The Conformational Equilibria and Vibrational Spectra, Including Infrared Matrix Isolation Spectra, of Chloroacetyl Chloride and Chloroacetyl Bromide.

Infrared spectra of the title compounds as liquids and as unannealed and annealed solids at 85 K were recorded in the 4000 - 50 cm-1 range. Matrix isolation spectra in argon and in nitrogen matrices were obtained at 14 K in a 1:500 ratio, with gas mixtures heated to various temperatures between 313 and 900 K. With CsI windows in the helium cryostat the matrix isolation spectra were restricted to the region above 200 cm-1. After annealing to 30-34 K the spectra changed considerably, and the high-energy conformer (gauche) vanished, indicating barriers lower than 8 kJ mol-1. Raman spectra of the compounds were recorded as liquids at various temperatures and as amorphous and crystalline solids. From the variable-temperature measurements the enthalpy differences between the conformers were found to be: 8.4 +/- 0.4 (liquid, Raman), 3.2 +/- 0.5 ('vapour', Ar matrix, IR) and 4.3 +/- 0.5 kJ mol-1 ('vapour', N2 matrix, IR) for chloroacetyl chloride and 8.5 +/- 0.4 (liquid, Raman), 5.1 +/- 0.5 ('vapour', Ar matrix, IR) and 4.7 +/- 0.5 kJ mol-1 ('vapour', N2 matrix, IR for bromoacetyl chloride, the anti conformer invariably being the more stable.