BUTADIENE .2. EXAMINATION OF THE ENERGETIC PREFERENCE FOR COPLANARITY OF DOUBLE-BONDS - COMPARISON OF BUTADIENE, ACROLEIN, AND VINYLAMINE

Experimental and theoretical studies show a 4-5 kcal/mol barrier to rotation for butadiene. What causes this barrier and what other features of butadiene change upon rotation? We have examined this question in terms of Huckel theory and Simpson's resonance force model. The latter predicts an important component from electron correlation which was not found. Huckel theory predicts significant double bond character for the central bond of planar butadiene. Butadiene and the related compounds acrolein and vinylamine were compared to the unconjugated model compounds, butane and the three butenes. Geometries, force constants, and integrated charge distributions of the conjugated compounds were found to be similar to their unconjugated 90-degrees structures and to the model compounds. Integration of the charge density at the center of the single bond for the two pi-MO's of butadiene relative to its 90-degrees isomer did reveal significant extra pi-electron density. However, the sigma-electrons were polarized in the opposite direction so as to minimize electron repulsion. The sigma-pi-interaction explains why only small changes in bond lengths or force constants are seen. The electrostatic potential maps for the three compounds also were studied and were converted to their equivalent point charge models. Electrostatic potential differences among butadiene, acrolein, and vinylamine were found to be consistent with their reactivity differences.