Infrared and Raman spectra, conformational stability, vibrational assignment, and ab initio calculations of 2-bromo-3-chloropropene

The infrared (3400-70 cm(-1)) and Raman (3300-20 cm(-1)) spectra of 2-bromo-3-chloropropene, H2C=C(Br)CH2Cl, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded. Variable temperature (-60--100 degrees C) studies of the infrared spectrum (3400-400 cm(-1)) of 2-bromo-3-chloropropene dissolved in liquid xenon have been performed. Utilizing two sets of conformer doublets, the enthalpy difference has been determined to be 197 +/- 20 cm(-1) (563 +/- 57 cal mol(-1)), with the anti conformer the more stable form (Cl atom cis to the double bond). Both anti and gauche conformers have been identified in the liquid phase, with the gauche conformer remaining in the annealed solid. The fundamental asymmetric torsional mode for the anti conformer is observed at 88.9 cm(-1). A complete vibrational assignment is proposed based on group frequencies and relative infrared and Raman intensities. The conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with the RHF/LANL-1DZ, MP2 LANL-IDZ and RHF/4-31G*/MIDI-4* basis sets. Normal coordinate analyses have also been performed with force fields determined from the RHF/4-31G*/MIDI-4* basis set. The calculated frequencies support the proposed vibrational assignment. These data are compared to corresponding data for some similar molecules.