Conformational stability, far-infrared spectra, barriers to internal rotation, vibrational assignment and RHF/STO - 3G* calculations of 1 -bromo- 2 -fluoroethane

Abstract

The far-IR spectrum (350-35 cm −1) of gaseous 1 -bromo-2-fluoroethane, BrCH 2CH 2F, has been recorded at a resolution of 0.10 cm −1. The fundamental asymmetric torsional frequencies of the more stable trans (two halogen atoms oriented trans) and high energy gauche conformers have been observed at 125.3 cm −1 and 111.3 cm −1, respectively, and the asymmetric torsional potential function governing internal rotation about the CC bond has been determined. This potential function gives values for the torsional potential coefficients of V 1 =584±6, V 2 =-147±4, V 3 =1217±11, V 4 =138±2, and V 4 = −21+4 cm −1, and a dihedral angle (∠FCCBr) of 67°0 for the gauche conformer. The trans to gauche, gauche to gauche, and gauche to trans barriers have been determined to be 1356 cm −1, 1418 cm −1 and 973 cm −1, respectively, with an energy difference between the conformations of 383±21 cm −1 (1.09±10.06 kcal mol −1). From studies of the Raman spectra at variable temperatures the conformational energy difference has been determined to be 350 ± 87 cm −1 (1.00±0.25 kcal mol −1) with the trans more stable and 300±46cm −1 (0.86±0.13 kcal mol −1) with the gauche more stable for the gas and liquid, respectively. A complete assignment of the vibrational spectra including the IR (3500-400 cm') spectra of the gas and solid and the Raman (3200-10 cm −1) spectra of the gas, liquid and solid is proposed. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies which have been determined from experiment, are compared to those obtained from ab initio Hartree-Fock gradient calculations and to the corresponding quantities obtained for some similar molecules.