The second stable conformer of 1,3-butadiene. Geometry optimizations with configuration interaction and coupled cluster methods

Abstract

The structures of s-cis and gauche butadiene have been determined using methods that include the effects of dynamical electron correlation. Optimized geometries, energies and harmonic vibrational frequencies are presented and discussed at the various levels of theory employed. The information yielded by the highest theoretical level used leads to a prediction that the gauche conformer is a minimum on the potential energy surface, lying 0.8 kcal/mol below the s-cis conformer which is predicted to be a transition state. This barrier to planarity is reduced to 0.5 kcal/mol with inclusion of zero-point vibrational energy effects. It is postulated that the gauche structure is favoured since the reduction in steric interactions in this conformation overcomes any loss of conjugation relative to the planar structure. Isoprene and dimethyl-butadiene have also been shown to display these effects in a more pronounced manner.