Theoretical group 14 chemistry, Part 3. A DFT study of Ge4R6

The application of the newly developed all-electron 6-31G(d) basis set, which proved to be much more reliable than previous implementations, to different isomers of Ge4R6 (R=H, Me, Ph, 2,6-dimethylphenyl) reveals a significant dependency of their relative energies and structures on the substituents R. The initially most stable 'bicyclic' structure 1 becomes less favorable with sterically increased demand of R. The energy gap between the acyclic structures of cis- and trans-1,3-tetragermabutadiene 3 and the minimum is significantly reduced and is expected to reverse with even bulkier groups and therefore corresponds well with the experimental results. At the level of theory used herein, no bond stretch isomerism for bicyclo[1.1.0]tetragermanes can be observed, only the long bond isomer 1lb is found. Although the behavior of the Ge4R6 isomers is comparable to that of the silicon counterparts, it becomes apparent that the germanium structures are more flexible and less susceptible to steric strain.