STUDY OF ENHANCED IVR IN MOLECULES WITH HINDERED INTERNAL ROTATION USING HIGH RESOLUTION INFRARED SPECTROSCOPY

Author Institution: Department of Chemistry, Princeton UniversityUsing an optothermal molecular beam spectrometer we have measured the $\nu_{1}$ and $2\nu_{2}$ spectra of several terminal acetylenes. We have studied a series of small acetylenes, propynal, 3-fluoropropynal, and vinyl acetylene, where the density of states increases through the series. These spectra show numerous weak, local perturbations characteristic of the IVR process. From these assigned spectra we can determine the homogeneous lifetime of the initial vibrational excitation. We have also attempted to record the spectra of the methyl substituted compounds butynone and 3-methyl butenyne, related to the previous compounds by placing a methyl group on the third carbon. In the fundamental region of $\nu_{1}$ we observe no spectrum, indicating that the IVR lifetime is decreased by the addition of the methyl group. We are, however, able to obtain a spectrum in the ethylene region of 3-methyl butenyne suggesting that these vibrations are more resistant to the IVR rate increase brought about by the addition of the methyl group."