Carbon-carbon rotational barriers in butane, 1-butene, and 1,3-butadiene

The rotational barriers for n-butane, 1-butene, and 1,3-butadiene were calculated at the G2 and CBS-Q theoretical levels. The thermodynamic functions were obtained with explicit calculation of the effect of the rotational modes. The G2 difference in energy between the syn and anti forms of butane at 298 K is 5.1 kcal/mol, which is significantly larger than the experimental estimate. However, it is shown that a reliable experimental estimate of the barrier cannot be made based on the currently available data. The structural changes on rotation are examined and are related to the changes found for C-C bond relation in ethane. The G2 model reproduces the observed relative energies for both 1-butene and 1,3-butadiene within the experimental uncertainties, and the CBS-Q model also gives generally satisfactory results.