Spectra and structure of organophosphorus compounds .57. Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of n-propyltetrafluorophosphorane

The Raman (3100 to 50 cm(-1)) and infrared (3100 to 50 cm(-1)) spectra of gaseous and solid n-propyltetraflourophosphorane, C3H7PF4, have been recorded. Additionally, the Raman spectra of the liquids have been obtained with qualitative depolarization ratios. From these data, two conformers, trans and gauche, have been identified in the fluid states. The trans conformer is shown to be the most stable rotamer in both the gaseous and liquid states, and the only conformer present in the solid. The enthalpy difference between the conformers has been determined from Raman variable temperature studies to be 107 +/- 20 cm(-1) (306 +/- 57 cal/mol) for the vapor and 168 +/- 38 cm(-1) (480 +/- 109 cal/mol) for the liquid. A complete vibrational assignment is proposed for the trans conformer as well as for the fundamentals of most of the heavy atom motions for the gauche conformer. The conformational stabilities and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio calculations employing the RHF/3-21G(*) and RHF/6-31G(*) basis sets. Additionally, the conformational stabilities and structural parameters have also been determined with the 6-31G(*) basis set with electron correlation at the MP2 level. The barriers to internal rotation of the hydrocarbon skeleton and the PF4 moiety, as well as the barrier to Berry pseudorotation of the axial and equatorial fluorine atoms, have been determined using ab initio calculations at various levels. These results are compared to corresponding quantities for some similar molecules.