LASER-INDUCED CROSS-LINKING OF SILOXANES - A SEMIEMPIRICAL MOLECULAR-ORBITAL STUDY

The semi-empirical molecular orbital method AM1 has been used as a tool for the prediction of radical intermediates and crosslink structures that may be of importance in the direct laser excitation of siloxanes. Special emphasis was put on configuration interaction (CI) in order to calculate energies and geometries of excited triplet states as well as potential curves of triplet reactions. Heats of formation of intermediates and reaction enthalpies of monomolecular fragmentation reactions of T1 excited states of 2-phenyl-1,1,1,2,3,3,3-heptamethyltrisiloxane and octamethyltrisiloxane were calculated. In the former case bimolecular reactions were also considered. Dissociation of a methyl group was found to be the most probable monomolecular fragmentation. Consecutive reactions of the silyl radical formed lead to crosslink structures like Si-CH2-Si and Si-C6H4-Si. Biphenyl- and phenylcyclohexadienyls are the most probable crosslink structures of bimolecular reactions with phenyl exciplexes as intermediates. Experimental evidence is given for these crosslink structures.