MICROWAVE-SPECTRUM, INTERNAL-ROTATION, AND DIPOLE-MOMENTS OF 3-METHYL-1-BUTENE

Microwave rotational spectra have been assigned for species of 3-methyl-1-butene in which the vinyl group is trans and gauche to the isopropyl group. Rotational transitions have also been assigned in the first three excited states of the torsional motion of the vinyl group in the trans conformer and in the first two excited states of the corresponding torsional mode of the gauche species. From relative intensity measurements the torsional excitation energies have been estimated to be 90 ± 10 and 104 ± 10 cm-1 for the trans and gauche species, respectively. The gauche-trans energy difference has been estimated to be 130 ± 20 cm-1. The excitation energies and energy difference have been used to estimate the torsional potential constants. By analysis of Stark effects the dipole moments have been determined to be for the trans conformer μa = 0.312 ± 0.003 D, μb = 0 (assumed), μc = 0.071 ± 0.042 D, and μT = 0.320 ± 0.010 D. For the gauche conformer they are μa = 0.367 ± 0.004 D, μb ≃ 0, μc = 0.154 ± 0.006 D, and μT = 0.398 ± 0.004 D. © 1979 American Chemical Society.