Spectra and structure of organophosphorus compounds .59. conformational stability from temperature dependent FT-IR spectra of xenon solutions and ab initio calculations for ethyldifluorophosphine

The mid-infrared spectra (3200-400 cm(-1)) of ethyldifluorophosphine, CH3CH2PF2, dissolved in liquified krypton as a function of temperature (-105 to -150 degrees C) have been recorded. Using the conformer pairs (gauche/trans) observed at 679/651, 749/499, 679/499, and 749/651 cm(-1), the enthalpy difference has been determined to be 80 +/- 7 cm(-1) (228 +/- 20 cal mol(-1)) with the trans (methyl group trans to the phosphorus lone pair) conformer being more stable than the high energy gauche form. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for both conformers from RHF/6-31G* and/or MP2/6-31G* ab initio calculations. The trans form is predicted to be the more stable conformer from both ab initio calculations in agreement with the experimental results. The other calculated quantities are compared with the experimentally determined values where applicable and with corresponding results obtained for some similar molecules. (C) 1997 Elsevier Science B.V.