Knowledge Bank

The high resolution spectra of the fundamental and first overtone of the C-H stretch in tertbutylacetylene-$d9$ trimethylsilyacetylence-$d9$ and tertbutylacetyene -$f9$(4,4,4-triflouro 3,3-bis (trifluoromethyl)butyne) 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 $analogues1$. It is found that for both molecules, at both levels of excitation, deuterating (or fluorinating) the methyl rotors results in an increased rate of IVR. Many of the changes in IVR rates for this series of molecules can be rationalized by changes in the density of cubic and quartic resonances. The fluorinated compound however, appears to the an anomaly and other arguments must be invoked to explain the increased IVR rate. Based on the calculated number of symmetry allowed background states, it is evident that some energy from the initially excited C-H Stretch must flow into the torsional modes of the methyl rotors.