Vibrational spectra and structure from ab initio calculations of tetrafluorodiphosphine

Ab initio calculations have been carried out for tetrafluorodiphosphine, P2F4, up to the MP2/6-311 + G* level. The harmonic force constants and vibrational frequencies along with infrared and Raman intensities for the most stable trans conformer have been obtained from these calculations which provide a basis for the vibrational assignment. Several reassignments of the fundamentals from those previously given were required from these data. The calculations indicate that the gauche conformer should be a stable rotamer but more than 800 cm(-1) (2.3 kcal/mol) higher in energy which helps explain why this conformer has not been observed in the vibrational spectrum of the sample at ambient temperature. The predicted P-F distances from the MP2/6-311 + G* calculations are more than 0.031 Angstrom longer than the experimentally determined values. The potential function governing conformer interchange and the change in geometric parameters with internal rotation have been predicted from the ab initio calculations. The results are compared to the corresponding results from similar molecules.