RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT AND AB-INITIO CALCULATIONS OF TRANS-1-BROMOBUT-2-ENE

Raman (3200-10 cm-1) and infrared (3200-50 cm-1) spectra were recorded for the fluid and solid phases of trans-1-bromobut-2-ene (crotyl brom ide), trans-CH3CH=CHCH2BR, and a complete vibrational assignment is pr oposed. The fundamental asymmetric torsion for the more stable gauche conformer was observed in the far-infrared spectrum of the gas as a hy brid band centered at 75.0 cm-1. The enthalpy difference between the g auche and syn conformers is estimated to be 434 +/- 111 cm-1 (1.24 +/- 0.32 kcal mol-1) in the gas phase from the relative intensities of th e Raman lines utilizing the differences in the Raman activities from t he ab initio calculations with the RHF/STO-3G basis set. The enthalpy difference was determined experimentally for the liquid from the rela tive intensities of the C-Br stretches as a function of temperature. T he determined value is 370 +/- 77 cm-1 (1.06 +/- 0.22 kcal mol-1) with the gauche conformer the more stable form. A reasonable range of valu es was obtained for the coefficients of the potential function governi ng internal rotation about the C-C bond. The three-fold barrier govern ing internal rotation of the CH3 group was determined from the far-inf rared spectrum of the gas. All of these data are compared to the corre sponding quantities obtained from ab initio Hartree-Fock gradient calc ulations employing the RHF/STO-3G and RHF/LANL1DZ basis sets. Additio nally, complete equilibrium geometries were determined for both rotame rs. The results are discussed and compared with the corresponding quan tities for some similar molecules.