Spectra and structure of phosphorus—boron compounds—XXVI. Infrared and Raman spectra, conformational stability and normal coordinate analysis of ethyldichlorophosphine-borane

Abstract

The Raman (3500-10 cm −1) and infrared (3500-50 cm −1) spectra of solid ethyldichlorophosphine-borane, CH 3CH 2P(BH 3)Cl 2 and its deuterated analog, CH 3CH 2P(BD 3)Cl 2 have been recorded. Additionally, the infrared spectra of the gases and the Raman spectra of the liquids have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several distinct Raman lines disappear on going from the liquid to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers, with the trans conformer being more stable in the liquid phase, and the only one present in the solid phase. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be nearly zero. Based on Raman depolarization data, group frequencies, isotopic shift factors and infrared band contours, a complete vibrational assignment has been proposed 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 BH 3 torsion has been observed at 188 cm −1, while the BD 3 torsion was not observed. The methyl torsions in the spectra of the solids have been observed at 209 and 202 cm −1 for the “light” and deuterated species, respectively. From the torsional data, barriers to internal rotation have been calculated. The asymmetric torsional mode has been observed for the trans conformer in the infrared spectra of the gas phase at 108 and 104 cm −1 for the BH 3 and BD 3 species, respectively. These results are compared with similar quantities for some corresponding organophosphine—borane compounds.