Knowledge Bank

The 3 MHz resolution infrared spectra of the $10\backslash mum$ bands of the gauche conformer of 1,2-difluoroethane (HFCl52) and the $C1$ symmetry conformer of 1,1,2-trifluoroethane (HFCl43) have been measured using a molecular-beam electric-resonance optothermal spectrometer with a tunable microwave-sideband $CO2$ laser source. For 1,2-difluoroethane, two bands have been studied, the $\nu 17$ B-symmetry C-F stretch at $1077.3cm-1$ and the $\nu 13$ B-symmetry $CH2$ rock at $896.6cm-1$. Both bands are effectively unperturbed, being fit by a standard asymmetric-rotor Hamiltonian to better than 0.5 MHz. Two bands have also been studied for 1,1,2-trifluoroethane, the $\nu 11$ symmetric $CF2$ stretch at $1077.2cm-1$ and the $\nu 13$ C-C stretch at $905.1cm-1$. In contrast to 1,2-difluoroethane, only one of the two bands, $\nu 11$, is unperturbed and fit to near experimental precision. The $\nu 13$ vibration is weakly perturbed by an interaction with a nearby state. This perturbation leads to a doubling or splitting of the lines, which we interpret as a lifting of the degeneracy of the symmetric and antisymmetric tunneling states associated with tunneling between the two $CI$ forms. For the $J,Ka$ states studied, the splittings are as large as 37 MHz. Combining this observation with calculations from an empirical torsional potential leads to the conclusion that the most likely perturbing state is $\nu 17+7\nu 18$, where $\nu 17$ is the $CF2$ twist and $\nu 18$ is the torsion. The matrix element responsible for this interaction exchanges 9 vibrational quanta!