CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENTS AND AB-INITIO CALCULATIONS OF 3-METHYLBUT-1-ENE

The Raman spectra (3200-10 cm-1) of gaseous, liquid and solid and infrared spectra (3200-40 cm-1) of gaseous and solid 3-methylbut-1-ene, H2C=CHCH(CH3)2, were recorded. From the vibrational spectra of gaseous 3-methylbut-1-ene, below 200 cm-1, the asymmetric torsional mode of the more stable trans and higher energy gauche conformers were assigned. From studies of the Raman spectrum of the gas with variable temperatures the trans conformer has been determined to be more stable than the gauche form by 155 +/- 31 cm-1 1443 +/- 89 cal mol-1 (1 cal = 4.184 J)]. Similar studies for the liquid indicate that the two conformers are nearly equivalent in energy and for the annealed solid only the gauche conformer persists. Optimized structural parameters, obtained from ab initio calculations with the MP2/6-31G** basis set, were used to obtain the kinetic energy terms with structural relaxation for the asymmetric internal rotation. The coefficients of the potential function governing the conformational interchange are V1 = 198 +/- 15, V2 = 68 +/- 6, V3 = 748 +/- 1 and V4 = -55 +/- 2 cm-1, and this potential has trans to gauche, gauche to gauche and gauche to trans barriers of 816, 787 and 657 cm-1, respectively. Barriers to internal rotation of the methyl rotors of 1169 cm-1 for the trans conformer and 1290 +/- 30 and 1525 +/- 50 cm-1 for the gauche conformer were determined from the low-frequency Raman and far-infrared spectra of the gas. Vibrational assignments are provided which are based on infrared band contours Raman depolarization values, group frequencies and normal coordinate calculations. The conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies which were determined from experimental data are compared with those obtained from ab initio calculations.