Mechanism of Photodegradation of Polysilanes: A Relaxed Cross Section of the Conical Intersection Hyperline in 2-MethylTrisilane

The potential energy surface of the S-1 state of 2-methyltrisilane has been, studied via MC-SCF and multireferenee MP2 methods. Remarkably, no conventional stationary points appear to exist on this surface. All attempts to minimize the energy with respect to all geometrical coordinates led to a region where the excited and ground states conically intersect and very fast radiationless decay becomes possible. The lowest energy point on the (n - 2)-dimensional conical intersection was optimized and lies at a geometry where the system is fragmented. For this reason, a relaxed cross section. of the (n - 2)-dimensional intersection space was computed between the Franck-Condon region and a point on the conical intersection at a fragmented geometry. The various photodegradation products observed after irradiation of short permethylated silanes can be tentatively rationalized on the basis of the electronic and molecular structures of the points on this relaxed cross section.