Influence of methyl group deuteration on the rate of intramolecular vibrational energy relaxation

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Abstract
The high resolution spectra of the fundamental and first overtone of the acetylenic C–H stretch in tert.-butylacetylene-d9 and (trimethylsilyl)acetylene-d9 have been measured using optothermal detection of a collimated molecular beam. IVR lifetimes determined from the homogeneously broadened lineshapes are compared to those of their undeuterated analogues. It is found that for both molecules, at both levels of excitation, deuterating the methyl rotors results in an increased rate of IVR. The results indicate that the previously suggested methyl rotor effect, as an enhancer for IVR, plays a secondary role to increasing the number of low order resonances to which the C–H stretch can couple. Although the torsional modes are important for the molecules to exhibit statistical case IVR and contribute to the filled-in homogeneous lineshapes, the rate of energy relaxation seems to be dominated by the number of low order resonances.