Fourier transform spectrum of the in-plane CH3-rocking fundamental and vibrational coupling in C-13 methanol

Abstract

The Fourier transform infrared spectrum of the in-plane CH3-rocking fundamental of CH313OH has been investigated at 0.002 cm−1 resolution. The rocking band is principally of parallel character and has a double-peaked Q branch and relatively wide spread subbands indicative of a substantial change in torsional barrier height. All A subbands from K=0 to 11 and all but one E subband from K=0 to 9 have been assigned in the n=0 torsional state and fitted to J(J+1) power-series expansions to obtain the subband origins and excited-state energy structure. The effects of vibrational interactions between the CH3-rocking and CO-stretching modes are prominent in the spectrum. Coriolis coupling between rocking (K−1) and CO-stretching K levels is observable for K⩾6, and makes significant contributions to the subband origins and effective B values. Several J-localized perturbations due to level-crossing resonances with CO-stretch states have been observed and characterized. Two reported strong far-infrared laser lines optically pumped by the 10R(26) CO2 laser line have been found to arise through such a ΔK=3 level-crossing resonance. Modeling of the rocking-state torsion-K-rotation energies yields a height of V3r=469.2(38) cm−1 for the torsional potential barrier, a 26% increase over the ground state. The asymmetry K-doubling pattern in the excited state is qualitatively consistent with this barrier for K=2 to 4, but the K=5 rocking substate displays strongly enhanced splitting.