FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND VIBRATIONAL COUPLING IN THE OH-BENDING BAND OF (CH3OH)-C-13

We present in this work a high-resolution Fourier transform infrared s tudy of the OH-bending vibrational band of (CH3OH)-C-13. We have inves tigated the 1070-1400 cm(-1) spectral region at 0.002 cm(-1) resolutio n using the modified Bomem DA3.002 Fourier transform spectrometer at t he Steacie Institute for Molecular Sciences at the National Research C ouncil of Canada in Ottawa. This study has led to (i) determination of excited-state J(J + 1) subband expansion coefficients and (ii) charac terization of a variety of interactions coupling the different vibrati onal modes, notably a strong Fermi resonance between the OH bend and t he 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 usi ng closed loops of IR and FIR transitions. The subbands have been fitt ed 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 in teraction 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 (n tau K)(u psilon) = (122)(OH) A OH-bending doublet was found to be anomalously s mall, and the splitting of the (122)(r)A CH3-rocking doublet is observ ed to be enhanced. We have identified a network of intermode interacti ons causing this unusual behavior, but a quantitative analysis of the vibrational coupling is restricted by limited knowledge of the unpertu rbed positions of the interacting levels. All these interactions provi de relaxation channels for intramolecular vibrational redistribution a mong the lower vibrational modes in (CH3OH)-C-13. Another important fi nding is that the torsion-K-rotation energy curves in the OH-bending s tate display an inverted pattern compared to the ground state. (C) 199 8 Academic Press.