Conformational stability of CH3CH2P(Z)F-2 (Z = O,S) from temperature dependent FT-IR spectra of rare gas solutions and r(0) structural parameters

Variable temperature (from -55 to -150 degrees C) studies of the infrared s pectra (3500-400 cm(-1)) of ethylphosphonic difluoride, CH3CH2P(O)F-2 and e thylphosphonothioic difluoride, CH3CH2P(S)F-2 dissolved in liquid xenon or krypton have been recorded. From these data, the enthalpy differences have been determined to be 76 +/- 9 cm(-1) (0.91 +/- 0.11 kJ/mol), for CH3CH2P(O )F-2 with the trans conformer the more stable rotamer and 53 +/- 7 cm(-1) ( 0.63 +/- 0.08 kJ/mol) for CH3CH2P(S)F-2 but with the gauche conformer the m ore stable form. Complete vibrational assignments are presented for both mo lecules, which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G(d) calculations. The optimized geometries, conforma tional stabilities, harmonic force fields, infrared intensities, Raman acti vities, and depolarization ratios have been obtained from RHF/6-31G(d) and/ or MP2/6-31G(d) ab initio calculations. These quantities are compared to th e corresponding experimental quantities when appropriate as well as with so me corresponding results for some similar molecules. The r(o) adjusted stru ctural parameters have been obtained for both molecules from a combination of the microwave rotational constants and ab initio predicted parameters. T he corresponding r(o) structural parameters have been obtained for some sim ilar molecules for comparison. (C) 2000 Elsevier Science B.V. All rights re served.