Structure and isomerization of heterapentalene compounds containing hypervalent bonds with central nitrogen, phosphorus, or arsenic atom

The possibility for a specific type of isomerization (electromorphism) to occur in conjugated bicyclic organic compounds containing Group V elements was studied by the ab initio (RHF/6-31G**, MP2(full)/6-31G**), and DFT (B3LYP/6-31G**) methods. Compounds 2 (X = N, P, As) were found to exist in a monocyclic planar form with intramolecular donor-acceptor N...O coordination (X = N) and as aromatic heterapentalene structures with hypervalent O-X-O bonds (X = P, As). According to calculations, no isomerization of planar heteroaromatic structures into pyramidal ones occurs. The strength of the O-X-O hypervalent bond and the aromaticity of heterapentalene structures 2 with ten pi-electrons increase on going from X = N to X = P. Correct estimation of these effects requires the inclusion of electron correlation.