Concurrence between Current Density, Nucleus-Independent Chemical Shifts, and Aromatic Stabilization Energy: The Case of Isomeric [4]- and [5]Phenylenes

Abstract

The 17 isomers of the [4]- and [5]­phenylenes have been studied with three different computational levels of current-density analysis (CDA) and by calculation of the out-of-plane contribution to nucleus-independent chemical shifts (NICS_πzz). Current-density maps for these isomeric phenylenes are typically dominated by strong paratropic ring currents in four-membered rings. The relative energies of the isomers, which differ only through the effects of differential strain and aromaticity, were computed at the B3LYP/6-311G* computational level. It was found that the three levels of CDA correlate well among themselves and with NICS_πzz. The latter correlation is improved when the ring sum ∑NICS_πzz for each isomer is correlated to the ring-current sum ∑<i>J</i> extracted from CDA. The strain-corrected relative energies of the isomers correlate linearly with ∑NICS_πzz. In particular, the compatibility of different summed quantities with easily computed Hückel–London ring currents suggests a simply calculated measure for dealing with global aromaticity of polycyclic systems