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

Abstract

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.

Is (benzene)Cr(CO)3 really more aromatic than benzene?

“…16 It is interesting to note that isotropic NICS criteria (NICS(0), NICS(1) and NICS(0) π ) found (CO) 3 Cr(benzene) to be more aromatic than benzene. 17 Figures 5 and 6 explain these findings. Isotropic NICS values contain the σ electrons contribution (irrelevant to aromaticity) which, in this case, musk the effect of the π electrons.…”

Quantifying aromaticity with electron delocalisation measures

“…In 2008, we proposed to extend this idea to the field of aromaticity suggesting the use of a test set as a tool to assess the performance of aromaticity descriptors. 50,139 To this end, we proposed a series of tests with well-known trends that can be used to appraise the performance of existing and new indices of aromaticity (see Fig. 1).…”

“…Multicentre electronic delocalisation indices and NICS are among the few indicators that fulfil these two requirements for rings of arbitrary size. 139,154 The new aromaticity test consists of the 4-MR series of valence isoelectronic species [XnY4-n] q± (X, Y = Al, Ga, Si, and Ge; n = 0-4), which should exhibit particular aromaticity trends. 154 Let us focus on the series from Al4 2-to Ge4 2+ ; similar trends are expected for the other series.…”

On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

“…Moreover, the BV(ELF  ) of the peripheral 14-electron loop (0.304) has increased compared to 14, revealing some -electron pair localization to certain bonds, these being the CC bonds in the 1,3-butadiene segments of the terminal hexagons. The features observed in 1 and 15 are naturally found in tetracene (17) and pentacene (21). Noteworthy, as the [n]acenes become longer the BV(ELF  ) of the CC bonds between hexagons become gradually lower, and it is thus questionable if any of the internal hexagons can be considered as aromatic.…”

“…Many different methods to assess the degree of aromaticity exist [19], and recently, Solà and coworkers concluded that aromaticity indices connected to electron delocalization properties are more revealing than those based on geometric, energetic, or magnetic properties [20,21]. Topological analyses of the electron localization function (ELF) [22], and in particular the -contribution, ELF  , can be used to directly link molecular electronic structure properties with (anti)aromaticity.…”