High resolution spectroscopy of 1,2‐difluoroethane in a molecular beam: A case study of vibrational mode‐coupling

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

High-resolution microwave and infrared molecular-beam spectra have been measured for 1,1,2,2-tetrafluoroethane (HFC134). For the higher energy, polar, C2 symmetry, gauche conformer, microwave spectra have been recorded for the normal and mono-13C isotopomers and analyzed to determine a C-C bond length of 1.512(4) Å, in good agreement with a recent ab initio value (MP2/6-31G**) of 1.515 Å [S. Papasavva, K. H. Illinger, and J. E. Kenny, J. Phys. Chem. 100, 10100-10110 (1996)]. A tunable microwave-sideband CO2 laser and electric-resonance optothermal spectrometer have been used to measure the infrared spectrum of the nu6, 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 C2 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 = Kc = 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 nu16 vibration of the anti conformer near 1127 cm-1. An a- and c-type hybrid band is observed, consistent with a Bu symmetry mode. Previous low-resolution studies have attributed the 1127-cm-1 mode to either a Bu or an Au 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. Copyright 1998 Academic Press.