Three Queries about the HOMA Index

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.

Substituent Effect in the Cation Radicals of Monosubstituted Benzenes

“…Indeed, the HOMA (Harmonic Oscillator Model of Aromaticity) geometrical aromaticity index [129,130] of monosubstituted and meta and para homo-disubstituted benzenes varies little with the substituent and is only slightly lower than 1 [131][132][133][134]. Nevertheless, the effect of six identical substituents can be significant [135]. In the monosubstituted benzenes in the first excited singlet S 1 state, the ring is at most slightly distorted [73].…”

Geometrical, electrical, and energetic parameters of hetero-disubstituted cumulenes and polyynes in the presence and absence of the external electric field

“…On the one hand, the longer the path, the longer the "charge" trip, and on the other hand, the higher the alternation (variance of the bond lengths), the higher the barriers during the trip and the harder the communication between the C 1 and C 10 atoms. However, the Harmonic Oscillator Model of Aromaticity (HOMA) descriptor [45,46] combines the size of the system and the bond alternation in one value, which was shown to be useful for the comparison of also acyclic, linear, or even branched structures [28,47,48]. For the unsaturated polymers such as polyenes or polythiophenes, the HOMA index seems to be a fair measure of the π-electron delocalization and thus to be a good simple tool for pre-evaluation of conductivity [28].…”

On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes

“…Magnetic index of aromaticity NICS [7,9] was used to determine the aromaticity of the studied tautomers and monohalogenoderivatives of benzene as reference. In addition, geometric indexes of aromaticity, HOMA [12][13][14] and HOMHED [35], were calculated. NBO charges [36] were calculated at optimized geometries.…”

“…The number of delocalized electrons could be determined according to Hückel's formula 4n + 2, where n = 1 for benzene. There are two commonly used theoretical indexes of aromaticity, the geometric one (HOMA, and closely related parameters, derived from structural dimensions [11][12][13][14]) and magnetic [7,9,15,16] (NICS). For benzene, HOMA is close to 1.…”