THE 2 CONFORMERS OF 2,3-DIGERMABUTADIENE

1,3-Butadiene and its 2,3-digerma analogue H2C=HGe-GeH=CH2 are investigated through ab initio calculations. Geometries are determined at the SCF-DZP level, and energies are refined at the MP4 level. Substitution of carbon by germanium at positions 2,3 has two main effects. First, the central bond conjugation is significantly decreased. Second, the relatively long Ge-Ge central bond removes the steric hindrance that was causing the s-cis isomer of butadiene to distort into a nonplanar gauche form. Therefore, 2,3-digermabutadiene has two stable planar conformers, close in energy, corresponding to C-2h s-trans and C-2v s-cis forms. The s-cis isomer is unfavored by 0.4 kcal/mol. The rotational barrier from the trans isomer is calculated at 1.6 kcal/mol. Due to planar sigma repulsion, the coordinate corresponding to rotation around the central Ge-Ge bond in the s-cis isomer is very flat on the potential surface and is associated with a very low-frequency normal mode. Various approaches, based on geometry or energy criteria, are used to evaluate the extent of pi conjugation in butadiene and 2,3-digermabutadiene. They suggest that the pi-conjugation in the latter is about half that in butadiene.