The microwave spectrum, dipole moment, and barrier to internal rotation of 3-methyl-2-butenenitrile

The microwave spectrum of 3-methyl-2-butenenitrile was investigated in the frequency range of 18.0 to 39.0 GHz, and the spectrum which has been obtained is characteristic of that of a nearly prolate rotor. Only a-type transitions were observed and R-branch assignments have been made for the ground vibrational state, as well as three vibrationally excited states. The rotational constants in the ground vibrational state were determined to be: A=8379.36±0.66, B=2184.87±0.01, and C=1769.96±0.01 MHz. With seven reasonably assumed structural parameters, a diagnostic least-squares adjustment was used to fit the three rotational constants, and the five remaining structural parameters were obtained: cis, r(C–CH3)=1.503±0.024 Å; trans, r(C–CH3)=1.504±0.029 Å; cis, ∢C=C–CH3=120.6±0.1°; trans, ∢C=C–CH3=120.3±0.1°; and ∢C=C–CN=124.4±0.1°. The dipole moment components were determined from the Stark effect to be ‖μa‖=4.59±0.11 D, ‖μb‖=0.40±0.07 D, and ‖μt‖=4.61±0.13 D. Splittings due to the coupling of internal rotation of the cis methyl group with the overall rotation of the molecule were observed in one of the vibrational satellites, and a subsequent analysis yielded a barrier of 790 cm−1 (2.26 kcal/mol) for the cis methyl group. All of these results are compared to similar quantities in some corresponding molecules.