THE 2 CONFORMERS OF 2,3-DIGERMABUTADIENE

1,3-Butadiene and its 2,3-digerma analogue H2C=HGe-GeH=CH2 are investi gated through ab initio calculations. Geometries are determined at the SCF-DZP level, and energies are refined at the MP4 level. Substitutio n 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 w as causing the s-cis isomer of butadiene to distort into a nonplanar g auche form. Therefore, 2,3-digermabutadiene has two stable planar conf ormers, 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 b arrier from the trans isomer is calculated at 1.6 kcal/mol. Due to pla nar sigma repulsion, the coordinate corresponding to rotation around t he central Ge-Ge bond in the s-cis isomer is very flat on the potentia l surface and is associated with a very low-frequency normal mode. Var ious approaches, based on geometry or energy criteria, are used to eva luate the extent of pi conjugation in butadiene and 2,3-digermabutadie ne. They suggest that the pi-conjugation in the latter is about half t hat in butadiene.