Three Queries about the HOMA Index.

Jan Cz Dobrowolski

doi:10.1021/acsomega.9b02628

Abstract

HOMA (Harmonic Oscillator Model of Aromaticity) is a simple, successful, and widely used geometrical aromaticity index. However, HOMA can also be used as a general molecular descriptor appropriate for any type of molecule. It reaches the global maximum for benzene, whereas the potent magnetic aromaticity NICS index has no lower or upper limits. Hence, questions arise and go beyond mere differences between the geometric and magnetic aspects of aromaticity: (1) Does a molecule of aromaticity greater than that of benzene, but undisclosed by the HOMA definition, exist? (2) Can the Kekuléne cyclohexatriene moiety with HOMA = 0 exist as a part of a larger system? (3) Can the geometrical aromaticity index be defined better? Our answer to the first query is “It is not likely enough”, to the second, “Why not define HOMA using a less mysterious molecule than cyclohexatriene?”, and to the third, “It is possible to construct another fair geometrical index, but is it better for evaluating aromaticity?” To find these answers, we have studied: (1) the HOMA and NICS indices of over 50 hexahomosubstituted benzenes, (2) the HOMA, as well as EN and GEO, indices of over 100 triply fused hexasubstituted benzenes, and (3) the HOMA and new Geometrical Auxiliary Index (GAI) , of different unsaturated and saturated, aromatic and aliphatic hydrocarbons including all alkane constitutional isomers composed of up to nine carbon atoms.

Highlights

Modern quantum-mechanics-based computational chemistry is focused on obtaining exact data
■ RESULTS AND DISCUSSION Does an “Overaromatic” Carbon Ring Exist? It is clear that search for a molecule with the aromaticity exceeding that of benzene cannot be based on the HOMA index alone: it reaches the global maximum for benzene by definition
The NICS(1) or NICS(0) indices serve us as preselection parameters: if they indicate that the magnetic aromaticity is greater than for benzene, the system is analyzed further

Summary

■ INTRODUCTION

Modern quantum-mechanics-based computational chemistry is focused on obtaining exact data. There are plenty of organic physical chemistry (semi)empirical descriptors, such as substituent-, solvent-, reactivity-, aromaticity-, topological indices, etc., that may be helpful for directing the research and modeling of the properties of interest. It has recently been shown that HOMA can be used as a general molecular index adequate for unsaturated or saturated, cyclic or acyclic, linear or branched (hydrocarbon) molecules.[7,8]. We proposed calling this generalized property the savoricity. In subst/mol NICS(1)zz NICS(1) NICS(0) d(CC) HOMA subst/mol NICS(1)zz NICS(1) NICS(0) d(CC) HOMA

C CF OCl

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES