VIBRATIONAL-SPECTRUM, AB-INITIO CALCULATIONS, ASSIGNMENTS OF FUNDAMENTALS, BARRIERS TO INTERNAL-ROTATION AND STABILITIES OF CONFORMERS OF 1,2-DICHLOROPROPANE

The infrared and Raman spectrum of 1,2-dichloropropane has been reinve stigated and is reported in the gas, liquid, amorphous solid and two c rystal modifications. All three conformers are present in disordered p hases and many bands are composites of overlapping contributors. Assig nments of observed bands are substantiated with the aid of ab initio c alculations utilizing the RHF/3-21G and RHF/6-31G basis sets. Electro n correlation was approximated by an MP2/6-31G calculation for all th ree 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 ch lorine 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 degrees C. In contrast, an enth alpy difference of 1038 cm(-1) was obtained from the ab initio calcula tions 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 entha lpy difference showed experimental variation with temperature, corresp onding to a more highly associated liquid at -105 degrees C than at 60 degrees C. The scaled harmonic force fields obtained using the MP2/6- 31G calculation are reported for all three conformers together with p otential energy distributions and scaled and unsealed frequencies calc ulated from these force fields. These results are compared to those ob tained for some similar molecules.