Coherence-converted population transfer infrared-microwave double-resonance spectroscopy is used to record the infrared spectra of jet-cooled CH(3)OH and CH(3)OD. Population transfer induced by a pulsed IR laser is detected by Fourier transform microwave spectroscopy background-free using a two-MW pulse sequence. The observed spectrum of CH(3)OH in the nu(3) symmetric CH stretch region contains 12 interacting vibrational bands, whereas in CH(3)OD, only one vibrational band is observed in the same interval (2750-2900 cm(-1)). The bright state, responsible for the transitions observed in this region, is not just nu(3) but also contains an admixture of the binary CH bending combinations, particularly 2nu(5). The lack of interacting bands in CH(3)OD confirms that in CH(3)OH the binary combinations of the OH bend (nu(6)) and a CH bend (nu(4), nu(5), nu(10)) act as doorway states linking the bright state to higher order combination vibrations involving torsional excitation. A time-dependent interpretation of the frequency-resolved spectra reveals a fast (approximately 200 fs) initial decay of the bright state followed by a slower (1-2 ps) redistribution among the lower frequency modes.

The four-dimensional model Hamiltonian of Wang and Perry [J. Chem. Phys. 109, 10795 (1998)] is used to compare the approximate adiabatic separation of the torsion and CH stretches in methanol to an exact solution of the same Hamiltonian. The adiabatic approximation accounts for the pattern of the energy levels in the lowest torsional states, including the inverted tunneling splittings, but does not account for the pattern of systematic two- and four-fold near degeneracies at high torsional excitation. In the adiabatic basis, the nonadiabatic couplings mix the torsional and vibrational degrees of freedom and hence are a source for intramolecular vibrational redistribution (IVR). These IVR matrix elements are found to decrease by only a factor of 2 or 3 with each higher coupling order, in agreement with the results of Pearman and Gruebele [Z. Phys. Chem. Munich 214, 1439 (2000)]. This gentle scaling behavior, which contrasts with a steeper falloff with coupling order in more rigid molecules, points to a more important role for direct high-order couplings in torsional molecules. In this model, the scaling behavior derives from a single coupling term that is low order in the torsional angular momentum in combination with one-dimensional torsional functions that include contributions from many torsional angular momenta.

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The vibrational structure of 2-ethyl-1-hexanol is of great interest because of its industrial and military applications. However, detailed spectral analysis is challenging due to its flexibility. This paper reports a detailed analysis of the gas and liquid phase vibrational spectra of 2-ethyl-1-hexanol using the Fourier transform infrared spectroscopy and Raman experimental data. By performing a detailed exploration of the conformational space in this work, the theoretical spectra reproduced almost all experimental details observed, and assigned internal valence coordinates to all of the experimentally observed bands in the floppy 2-ethyl-1-hexanol molecule. Relative contributions from the various internal valence coordinates to the experimental vibrational bands are directly compared between the liquid phase Raman band and the gas and liquid phase infrared band.

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