Jr. Durig et al., CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENTS AND AB-INITIO CALCULATIONS OF 3-METHYLBUT-1-ENE, Journal of Raman spectroscopy, 24(9), 1993, pp. 555-572
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.