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


The Fourier transform infrared spectrum of the in-plane CH3-rocking fundamental of 13CH3OH has been investigated at 0.002 cm21 resolution. The rocking band is principally of parallel character and has a double-peaked Q branch and relatively wide spread subbands indicative of a substantialchange in torsional barrier height. All A subbands from K50 to 11 and all but one E subband fromK50 to 9 have been assigned in the n50 torsional state and fitted to J(J11) power-seriesexpansions to obtain the subband origins and excited-state energy structure. The effects ofvibrational interactions between the CH3-rocking and CO-stretching modes are prominent in thespectrum. Coriolis coupling between rocking (K21) and CO-stretching K levels is observable forK>6, and makes significant contributions to the subband origins and effective B values. SeveralJ-localized perturbations due to level-crossing resonances with CO-stretch states have beenobserved and characterized. Two reported strong far-infrared laser lines optically pumped by the10R(26) CO2 laser line have been found to arise through such a DK53 level-crossing resonance.Modeling of the rocking-state torsion-K-rotation energies yields a height of V3 r 5469.2(38) cm21for the torsional potential barrier, a 26% increase over the ground state. The asymmetry K-doublingpattern in the excited state is qualitatively consistent with this barrier for K52 to 4, but the K55 rocking substate displays strongly enhanced splitting. © 1997 American Institute of Physics.@S0021-9606~97!01929-6#