FAR-IR SPECTRUM, CONFORMATION STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT, AND AB-INITIO CALCULATIONS OF 3-CHLORO-2-METHYLPROPENE

The far-IR spectrum from 375 to 30 cm(-1) of gaseous 3-chloro-2-methyl propene, CH2=C(CH3)CH2Cl, has been recorded at a resolution of 0.10 cm (-1). The fundamental asymmetric torsional mode for the gauche conform er is observed at 84.3 cm(-1) with three excited states falling to low er frequency. For the higher energy s-cis conformer, where the chlorin e atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm(-1) with two excited states falling to lower frequency. Utili zing the s-cis and gauche torsional frequencies, the gauche dihedral a ngle and the enthalpy difference between conformers, the potential fun ction governing the interconversion of the rotamers has been calculate d. The determined potential function coefficients are (in reciprocal c entimeters): V-1=189+/-12, V-2=-358+/-11, V-3=886+/-2 and V-4=-12+/-2 with an enthalpy difference between the more stable gauche and s-cis c onformers of 150+/-25 cm(-1) (430+/-71 cal mol(-1)). This function giv es values of 661 cm(-1) (1.89 kcal mol(-1)), 1226 cm(-1) (3.51 kcal mo l(-1)) and 812 cm(-1) (2.32 kcal mol(-1)), for the s-cis to gauche, ga uche to gauche, and gauche to s-cis barriers, respectively. From the m ethyl torsional frequency of 170 cm(-1) for the gauche conformer, the threefold barrier of 678 cm(-1) (1.94 kcal mol(-1)) has been calculate d. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained f rom ab initio calculations with both the 3-21G and 6-31G* basis sets. A normal-coordinate analysis has also been performed with a force fie ld determined from the 3-21G basis set. These data are compared with the corresponding data for some similar molecules.