USING TORSIONAL SPLITTINGS TO PROBE VIBRATIONAL MODE MIXING: THE $10 \mu m$ INFRARED SPECTRUM OF PROPENE

Author Institution: Universitat Ulm, Abteilung f\""{u}r Physikalische Chemie; Molecular Physics Division, National Institute of Standards and TechnologyThe infrared spectrum of propene $(CH_{3}CH=CH_{2})$ in the region between $900 - 1100 cm^{-1}$ has been measured at a resolution of 2 MHz using an electric-resonance optothermal spectrometer and a microwave-sideband $CO_{2}$ laser. Spectra have been assigned for the A' fundamental, $v_{13}$ at $919.30 cm^{-1}$, and the A' fundamentals, $\nu_{17}$ at $1045.21 cm^{-1}, \nu_{18}$ at $990.78 cm^{-1}$, and $v^{19}$ at $912.67 cm^{-1}$. The observed $J^{\prime}=0 A/E$ tunneling splittings are -85.5 MHz for $\nu_{18} 40.9$ MHz for +191.3 MHz for $\nu_{18}$, and +282.8 MHz for $\nu_{19}$, compared to the ground-state value of +104.6 MHz. The range of torsional splittings observed for the vibrationally excited states are evidence of coupling of the various vibrations to background states in which the torsional mode is excited. The other possibility that the observed splittings are indicative of a change in torsional barrier upon vibrational excitation is not reasonable since for two of the vibrations the E state is below the A state in energy, compared to the ground-state ordering which has the A state below the E state, and for the other two vibrations unreasonably large changes in the ground-state barrier are required to reproduce the observed splittings. For $\nu_{17}$ and $\nu_{19}$, transitions are observed to background perturbing states. The present results indicate that caution should be used when interpreting tunneling splittings in vibrationally excited states as resulting from a change in torsional barrier from that of the ground state