STRUCTURE, TOPOLOGICAL ELECTRON-DENSITY ANALYSIS AND AROMATICITY OF 4-HETEROSUBSTITUTED METHYLENECYCLOPROPENES - CH2 = CCH = CH, NH = CCH = CH, O = CCH = CH, SIH2 = CCH = CH, PH = CCH = CH AND S = CCH = CH

The structures of a series of heterosubstituted methylenecyclopropenes and methylenecyclopropanes were optimized at the HF/6-31G* level. All methylenecyclopropenes are planar except for the silicon analogue, which is bent at botl C-3 and Si. The planar silicon structure is a transition state. The relative aromaticity of these compounds were evaluated using the C-1-C-3 bond length, the integrated charge on C-3 and the heteroatom, and delocalization energy. Second-row systems have slightly larger delocalization energies than their first-row counterparts, owing to their larger polarizability. Using these criteria, methylenecyclopropene and the silicon analogue are not aromatic and the N, O, P and S analogues are moderately aromatic. The planar silicon analogue is antiaromatic and bends from planarity to reduce this antiaromaticity.