Conformational stability of CH3CH2PH2BH3 from temperature dependent FT-IR spectra of xenon solutions and r(0) structural parameters

Variable temperature ( - 55- - 100 degrees C) studies of the infrared spect ra (3500-400 cm(-1)) of ethylphosphine-borane, CH3CH2PH2BH3, and ethylphosp hine-borane-d(5) dissolved in liquid xenon have been recorded. From these d ata, the enthalpy difference has been determined to be 86 +/- 8 cm(-1) (1.0 3 +/- 0.10 kJ/mol), with the trans conformer the more stable rotamer. Compl ete vibrational assignments are presented for both conformers, which are co nsistent with the predicted frequencies obtained from the ab initio MP2/6-3 1G(d) calculations. The optimized geometries, conformational stabilities, h armonic force fields, infrared intensities, Raman activities, and depolariz ation ratios have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab in itio calculations. These quantities are compared to the corresponding exper imental quantities when appropriate as well as with some corresponding resu lts for some similar molecules. The r(o) structural parameters have been ob tained from a combination of the previously reported microwave rotational c onstants and ab initio predicted parameters. (C) 1999 Elsevier Science B.V. All rights reserved.