Spectra and structure of phosphorus—boron compounds Part XXV. Infrared and Raman spectra, conformational stability and normal coordinate analysis of ethyldimethylphosphine—borane

Abstract

The Raman (10 to 3500 cm −1) and infrared (50 to 3500 cm −1) spectra of solid ethyldimethylphosphine—borane, CH 3CH 2(CH 3) 2P(BH 3) and its deuterated analog, CH 3CH 2(CH 3) 2P(BD 3) have been recorded. Additionally, the Raman spectra of the liquid phases have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several bands were observed to disappear upon going from the liquid to the solid phase, it is concluded that the molecule exists as a mixture of the trans and gauche conformers with the trans conformer being the only one present in the solid phase. Employing Raman depolarization data, group frequencies and isotopic shift data, the spectrum of the solid phase has been interpreted on the basis of C s symmetry for the trans conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The asymmetric torsion has been assigned to the bands at 127 cm −1 in both the -d 0 and -d 3 compounds. From a variable temperature Raman study it is concluded that the gauche conformer is the more stable form in the liquid phase and the enthalpy difference between the two conformers is estimated to be 590 ± 100 cm −1 (1.7 ± 0.3 cal mol −1). These results are compared with corresponding results for several other organophosphine—borane compounds.