absorption spectroscopy in the $\nu_{11}$ asymmetric CH-stretch region ($2965~to~3005$\,cm$^{-1}$) with resolution of 0.003\,cm$^{-1}$. The 580 lines, assigned by ground state combination differences, represent 27 subbands with $|K^{\prime}|$ $\leq$ 2 for the $A$, $B$, $E_1$ and $E_2$ symmetries. Several of the observed subbands are split by perturbations. The analysis of spectrum shows that the patterns of the torsion-inversion tunneling splittings are qualitatively different from the ground state. In addition, the low - $J$ splittings between $|K^{\prime}|$ =~$ +|K^{\prime}|$ and $-|K^{\prime}|$ are greatly reduced relative to the ground state in both the $E_1$ and the $E_2$ species, indicating that torsion-rotation coupling is suppressed in the $\nu_{11}$ CH-stretch excited state