Conformational stability of CH3CH2PH2BH3 from temperature dependent FT-IR spectra of xenon solutions and r0 structural parameters.

Abstract Variable temperature (−55–−100°C) studies of the infrared spectra (3500–400 cm −1 ) of ethylphosphine-borane, CH 3 CH 2 PH 2 BH 3 , and ethylphosphine-borane-d 5 dissolved in liquid xenon have been recorded. From these data, the enthalpy difference has been determined to be 86±8 cm −1 (1.03±0.10 kJ/mol), with the trans conformer the more stable rotamer. Complete vibrational assignments are presented for both conformers, which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G(d) calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, 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 the corresponding experimental quantities when appropriate as well as with some corresponding results for some similar molecules. The r 0 structural parameters have been obtained from a combination of the previously reported microwave rotational constants and ab initio predicted parameters.