LASER-PULSE PHOTOLYSIS OF 7-SILANORBORNADIENE IN SOLUTION - EXPERIMENTAL AND AM1 STUDIES OF COMPLEXATION BETWEEN SILYLENES AND CHBR3

The quantum yield (0.95 +/- 0.1) for the decomposition of -1,4,5,6-tet raphenyl-2,3-benzo-7-silanorbornadiene (Ib) in hexane at room temperat ure has been determined by the laser pulse photolysis technique. React ion rate constants of the generated dimethylsilylene with bromoform (4 .4 x 10(7) M(-1) s(-1)) and Ib (5.7 x 10(9) M(-1) s(-1)) have been mea sured. It is suggested that the reaction of Me(2)Si: with CHBr3 occurs via the formation of an intermediate complex (lambda(max) = 338 nm, e psilon = 1280 M(-1) cm(-1)). The bimolecular rate constant for decay o f the complex (2k = 1.61 x 10(9) M(-1) s(-1)) has also been estimated. Semi-empirical PM3 calculations of the model reaction between singlet SiH2 and CHBr3 show the formation at the first step of the reaction o f a donor-acceptor complex stabilized by interactions between a vacant p-AO of SiH2 and a lone electron pair of one of the Br atoms. Such a complex should be rather stable both toward dissociation into the star ting reagents (E = 23.3 kcal/mol) and further rearrangement into the i nsertion product, Br(H-2)-SiCHBr2 (E = 22.3 kcal/mol).