Mono BN-substituted analogues of naphthalene: a theoretical analysis of the effect of BN position on stability, aromaticity and frontier orbital energies

Abstract

All isomeric BN isosteres of naphthalene have been studied theoretically, at the B3LYP/6-311+G(d,p) level, in order to investigate the effect of the BN position in a molecule on relative stability, aromaticity and frontier orbital energies. The results show that the orientational isomers of m,n/n,m type have similar aromaticity, similar HOMO-LUMO gaps and similar stability, though the latter only when both heteroatoms occupy external ring positions. This latter finding is explained by an analysis of partial atomic charges in the parent hydrocarbon and final BN-compound. Racing the BN pair in one ring results in larger stability and larger HOMO-LUMO gaps than when it is placed in separate rings. The stability order of the former series is predictable on the basis of charge distribution in the nitrogen-polarized naphthalenic system and is related to partial atomic charges at the boron and nitrogen atoms in a BN-heterocycle. The HOMO-LUMO gap of the parental hydrocarbon is retained only in the 1,4-isomer, it is slightly increased in 1,2/2,1 and 5,10 isomers and more or less decreased in all other isomers. The aromaticity of a benzene unit in naphthalene is retained/increased only in a carbocycle, when it can be represented as having a pi-electronic sextet. The aromaticity of heterocyclic rings is more or less reduced relative to naphthalene and is the weakest in the rings featuring the push-pull pi-electronic structure while the BN pair resides at the external ring positions.