Nucleus-Independent Chemical Shifts (NICS):  Distance Dependence and Revised Criteria for Aromaticity and Antiaromaticity

Abstract

Nucleus-independent chemical shifts (NICS) have been used extensively for the identification of aromaticity properties of molecules, ions, intermediates, and transition states since their introduction in 1996 by Schleyer et al. Initially, probes (bq's) were placed at the centers of systems (NICS(0)) and later, 1A above the molecular planes (NICS(1)). However, contradicting assignments of aromaticity by NICS and other methods were found for some systems. In this article, an alternative NICS-based method is introduced. The method is based on scanning NICS values over a distance and separating them into in-plane and out-of plane contributions. The shapes of the plots of the chemical shifts and their components as a function of the distance of the NICS probe (bq) from the molecular plane give a clear indication of diamagnetic and paramagnetic ring currents. This method is applied to several (4n + 2)- and 4n pi-electron systems (molecules and ions) in the singlet and triplet electronic states, including some of the problematic systems mentioned above. It is also shown that relative aromaticities of rings in polycyclic systems (local aromaticities) cannot be estimated by comparing NICS or NICS-scan values.

Stabilisation of tetrabromo- and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals

“…The NICS profiles for neutral quinone species are typical for nonaromatic molecules, 45 while those for hydroquinone dianions are typical for aromatic species, although notably less aromatic than the prototype benzene. The profiles obtained for semiquinone molecules are halfway between these typical cases, as one could expect on the basis of the resonance structures in Scheme 1.…”

“…Aromaticity of tetrahaolgenosemiquinone radical anions was assessed by the component of a shielding tensor 45 (nucleus independent chemical shift, NICS) that is orthogonal to the ring plane calculated along the axis perpendicular to the semiquinone ring and passing through its centroid (Fig. 4).…”

“…Both quantities are commonly used as quantitative aromaticity indicators. 47,48 The aromaticity indicators were calculated for the neutral quinone, semiquinone radical anion and hydroquinone dianion both with the hydrogen atoms on the quinone ring left unsubstituted and substituted by Cl and Br.The NICS profiles for neutral quinone species are typical for nonaromatic molecules, 45 while those for hydroquinone dianions are typical for aromatic species, although notably less aromatic than the prototype benzene. The profiles obtained for semiquinone molecules are halfway between these typical cases, as one could expect on the basis of the resonance structures in Scheme 1.…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…Following suggestions by Pauling and Londsdale, London proposed in 1937 a very appealing model that accounts for these effects in terms of ring currents that are induced by the external magnetic field in the system of cyclically delocalized p electrons. [5] There has been a vigorous debate in recent years about whether and to what extent these ring currents contribute to the deshielding or shielding of protons attached to aromatic or antiaromatic rings, respectively, [6][7][8] as manifested in 1 H NMR spectra. At the focus of this debate stand the "nuclear independent chemical shifts" (NICS values) [9] that can be computed at any point in the space surrounding a molecule.…”

Substituent Effects on the Energetics and Aromaticity of Aminomethylbenzoic Acids

“…For all of the studied compounds, the minimum NICS value occurs at a distance from the ring of about +0.8 and -0.8 Å, rather than at about 1 Å, as is the case for many aromatic systems. 6,25 Because NICS(1) has become a standard for the reported NICS data, NICS(1) will be used from this point onward.…”