Torsional splittings in the diode laser slit-jet spectra of the v(6) fundamental of 1-chloro-1,1-difluoroethane (HCFC-142b)

High resolution infrared spectra (0.001 cm(-1) FWHM) have been measured for mixtures of 1-chloro-1,1-difluoroethane in Ne, expanded in a supersonic planar jet. The v(6) fundamental, infrared active with a dominant parallel transition moment, exhibits a remarkable splitting of about 0.035 cm(-1) for both Cl-35 and Cl-37 isotopomers. Several mechanisms of interaction Of v(6) with states with high torsional excitation are critically examined to explain the observed effect. It is concluded that the observed torsional splitting patterns can be explained in terms of a torsional Coriolis interaction between v(6) and a highly excited torsional mode, 6v(18). A full numerical analysis, performed including a torsional Coriolis term in the Hamiltonian, shows that the interaction mechanism requires a torsional barrier height of about 1270 cm(-1).