RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT AND AB-INITIO CALCULATIONS OF TRANS-1-BROMOBUT-2-ENE
Jr. Durig et al., RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT AND AB-INITIO CALCULATIONS OF TRANS-1-BROMOBUT-2-ENE, Journal of Raman spectroscopy, 24(11), 1993, pp. 709-724
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.