Fourier Transform Infrared Spectroscopy and Vibrational Coupling in the OH-Bending Band of 13CH3OH.

We present in this work a high-resolution Fourier transform infrared study of the OH-bending vibrational band of 13CH3OH. We have investigated the 1070-1400 cm-1 spectral region at 0.002 cm-1 resolution using the modified Bomem DA3.002 Fourier transform spectrometer at the Steacie Institute for Molecular Sciences at the National Research Council of Canada in Ottawa. This study has led to (i) determination of excited-state J(J + 1) subband expansion coefficients and (ii) characterization of a variety of interactions coupling the different vibrational modes, notably a strong Fermi resonance between the OH bend and the torsionally excited CH3-rocking mode. The OH-bending band is widely spread with Q subbranches grouped in two peaks at about 1312 and 1338 cm-1. The lower levels for all assigned subbands were confirmed using closed loops of IR and FIR transitions. The subbands have been fitted to J(J + 1) power-series expansions in order to obtain the subband origins and the state-specific energy expansion coefficients for both the OH-bending and excited torsional CH3-rocking states. The strong interaction between the OH-bending state and the first excited torsional CH3-rocking state gives rise to several "extra" forbidden subbands due to intensity borrowing. The asymmetry splitting of the (ntauK) v = (122)OH A OH-bending doublet was found to be anomalously small, and the splitting of the (122)rA CH3-rocking doublet is observed to be enhanced. We have identified a network of intermode interactions causing this unusual behavior, but a quantitative analysis of the vibrational coupling is restricted by limited knowledge of the unperturbed positions of the interacting levels. All these interactions provide relaxation channels for intramolecular vibrational redistribution among the lower vibrational modes in 13CH3OH. Another important finding is that the torsion-K-rotation energy curves in the OH-bending state display an inverted pattern compared to the ground state. Copyright 1998 Academic Press.