FTIR SPECTROSCOPY OF THE SYMMETRIC METHYL-DEFORMATION BAND OF $^{13}CD_{3}OH$

Abstract

The methyl-deformation bands of $^{13}CD_{3}$OH have been studied by high-resolution Fourier transform spectroscopy in the region $1030-1180 cm^{-1}$, lying above the strong $C-O$ stretching band. The symmetric-deformation band is the umbrella mode and shows a reasonably well-developed P, Q and R branch structure. The R-branch spacing is clearly larger than that of the P branch, showing that the B-value increases in going to the excited umbrella-bending state. Numerous individual P, Q and R branch series have been identified and analyzed, along with interesting Fermi and Coriolis resonances observed in the spectrum. The torsional energy levels of the excited methyl-deformation state follow the oscillatory pattern with K found for the ground state, but with a slightly increased amplitude. This implies that the barrier to internal rotation is somewhat lower in the excited state, contrary to results found for other vibrational modes. A least squares fit to the IR series origins, with the ground state molecular constants held fixed, gives an excited deformation state barrier of $V_{3} = 325(16) cm^{-1}$, a 12% decrease from the ground state value of $370 cm^{-1}$. However, $V_{3}$ is strongly correlated in the fit with the framework moment of inertia $I_{a1}$, which shows a suspiciously large 11% increase, hence the actual value of $V_{3}$ is still preliminary and should be viewed with caution.""