THE VIBRATIONAL-SPECTRUM AND ROTATIONAL-CONSTANTS OF DIFLUOROETHYNE FC=CF - MATRIX AND HIGH-RESOLUTION INFRARED STUDIES AND ABINITIO CALCULATIONS

Infrared spectra of the short-lived difluoroethyne molecule have been recorded in neon and argon matrices between 200 and 5000 cm-1. Fourier transform infrared spectra with a resolution of 0.004 cm-1 have been measured in the gas phase around 1350 cm-1 (nu-3, nu-2 + nu-4 + nu-5, hot bands) and 2150 cm-1 (nu-2 + nu-3, nu-1 - nu-5, hot bands). The high resolution study yields rotational parameters of the ground and all singly excited vibrational states. The interpretation of the experimental data has been guided by ab initio calculations at the SCF (self-consistent-field) level and the correlated MP2 level (Moller-Plesset second order perturbation theory) employing three different large basis sets. The theoretical calculations provide the SCF and MP2 harmonic fields as well as the SCF anharmonic force field of FCCF. The agreement between the available theoretical and experimental results is generally quite good, with the exception of the spectroscopic constants involving the trans-bending mode nu-4 where more theoretical work is required. The combined use of theoretical and experimental information leads to an estimate of the equilibrium structure [D infinity h, r(e) (CC) = 1.1865 angstrom, r(e) (CF) = 1.2832 angstrom] and to recommended "best" values for the wave numbers of all fundamental vibrations based on the matrix and high resolution infrared data and some ab initio anharmonicity constants. The present study demonstrates the advantages of a combined theoretical and experimental approach to the spectroscopy of short-lived molecules.