Quantum-chemical investigation of structure and valence isomerizations of perdehydrotetrasilacyclobutadiene

The structure and valence isomerizations of perdehydrotetrasilacyclobutadiene (Si4) are discussed on the basis of concepts of cyclic π- and σ-conjugation. Calculations by the MINDO/3 and MNDO methods established two types of geometric configurations with D/sub 4h/ symmetry for both the singlet and triplet states of perdehydrotetrasilacyclobutadiene, differing in inversion of the HOMO and LUMO. Neither D/sub 4h/ configuration of the singlet state corresponds to minima on the potential energy surface, and these configurations are distorted, one toward a rectangular structure with D/sub 2h/ symmetry with alternation of bond lengths in the ring, and the other toward a rhombic bicyclic structure with D/sub 2h/ symmetry. The latter structure corresponds to the global minimum on the potential energy surface of the Si4