Beyond NICS

Abstract

A ring current model describing the flow of delocalized electrons, induced in unsaturated molecules by a magnetic field perpendicular to the molecular plane, has been developed to define simple and practical indices of current strength, which provide a reliable measure of aromaticity on the magnetic criterion. Comparison with corresponding ab initio estimates of current strength for a series of aromatic and antiaromatic systems is used to assess the quality of the proposed indices, which are shown to be preferable to others currently used, e.g., out-of-plane components of magnetizability, ξ∥, and NICS∥. It is also shown that these quantities are both biased by spurious geometrical parameters, which limit their practicality. In particular, NICS∥ depends on the inverse of the ring radius and it does not provide acceptable magnetotropicity scales.