High-Resolution Microwave and Infrared Molecular Beam Studies of the Conformers of 1, 1, 2, 2-Tetrafluoroethane

Abstract High-resolution microwave and infrared molecular-beam spectra have been measured for 1,1,2,2-tetrafluoroethane (HFC134). For the higher energy, polar, C 2 symmetry, gauche conformer, microwave spectra have been recorded for the normal and mono- 13 C isotopomers and analyzed to determine a C–C bond length of 1.512(4) A, in good agreement with a recent ab initio value (MP2/6-31G**) of 1.515 A [S. Papasavva, K. H. Illinger, and J. E. Kenny, J. Phys. Chem. 100, 10100–10110 (1996)]. A tunable microwave-sideband CO 2 laser and electric-resonance optothermal spectrometer have been used to measure the infrared spectrum of the ν 6 , C–C stretch of the gauche conformer near 906 cm −1 . Microwave-infrared double resonance and precise ground state combination differences provided by the microwave measurements guide the assignment of the spectrum. The observation of a c -type spectrum definitively establishes that the upper state vibration is of A symmetry in the C 2 point group. The spectrum is fit to a Watson asymmetric-top Hamiltonian to a standard deviation of 0.24 MHz. A weak perturbation shifts the line positions for transitions near J = K c = 20 by as much as 12 MHz. The identity of the perturber is unknown. Pulsed slit-jet diode-laser spectra have been recorded for the ν 16 vibration of the anti conformer near 1127 cm −1 . An a - and c -type hybrid band is observed, consistent with a B u symmetry mode. Previous low-resolution studies have attributed the 1127-cm −1 mode to either a B u or an A u symmetry vibration. A total of 522 nonblended transitions were assigned and fit to determine ground and excited state constants. The ground state constants of A = 5134.952(65) MHz, B = 3148.277(27) MHz, and C = 2067.106(43) MHz are the first experimental determinations of the rotational constants for this conformer. Here, type A standard uncertainties are given in the parentheses.