The structure of cis-butadiene

Abstract

We have optimized the s-cis and gauche structures of butadiene with methods that include the effects of dynamical electron correlation. On the basis of the structural parameters and vibrational frequencies we conclude that the conjugation along the carbon chain is little affected by torsion of up to ≈40°, but that the steric hindrance between the terminal hydrogen atoms is considerably reduced. Our best results indicate that the gauche conformer is the minimum on the Born-Oppenheimer potential energy surface by 0.7 kcal/mol, and this value is remarkably insensitive to improvements in the n-particle and one-particle bases. It is shown that dynamical effects, as estimated by the zero-point vibrational correction, reduce this energy difference to 0.4 kcal/mol. Harmonic vibrational frequency analyses of the s-cis and gauche structures are reported and contrasted with experiment. © 1989.