Raman and infrared spectra, barriers to internal rotation, vibrational assignment and ab initio calculations of 1‐bromo‐2‐methylpropene

Abstract

The Raman spectra (3200-20 cm-I ) of the liquid and solid phases of 1-hromo-2-methylpropene IBrHC=C(CH,),] and the infrared (3200-30 cm-') spectra of the gas and solid were recorded. Additionally, qualitative depolarization values were obtained from the Raman spectrum of the liquid. All of the normal modes were assigned based on hand contours, depolarization values, relative intensities and normal coordinate analysis. The torsional transition for the methyl group trans to the bromine atom was observed in the far-infrared spectrum of the vapor at 167 Em-', and from this wavenumber the barrier to internal rotation was calculated to be 641 cm-' (1.83 kcal mol-I). Optimized structural parameters, barriers to internal rotation and the wavenumbers for the vibrational fundamentals were calculated using the RHF/STO-3G*, RHFLANL-1 DZ and/or MP2/LANL-1DZ basis sets. All of the results are discussed and compared with the corresponding values obtained for some similar molecules.