The vibrational spectrum, normal coordinate analysis and conformational equilibrium of 3-chloro-2-methylpropene†

The infrared spectra of gaseous and liquid 3-chloro-2-methylpropene and the Raman spectra of the compound as gas, liquid and frozen solid have been recorded. Several bands present in the Raman spectrum of the liquid were found to be absent or greatly reduced in intensity in the spectrum of the solid phase, thus indicating the presence of a conformational equilibrium. The Raman polarization data show that the gauche conformer of this compound is the more stable. A study of the temperature dependence of the relative intensities of two Raman bands due to different conformers led to values of ΔHθ of 2.5±0.5 kJ mol−1 and ΔSθ of −4.8±2.0 JK−1 mol−1 for the conformational equilibrium in the liquid phase. These results are consistent with a conformational equilibrium of the type gauche⇌s-cis, as found for a number of related molecules. A complete vibrational assignment has been proposed for the spectral data, except for the unobserved methyl torsion, aided by a normal coordinate treatment.