Knowledge Bank

An electric-resonance optothermal spectrometer and a microwave-sideband $CO2$ 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 $980.2cm-1$ vibrational fundamentals of 3,3,3-trifluoropropene $(CF3CH=CH2)$. 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 $980.2cm-1$ 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 $CF3$ torsional frequency of $-70cm-1$ and the high torsional barrier of $-535cm-1$, 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.