Assessment of Ring Current Models for Monocycles
journal contributionposted on 06.03.2014, 00:00 by Guglielmo Monaco, Riccardo Zanasi
Interpretation of both measured and computed values of chemical shift in (poly)cyclic molecules is widely based on ring current models (RCMs). Few improvements have been considered to date for the reference RCM, consisting of an infinitely thin circular loop of current (ICLOC). In this paper, six analytical RCMs (three of which are proposed for the first time) have been discussed, and they have been graded by comparing their ability to reproduce the π and σ contributions to the ab initio ring current strengths and to the scans of the parallel component of the magnetic shielding (σ∥ = −NICS∥) for a set of 33 organic and inorganic monocycles. For π currents, two vertically displaced ICLOCs (ICLOC2 model) are the preferred choice to have a very good reproduction of the scans, while the toroidal widening of the loops, proposed long ago by Farnum and Wilcox, is unable to give a significant improvement. For σ currents, the best model is the novel ICLOC2C, formed by two concentric ICLOCs. An off-plane extremum in the NICS∥ scan is not a general indication for the presence or absence of π-aromaticity. In agreement with the models proposed here, such an extremum is absent in the NICS∥,π scans of large rings; however, it characterizes most of the NICS∥,π scans of small rings and can also appear in NICS∥,σ scans.