Microwave spectrum and rotational isomerism in butene-1

Abstract

The butene-1 molecule exists in two rotational isomers, the cis and the skew forms. The rotational constants of these isomers in the ground vibrational states were found to be A = 15 302.54, B = 5 574.92, and C = 4 303.14 MHz for the cis form and A = 22 557.33, B = 4 156.28, and C = 4 056.21 MHz for the skew form. The dipole moment was obtained through the Stark effect: μ a = 0.368 ± 0.004, μ b = 0.237 ± 0.006 and μ t = 0.438 ± 0.007 D for the cis and μ a = 0.345 ± 0.007, μ b = 0.081 ± 0.011, μ c = 0.059 ± 0.016 and μ t = 0.359 ± 0.011 D for the skew form. The structures of both forms were determined from the rotational constants of the deuterated as well as of the normal species. An opening of the skeletal CCC angles of the cis form was concluded and ascribed to intramolecular repulsion between the methyl and vinyl groups. The skew forms of a number of deuterated species show an anomalous Stark effect, which makes it possible to determine the rotational constant A. The barrier hindering the internal rotation of the methyl group was found to be 3.16 kcal/mole for the skew form, whereas a remarkable increase to 3.99 kcal/mole was obtained for the cis form. This high barrier is probably due to the repulsion between the methyl and vinyl hydrogen atoms. For the skew form, the effect of tunneling through the trans barrier was observed in the rotational spectra in the excited states of the CC torsion.