THE MOLECULAR-STRUCTURE OF A GERMACYCLOBUTANE BY GAS ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The molecular structure of 1,1,3,3-tetramethylgermacyclobutane has bee n determined by gas electron diffraction supplemented by ab initio cal culations at the HF/6-31G and MP2/6-31G* levels. Structure refinement of a C-S model with the differences between symmetry inequivalent Ge- C and C-C bond distances fixed at the HF values yielded the bond dista nces Ge-C = 197.5(3) and C-C = 156.7(2)pm (endocyclic) and Ge-C = 195. 9(3) and C-C = 152.9(2)pm (exocyclic). The endocyclic valence angles a re angle CGeC = 75.3(2)degrees, angle GeCC = 89.5(2)degrees and angle CCC = 100.7(3)degrees. The exocyclic angle CGeC = 108.8(8)degrees. The central ring is non-planar with a puckering angle of 24(2)degrees, th e barrier to planarity is estimated to be V-O= 4.2(8) kJ mol(-1). Opti misation of the structure of 1,1-dimethyl- germacyclopropane at the HF /6-31G level yields angle CGeC valence angles of 47.5 degrees (endocy clic) and 116.2 degrees (exocyclic). (C) 1997 Elsevier Science S.A.