Spectra and structure of organosphosphorus molecules. Part LX—Raman and infrared spectra, conformational stability,ab initio calculations and vibrational assignment of fluoromethyldichlorophosphine

Abstract

The Raman spectra (3500–30 cm−1) of gaseous, liquid and solid and the infrared spectra (3500–400 cm−1) of gaseous and solid fluoromethyldichlorophosphine, FCH2PCl2 , were recorded. The data indicate that two conformers are present in the fluid states where the gauche conformer is the predominant species but only the trans rotamer (the F atom on the methyl group is trans to the phosphorus lone pair of non-bonded electrons) remains in the crystalline solid. The enthalpy, ΔH, was determined to be 165±8 cm −1 (472±24 cal mol−1 ); 1 cal=4.184 J with the gauche conformer more stable from the sample dissolved in liquified xenon utilizing the bands assigned to the CH2 rocks for the two conformers. Additionally, utilizing six Raman lines in the spectrum of the liquid and adjusting for their ab initio -predicted relative intensities, the enthalpy difference was determined to be 156±67 cm−1 (447±193 cal mol−1), again with the gauche conformer the more stable form. These values are in agreement with the 273 cm−1 energy difference obtained from ab initio MP2/6–311++G** calculations. Complete vibrational assignments are proposed for both conformers based on infrared band contours, Raman and infrared relative intensities and group wavenumbers, which are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6–31G* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational wavenumbers are reported for both conformers from RHF/6–31G* and/or MP2/6–31G* ab initio calculations. The calculated quantities are compared with the experimentally determined values where applicable and also with some corresponding results for some similar molecules. Copyright © 1999 John Wiley & Sons, Ltd.