15min:

X. WANG, Y. MA, A. CHIROKOLAVA AND D. S. PERRY, Department of Chemistry, University of Akron, Akron, OH 44325-3601.

Jet-cooled high resolution infrared absorption spectra of the A₂ asymmetric C-H stretch in methanol, the ₉ band, were recorded from 2945 to 2991~cm^-1. A large part of the bending overtone 2₄ appears in the same region and was also recorded. The analysis has resulted in 38 subband assignments for the ₉ and 2₄ reaching K' up to 4 for ₉ and up to 2 for 2₄. A plot of the upper state torsional energies versus K' shows the expected cosine patterns that result from the interaction of the torsion with K-rotation; however the torsional tunnelling splitting at K'=~0 is inverted for ₉ with the E levels below the A levels. The A and E K'=~0 subband origins for ₉ are 2966.6437(4) and 2952.040(3)~cm^-1, respectively, and for 2₄ are 2958.3586(11)~cm^-1 and 2957.565(6)~cm^-1. The ₉ band origin (average of A and E) was found to be about 11~cm^-1 below the estimate from low resolution spectra (2970.0~cm^-1). A number of perturbations in the 2₄ band have been identified.

The inverted torsional structure of the ₉ band supports the local mode Hamiltonian that was developed to explain the inverted torsional structure of the ₂ asymmetric C-H stretch and the regular torsional splitting of the ₃ symmetric C-H stretch. The model takes into account the difference in the local C-H frequency between the trans and gauche positions. The local mode parameters are the local C-H frequency \omega~=~2934.0~cm^-1, the local-local coupling parameter \lambda~=~-42.2~cm^-1, and the stretch-torsion coupling parameter µ~=~12.9~cm^-1. This model yielded the correct A/E ordering and fit the 6 K'=~0 subband origins with a standard deviation of 0.4~cm^-1. Qualitative agreement with the K'-dependence of the torsional energies was obtained.