Bond stretch isomerism in rhombic C₂Si₂

The differences in bonding and electronic properties between the MP/6-31G* optimized disilicon dicarbide structures 1 and 2 can be related to bond stretch isomerism, which occurs through a level-crossing mechanism. Their energy differences are calculated at the MP4 and GVB/6-31G* levels. Electron density analysis indicates concentration of charge in the center of rhombic structure 1 with some multiple bonding character between its transannular carbons. In contrast, the higher energy isomer 2 has a depletion of charge in the center relative to the circumference of the rhombic structure.