INFRARED-SPECTRA OF THE 10-MU-M BANDS OF 1,2-DIFLUOROETHANE AND 1,1,2-TRIFLUOROETHANE - VIBRATIONALLY MEDIATED TORSIONAL TUNNELING IN 1,1,2-TRIFLUOROETHANE

The 3-MHz-resolution infrared spectra of the 10-mu m bands of the gauc he conformer of 1,2-difluoroethane (HFC152) and the C-1-symmetry confo rmer of 1,1,2-trifluoroethane (HFC143) have been measured using a mole cular-beam electric-resonance optothermal spectrometer with a tunable microwave-sideband CO2 laser source. For 1,2-difluoroethane, two bands have been studied, the upsilon(17) B-symmetry C-F stretch at 1077.3 c m(-1) and the upsilon(13) B-symmetry CH2 rock at 896.6 cm(-1). Both ba nds are well fit to a asymmetric-rotor Hamiltonian to better than 0.5 MHz. The upsilon(13) band is effectively unperturbed, while the upsilo n(17) band is weakly perturbed, as shown by the large change in centri fugal distortion constants from the ground state values. Two bands hav e also been studied for 1,1,2-trifluoroethane, the upsilon(11) symmetr ic CF2 stretch at 1077.2 cm(-1) and the upsilon(13) C-C stretch at 905 .1 cm(-1). One of the two bands, upsilon(11), is unperturbed and fit t o near the experimental precision. The upsilon(13) vibration, on the o ther hand, is weakly perturbed by an interaction with a nearby state. This perturbation leads to a doubling or splitting of the lines, due t o a perturbation-induced lifting of the degeneracy of the symmetric an d antisymmetric tunneling states associated with tunneling between the two equivalent C-1 forms. For the J, K-a states studied, the splittin gs are as large as 37 MHz. Combining this observation with published l ow-resolution far-infrared measurements of torsional sequence-band and hot-band frequencies and calculations from an empirical torsional pot ential allows us to identify the perturbing state as upsilon(17) + 6 u psilon(18). Here, upsilon(17) is the CF2 twist and upsilon(18) is the torsion, The matrix element responsible for this interaction exchanges eight vibrational quanta! (C) 1995 Academic Press, Inc.