CONFORMATIONAL STABILITY OF ETHYLDIFLUOROPHOSPHINE-BORANE FROM TEMPERATURE-DEPENDENT FT-IR SPECTRA OF XENON SOLUTIONS

Abstract Variable temperature (−55 to −100°C) studies of the infrared spectra (3500 to 400 cm −1 ) of ethyldifluorophosphine-borane, CH 3 CH 2 PF 2 BH 3 , 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 CH 3 CH 2 PF 2 BH 3 and CH 3 CH 2 PF 2 BD 3 , respectively, with the trans conformer (methyl group trans to the BH 3 or BD 3 group) the more stable form. The average Δ 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.