Microwave spectrum, barrier to internal rotation, and dipole moment of 1,2,3-pentatriene

Abstract

The microwave spectrum of 1,2,3-pentatriene, CH2=C=C=CHCH3, has been studied in the frequency region from 8 to 40 GHz. A-type R-branch transitions from J=3←2 to J=9←8 were assigned. Most of the observed lines are split slightly as a result of internal rotation. Analysis of the splittings by PAM leads to an internal barrier of 1341±14 cal/mol (or 56.1±0.6 kJ/mol). The effective rotational constants are A0=29 802(20) MHz, B0=2212.646(7) MHz, and C0=2085.023(4) MHz. The value of Ic−Ia−Ib shows the molecule is planar except for the hydrogen atoms in the methyl group. A new method has been developed for the determination of the electric dipole moment of a molecule from high-J transitions in which individual Stark components are not resolved. The dipole moments of 1,2,3-pentatriene are determined to be μa=0.50±0.05 D, μb=0.122±0.003 D, and μtotal=0.51±0.05 D. Comparison of obtained molecular constants with those for propene and 1,2-butadiene revealed that increase in the cumulative double bonds lowers the potential barrier to internal rotation and increases the dipole moment.