Conformational analysis from low-frequency vibrational data and ab initio calculations for 3-chloropropene

Abstract

The asymmetric torsion of 3-chloropropene, CH 2CHCH 2Cl, has been investigated by far-infrared and Raman spectra of the gas and the low-frequency Raman spectrum of the liquid at various temperatures. Both the gauche and cis (halogen to double bond) conformers are present in the fluid phases at ambient temperature and the gauche isomer is the predominant form in the liquid. The asymmetric torsion of the cis rotamer is observed in the far-infrared spectrum of the gas at 147.28 cm −1 with four accompanying “hot bands” and the corresponding fundamental of the gauche conformer is observed at 101.97 cm −1 also with four “hot bands”. From these data, a potential function governing the internal rotation of the asymmetric top has been determined with potential constants of: V 1 = −47±19, V 2 = 145±17, V 3 = 862±8, V 4 = 14±6, V 6 = −50±4 cm −1. This potential function is consistent with the cis form being thermodynamically preferred by 81 cm −1 (0.23 kcal mol −1), and a torsional dihedral angle of 122° for the gauche conformer away from the cis form. The calculated cis to gauche, gauche to gauche, and gauche to cis barriers are 969 cm −1 (2.77 kcal mol −1), 734 cm −1 (2.10 kcal mol −1) and 888 cm −1 (2.54 kcal mol −1), respectively. From relative intensities of the Raman lines of the liquid at 245 ( cis) and 289 ( gauche) cm −1 as a function of temperature, the enthalpy difference was found to be 178±11 cm −1 (509±31 cal mol −1) with the gauche form being more stable. The normal vibrations for both the cis and gauche rotamers have been calculated by ab initio Hartree-Fock gradient calculations employing the 3-21G* basis set. The calculated frequencies and force constants are compared to those previously reported. Potential surface calculations and structural determinations have been carried out employing both the 3-21G* and 6-31G* basis sets and compared to those obtained experimentally. These results are compared to similar quantities in some related molecules.