Vibrational analyses employing cartesian coordinates

Geometry-optimised ab initio (STO-3G minimal basis set) calculations have been carried out for S-trans and orthogonal forms of 1,3-butadiene. The central C-C bond is shorter (1.482 Å) in the S-trans form than the orthogonal form (1.504 Å). Calculations on the orthogonal form indicate that the additional bond length decrease destabilizes the σ bond. the stabilization of the S-trans form being associated with conjugative (or π delocalization) effects.