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 infr ared spectra (3200-40 cm-1) of gaseous and solid 3-methylbut-1-ene, H2 C=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 temperatur es 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 gauch e conformer persists. Optimized structural parameters, obtained from a b initio calculations with the MP2/6-31G* basis set, were used to obt ain the kinetic energy terms with structural relaxation for the asymme tric internal rotation. The coefficients of the potential function gov erning the conformational interchange are V1 = 198 +/- 15, V2 = 68 +/- 6, V3 = 748 +/- 1 and V4 = -55 +/- 2 cm-1, and this potential has tra ns to gauche, gauche to gauche and gauche to trans barriers of 816, 78 7 and 657 cm-1, respectively. Barriers to internal rotation of the met hyl rotors of 1169 cm-1 for the trans conformer and 1290 +/- 30 and 15 25 +/- 50 cm-1 for the gauche conformer were determined from the low-f requency Raman and far-infrared spectra of the gas. Vibrational assign ments are provided which are based on infrared band contours Raman dep olarization values, group frequencies and normal coordinate calculatio ns. The conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies which were determined from experim ental data are compared with those obtained from ab initio calculation s.