Electronic Structure of Thiirene and Silacyclopropene Substituted with Electropositive Groups

Abstract

It has been revealed by ab initio calculations at various levels that electropositive substituents (Li, BeH, and BH2) on silacyclopropene do not form bridge bond over the ring, as in the case of 1-BH2-phosphirene. Although in 1-lithiumsilacyclopropene Li occupies a position on the top of the ring, this structure, however, should be more appropriately described as a silacyclopropene anion Li+ ion pair, as shown by the MOs and Wiberg indices. For the various substituted thiirenes studied (with all combinations of the substituents Li, BeH, and BH2), instead of having a hypervalent sulfur, bridge bond structures (over the CC bond) are formed in all cases. For BeH,BH2-thiirene, the two substituents might position “above” or “behind” the ring, resulting in three different stable structures, and the most stable one has BeH forming a cage structure with the SCC ring. This bridge bond structure is similar to that in 1-BH2-phosphirene. Except for the case of having two BH2 groups, this cage structure has been found for all other substituent combinations. For this structure with two different substituent groups, the isomer with the more electropositive substituent group on the top of the ring is more stable. The bridge bond could be characterized by the participating AOs in both the highest doubly occupied a‘ and a‘‘ orbitals. Unlike in the case of 1-BH2-phosphirene, where dynamic electron correlation was needed to stabilize the bridge bond structure, the MO interaction alone was shown to be enough for the stabilization of this structure for the substituted thiirenes considered in this work.