HIGH-ORDER TORSIONAL COUPLINGS IN THE INFRARED SPECTRUM OF TRIFLUOROPROPENE

Author Institution: Universit\""{a}t Ulm, Abteilung f\""{u}r Physikalische Chemie; Molecular Physics Division, National Institute of Standards and TechnologyAn electric-resonance optothermal spectrometer and a microwave-sideband $CO_{2}$ laser have been used together with microwave-infrared double resonance to measure and assign the 2 MHz resolution infrared spectrum of the 963.4 and $980.2 cm^{-1}$ vibrational fundamentals of 3,3,3-trifluoropropene $(CF_{3}CH=CH_{2})$. Two c-type bands are observed, indicating that the two upper state vibrations are of A'' symmetry, in disagreement with previously proposed normal-mode assignments of the low-resolution infrared spectrum of trifluoropropene. The lower frequency band is well characterized by an asymmetrical-top Hamiltonian, except for the presence of a small perturbation affecting the $J^{\prime}=7, K^{\prime}=2$ asymmetry doublet arising from a state with a torsional splitting of greater than 750 MHz. In contrast, the $980.2 cm^{-1}$ shows a large number of perturbations affecting the $K^{\prime}=0, 1$ and 2 levels for $J^{\prime}> 9$, resulting from at least one background state with a torsional splitting significantly greater than 40 MHz. Because of the low $CF_{3}$ torsional frequency of $- 70 cm^{-1}$ and the high torsional barrier of $- 535 cm^{-1}$, the background states must have at least 5 quanta in the torsional coordinate to achieve the observed torsional splittings. This implies that the observed perturbations arise from couplings which interchange at least 8 quanta of vibrational excitation, suggesting that high-order vibrational interactions can play an important in the vibrational dynamics of molecules