Vibrational spectrum, ab initio calculations, assignments of fundamentals, barriers to internal rotation and stabilities of conformers of 1,2-dichloropropane

Abstract

The infrared and Raman spectrum of 1,2-dichloropropane has been reinvestigated and is reported in the gas, liquid, amorphous solid and two crystal modifications. All three conformers are present in disordered phases and many bands are composites of overlapping contributors. Assignments of observed bands are substantiated with the aid of ab initio calculations utilizing the RHF/3-21G and RHF/6-31G∗ basis sets. Electron correlation was approximated by an MP2/6-31G∗ calculation for all three conformers. From the temperature dependence of the Raman spectrum of the liquid, the bands at 528 and 671 cm −1 were uniquely assigned, respectively, to the conformer with the hydrogen atom trans to the chlorine atom (G −) and the chlorine atom trans to the chlorine atom (A), and an average enthalpy difference for the liquid phase was found to be 130 cm −1 (370 cal mol −1) at −57°C. In contrast, an enthalpy difference of 1038 cm −1 was obtained from the ab initio calculations for the isolated molecule using the RHF/3-21G basis set and 561 cm −1 (1.605 kcal mol −1) from the MP2/6-31G∗ calculation. The enthalpy difference showed experimental variation with temperature, corresponding to a more highly associated liquid at −105°C than at 60°C. The scaled harmonic force fields obtained using the MP2/6-31G∗ calculation are reported for all three conformers together with potential energy distributions and scaled and unscaled frequencies calculated from these force fields. These results are compared to those obtained for some similar molecules.