A Critical Assessment of the Performance of Magnetic and Electronic Indices of Aromaticity

Miquel Solà,Ferran Feixas,Jordi Poater,Eduard Matito,J Oscar C Jiménez-Halla

doi:10.3390/sym2021156

Miquel Solà, Ferran Feixas + Show 3 more

Open Access

https://doi.org/10.3390/sym2021156

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Abstract

The lack of reference aromatic systems in the realm of inorganic aromatic compounds makes the evaluation of aromaticity in all-metal and semimetal clusters a difficult task. To date, calculation of nucleus-independent chemical shifts (NICS) has been the most widely used method to discuss aromaticity in these systems. In the first part of this work, we briefly review our previous studies, showing some pitfalls of the NICS indicator of aromaticity in organic molecules. Then, we refer to our study on the performance of some aromaticity indices in a series of 15 aromaticity tests, which can be used to analyze the advantages and drawbacks of aromaticity descriptors. It is shown that indices based on the study of electron delocalization are the most accurate among those analyzed in the series of proposed tests, while NICS(1)zz and NICS(0)πzz present the best behavior among NICS indices. In the second part, we discuss the use of NICS and electronic multicenter indices (MCI) in inorganic clusters. In particular, we evaluate the aromaticity of two series of all-metal and semimetal clusters with predictable aromaticity trends by means of NICS and MCI. Results show that the expected trends are generally better reproduced by MCI than NICS. It is concluded that NICS(0)π and NICS(0)πzz are the kind of NICS that perform the best among the different NICS indices analyzed for the studied series of inorganic compounds.

Highlights

The concept of aromaticity was introduced to rationalize the structure, stability, and reactivity of benzene and related organic compounds
In the work of Reference [75], where the para-delocalization index (PDI) index was introduced as a new electronic aromaticity criterion, we found that for some six-membered rings (6-MRs) there was a bad correlation between nucleus-independent chemical shifts (NICS) and PDI values
Our results indicate that the multicenter indices perform generally better than NICS, especially the

Summary

Introduction

The concept of aromaticity was introduced to rationalize the structure, stability, and reactivity of benzene and related organic compounds. The concept of aromaticity can be used to explain structure, stability, and reactivity of classical organic compounds and of many organometallic and inorganic species, in particular, of all-metal and semimetal clusters. An in-depth analysis of all-metal and semimetal clusters requires evaluation of their aromaticity This evaluation is not unique since a plethora of different measures based on structural- [23,24], magnetic- [25], energetic- [26], and electronic-based [27,28] indices can be used to quantify aromaticity of a given set of compounds. The most widely employed method to analyze the aromaticity of all-metal and semimetal species is the NICS index [18,56,57,58,59,60,61] This magnetic-based descriptor of aromaticity was introduced by Schleyer and co-workers [45]. This feature is especially interesting to evaluate multifold aromaticity in all-metal clusters

Some Reported NICS Drawbacks

A Critical Assessment of Aromaticity Indices Using a Test Set

NICS Profiles as Aromaticity Criteria in Inorganic Clusters

Findings

Conclusions