Coupling Trans-Bent Double Bonds in Tetragermabutadiene

Abstract

The coupling of two trans-bent double bonds is examined theoretically through ab initio calculations on tetragermabutadiene H2GeHGeGeHGeH2. If a trans-bent arrangement is maintained for each GeHGeH2 fragment, there are two ways of coupling two trans-bent units, starting from an s-trans conformation around the central bond. The first one preserves an all-trans arrangement of the four pyramidalized germanium atoms. Along the rotational pathway around the central bond, this configuration has no symmetry (C1), except for dihedral angles of 0° (Cs) or 180° (Ci). As in butadiene, the potential curve along this coordinate is symmetrical with respect to 0° and 180°, with a preferred s-trans form, t-1 and two equivalent gauche forms, g-1, lying about 3 kcal/mol above in energy (MP4/DZP//SCF/DZP). The s-cis saddle point separating the two gauche forms is higher in energy than the barrier separating the s-trans and gauche forms. In the second coupling scheme, the molecule maintains a C2 symmetry axis for any torsion...