IR and FTMW-IR Spectroscopy and Vibrational Relaxation Pathways in the CH Stretch Region of CH₃OH and CH₃OD

Infrared spectra of jet-cooled CH₃OD and CH₃OH in the CH stretch region are observed by coherence-converted population transfer Fourier transform microwave-infrared (CCPT-FTMW-IR) spectroscopy (E torsional species only) and by slit-jet single resonance spectroscopy (both A and E torsional species, CH₃OH only). Twagirayezu et al. reported the analysis of ν₃ symmetric CH stretch region (2750–2900 cm^–1; Twagirayezu et al. J. Phys. Chem. A 2010, 114, 6818), and the present work addresses the more complicated higher frequency region (2900–3020 cm^–1) containing the two asymmetric CH stretches (ν₂ and ν₉). The additional complications include a higher density of coupled states, more extensive mixing, and evidence for Coriolis as well as anharmonic coupling. The overall observed spectra contain 17 interacting vibrational bands for CH₃OD and 28 for CH₃OH. The sign and magnitude of the torsional tunneling splittings are deduced for three CH stretch fundamentals (ν₃, ν₂, ν₉) of both molecules and are compared to a model calculation and to ab initio theory. The number and distribution of observed vibrational bands indicate that the CH stretch bright states couple first to doorway states that are binary combinations of bending modes. In the parts of the spectrum where doorway states are present, the observed density of coupled states is comparable to the total density of vibrational states in the molecule, but where there are no doorway states, only the CH stretch fundamentals are observed. Above 2900 cm^–1, the available doorway states are CH bending states, but below, the doorway states also involve OH bending. A time-dependent interpretation of the present FTMW-IR spectra indicates a fast (∼200 fs) initial decay of the bright state followed by a second, slower redistribution (about 1–3 ps). The qualitative agreement of the present data with the time-dependent experiments of Iwaki and Dlott provides further support for the similarity of the fastest vibrational relaxation processes in the liquid and gas phases.