Assessment of Ring Current Models for Monocycles

作者：Monaco Guglielmo^*; Zanasi Riccardo

来源：Journal of Physical Chemistry A, 2014, 118(9): 1673-1683.

DOI：10.1021/jp411194m

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 it and a contributions to the ab initio ring current strengths and to the scans of the parallel component of the magnetic shielding (sigma(parallel to) = NICS parallel to) for a set of 33 organic and inorganic monocycles. For pi 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 sigma currents, the best model is the novel ICLOC2C, formed by two concentric ICLOCs. An off-plane extremum in the NICS parallel to scan is not a general indication for the presence or absence of pi-aromaticity. In agreement with the models proposed here, such an extremum is absent in the NICS parallel to,pi scans of large rings; however, it characterizes most of the NICS parallel to,pi scans of small rings and can also appear in NICS parallel to,sigma scans.

• 出版日期2014-3-6