Conformational stability determination of chloromethyl thiirane from variable temperature FT-IR studies of rare gas solutions, structural parameters, and ab initio calculations

The infrared spectra (3500–400 cm-1) of rare gas solutions of chloromethyl thiirane (3chloropropylene sulfide), c-SC2H3CH2Cl, at variable temperatures (-55 to -150°C), have been recorded and only the gauche-2 and gauche-1 conformers are present in detectable amounts at ambient temperature. Utilizing six conformer pairs in the xenon solution, an enthalpy difference of 105±15 cm-1 (1.26±0.18 kJ mol-1) has been obtained with the gauche-2 conformer, the more stable form. From these data, it is estimated that 38% of the gauche-1 form is present at ambient temperature. Complete vibrational assignments are provided for both stable conformers. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities, and Raman activities have been obtained from MP2/6-31G(d) calculations with full electron correlation. These quantities have been compared to the experimental values when appropriate. The optimized geometries have also been obtained with several different ab initio basis sets with full electron correlation up to MP2/6-311+G(2d,2p). The adjusted-r0 structural parameters have been obtained for the gauche-1 conformer by combining the ab initio data with the previously reported microwave rotational constants. The predicted structural parameters for both conformers are compared to those reported from an electron diffraction study. These results are compared to the corresponding quantities of some similar molecules.