Studies of 1,2-Dihalo Shifts in Carbon-Carbon, Carbon-Silicon, and Silicon-Silicon Systems: A Computational Study

Ab initio exploration of the two unusual silicon-containing transition-state structures has been carried out to determine their bonding characteristics. Each has features that are unprecedented in silicon chemistry in that a fluorine- and chlorine-substituted pentacoordinate silicon atom is found to be adjacent to either a tricoordinate carbon or silicon (structures 1 and 2 in text). The relationship between these structures and well-studied doubly halogen bridged carbon-carbon species indicates that the silicon-containing systems are quite different, with pentacoordinate bonding to silicon as well as bonding between the fluorine and chlorine atoms and the adjacent tricoordinate centers. The nature of these interactions has been explored by various methods that assess the location of electron density. These methods give complementary views of the bonding in such unusual species. A large number of doubly halogen bridged carbon-carbon transition states have been investigated to probe the nature of both size and electronic effects in their ring-opening reactions. Similarly the nature of the reactions leading to products from the silicon-containing transition states has been probed. The entire potential energy surface for the closely related dichloro-substituted carbon-silicon, silicon-silicon, and carbon carbon species has been mapped as well.