GENERALIZED VALENCE-BOND STUDY OF ROTATIONAL SINGLET STRUCTURES AND PI-BOND-ENERGIES FOR SYSTEMS CONTAINING C=C, SI=SI, AND C=SI DOUBLE BONDS

Ab initio GVB(6/12)/6-31G* calculations were performed on A(2)X=YB2 ( A, B = Il, F; X, Y = C, Si) to obtain the optimized geometries for pla nar and twisted singlet structures, and to also calculate pi bond ener gies (rotational barriers). The nature of C-C, Si-Si, and C-Si pi bond s has been investigated. The results show that the C-C pi bond energy (E-pi(ethene) = 65.4 kcal/mol) decreases with increasing fluorine subs titution. The pyramidalization at the carbon or silicon center for the twisted structures decreases the pi bond energies in the substituted ethenes and their silicon counterparts. The Si-Si (E-pi(disilene) = 23 .2 kcal/mol) and C-Si (E-pi(silaethene) = 31.6 kcal/mol) pi bonds beco me much weaker. Fluorine substitution stabilizes both the diradical an d the dipolar twisted singlet structures.