False Identification of (Anti)aromaticity in Polycyclic Molecules in Ground and Excited States Through Incorrect Use of NICS

Abstract

Aromaticity is a key concept in physical organic chemistry. However, as it cannot be measured directly, it is assessed indirectly via other properties (energetic, electronic, geometric and magnetic). Although these properties describe aromaticity, they are not solely related to aromaticity as the observed values also can stem from, for example, magnetically induced local currents at certain atoms or groups, or strain in the σ-skeleton. This can lead to misinterpretations. Here, we highlight a pitfall in the (anti)aromaticity assessment of polycyclic molecules when it is mainly based on nucleus independent chemical shifts (NICSs). The NICS index can be misinterpreted to indicate ‘aromaticity’ or ‘antiaromaticity’ in nonaromatic rings as a result of paratropic or diatropic ring currents in adjacent rings. We explore if such false indications by NICS are (i) stronger in Baird-aromatic or -antiaromatic excited states (mainly triplet and quintet, but also singlet) than in closed-shell singlet ground states, and (ii) if a paratropic ring current in an adjacent ring causes stronger or weaker false ‘aromaticity’ than a diatropic one causes false ‘antiaromaticity’. Based on our computations we conclude that larger aromatic rings in all types of states (e.g., a triplet state Baird-aromatic cyclooctatetraene ring) have greater influence than smaller ones, yet, we see no indication that the effect is stronger in excited states. Instead, annulene rings are more influential in their paratropic (antiaromatic) states, regardless if ground or excited states, than in their diatropic (aromatic) ones.