Due to the different strengths of the GedC and CdC π bonds, 1,4-digermabutadiene H 2 GedHCsCHdGeH 2 is expected to exhibit stronger conjugation through its central C-C bond than does butadiene. Ab initio calculations are used to investigate this effect. Like butadiene, 1,4-digermabutadiene has a preferred planar s-trans form, with two equivalent gauche conformers lying 3 kcal/mol higher in energy. As in butadiene, the s-cis planar form is a saddle point relating the two gauche forms, corresponding to a barrier of 1 kcal/mol. The perpendicular form associated with the rotational barrier around the central C-C bond is found to be significantly higher in the 1,4-digerma derivative than in butadiene (10 vs 6 kcal/mol). Other indicators, based on geometry or energy criteria, actually suggest that π-conjugation in 1,4-digermabutadiene is about one-and-a-half that in butadiene. The ensuing enhanced weight of valence-bond forms reflecting electron arrangements like • H 2 GesHC)CHsGeH 2 • is expected to favor 1,4-couplings, such as the intramolecular cyclization into 3,4-digermacyclobutene. In its conrotatory thermally-allowed pathway, this rearrangement is found to require a weak activation barrier of about 1 kcal/mol above the starting gauche form. The strong exothermicity of such an internal ring closure, calculated at 44 kcal/mol from the gauche conformer, is due to bond energies and geometrical features in the final four-membered frame, in contrast with the endothermic butadiene-to-cyclobutene rearrangement. The 1,4-diradical character of 1,4-digermabutadiene is further attested by a weak torsional barrier around the GedC bond, estimated at less than 5 kcal/mol, and a low-lying triplet state, located at only 6 kcal/mol above the closed-shell singlet ground state. Figure 7. Energy profiles along CdX (left) and central C-C (right) torsional coordinates for 1,4-digermabutadiene (full curve) and butadiene (dashed curve).
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