Mechanism of silylene extrusion reaction in cyclic polysilane via triplet energy surface: a direct MO dynamics study

Dynamics of photoreaction of permethylcycloheptasilane (Me2Si)(7) on the triplet state surface, (Me2Si)(7) (T-1) --> (Me2Si)(6) + Me2Si (T-1), has been investigated by means of the direct MO dynamics method in order to elucidate mechanism of silylene extrusion from the Si-ring following the triplet energy transfer from a carbonyl compound. Full dimensional potential energy surface calculated at the PM3 method was employed in the dynamics calculation: the total energy and energy gradient on each atom were calculated at each time step during the reaction. The calculations showed that the seven-membered Si-ring is spontaneously changed to the six-membered ring without activation barrier on T-1 surface. The photoreaction of (Me2Si)(7) is significantly fast (reaction is completed within 1.0 ps), although heavy molecule (silylene radical) is dissociated. This is due to the fact that the T-1 surface is strongly repulsive for the coordinate between the silylene and the Si-ring. The mechanism of the silylene extraction from the Si-ring was discussed on the basis of theoretical results.