Conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 2-methylpropionyl chloride

Abstract

The Raman spectra (3600-10 cm −1) of gaseous, liquid and solid 2-methylpropionyl chloride ((CH 3) 2CHCClO) and the IR spectra (3600-40 cm −1) of gaseous and solid (CH 3) 2CHCClO have been recorded. In addition, qualitative depolarization ratios have been obtained from the Raman spectrum of the liquid. Variable-temperature (−60 to −100°C) studies of the IR spectra (3500-400 cm −1) of (CH 3) 2CHCClO dissolved in liquid xenon have been recorded. Utilizing three sets of conformer doublets, the enthalpy difference has been determined to be 240 ± 12 cm −1 (686 ± 34 cal mol −1). These data have been interpreted for the fluid phases and show that the gauche conformation (methyl group eclipses the carbonyl bond) is thermodynamically preferred over the high-energy trans conformation and is the only rotamer present in the spectra of the annealed solid. From the relative intensities of the Raman lines of the liquid at 687 and 563 cm −1 as a function of temperature, the enthalpy difference for the liquid has been determined to be 345 ± 93 cm −1 (986 ± 266 cal mol −1). A complete vibrational assignment is proposed, which is based on IR band contours, depolarization values, group frequencies and normal coordinates. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies determined experimentally are compared with those obtained from ab initio calculations and with the corresponding quantities obtained for similar molecules.