Far-IR spectrum, conformation stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 3-chloro-2-methylpropene

Abstract

The far-IR spectrum from 375 to 30 cm −1 of gaseous 3-chloro-2-methylpropene, CH 2=C(CH 3)CH 2Cl, has been recorded at a resolution of 0.10 cm −1. The fundamental asymmetric torsional mode for the gauche conformer is observed at 84.3 cm −1 with three excited states falling to lower frequency. For the higher energy s-cis conformer, where the chlorine atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm −1 with two excited states falling to lower frequency. Utilizing the s-cis and gauche torsional frequencies, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in reciprocal centimeters): 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 conformers of 150 ±25 cm −1 (430 ± 71 cal mol −1). This function gives values of 661 cm −1 (1.89 kcal mol −1), 1226 cm −1 (3.51 kcal mol −1) and 812 cm −1 (2.32 kcal mol −1), for the s-cis to gauche, gauche to gauche, and gauche to s-cis barriers, respectively. From the methyl torsional frequency of 170 cm −1 for the gauche conformer, the threefold barrier of 678 cm −1 (1.94 kcal mol −1) has been calculated. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3–21G * and 6–31G * basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G * basis set. These data are compared with the corresponding data for some similar molecules.