Conformational stability of ethyldifluorophosphine-borane from temperature-dependent FT-IR spectra of xenon solutions

Variable temperature ( - 55 to - 100 degrees C) studies of the infrared spectra (3500 to 400 cm(-1)) of ethyldifluorophosphine-borane, CH3CH2PF2BH3, dissolved in liquid xenon have been recorded. Similar data were also obtained for the corresponding borane-d(3) molecule. From these data, the enthalpy differences have been determined to be 107 +/- 15 cm(-1) (1.28 +/- 0.18 kJ mol(-1)) and 74 +/- 4 cm(-1) (0.86 +/- 0.05 kJ mol(-1)) for CH3CH2PF2BH3 and CH3CH2PF2BD3, respectively, with the trans conformer (methyl group trans to the BH3 or BD3 group) the more stable form. The average Delta H value from all eight determinations for both isotopomers is 91 +/- 8 cm(-1) (1.09 +/- 0.10 kJ mol(-1)). A complete vibrational assignment is proposed for the trans conformer for both isotopomers and several of the fundamentals for the gauche conformer have been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311++G** basis sets at the levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results are discussed and compared to the corresponding quantities for some similar molecules. (C) 1998 Elsevier Science B.V.