Abstract
The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono- and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index.
Highlights
The geometries of molecules, as well as their energetic and magnetic properties, provide principal information about electron distribution, that related to π -electron delocalization
The geometry-based HOMA (Harmonic Oscillator Model of Aromaticity) index was defined more than 30 years ago to describe quantitatively π-electron delocalization in homo- and heteroaromatic systems [20,21,22,23]
We reviewed the principal changes proposed for the reformulated HOMA index, which led to the unexpected values for π-electron systems containing heteroatoms
Summary
The geometries of molecules, as well as their energetic and magnetic properties, provide principal information about electron distribution, that related to π -electron delocalization. The negative rHOMA values have been observed for some tautomeric π-electron systems [18,26,27,28,29,30] All these discrepancies encouraged us to re-examine the geometry-based HOMA index. In some cases (e.g., tautomeric systems), experimental determinations may be difficult or even impossible For these reasons, we chose the quantum-chemical methods to estimate the bond lengths for all compound considered in this paper, for the reference molecules as well as for different π-electron delocalized acyclic and cyclic compounds containing the C, N, and/or O atoms. The use of the same level of theory for the reference molecules and for π -electron heteroatomic systems has this advantage that computational errors may cancel out in the procedure of the HOMED estimation We confirmed this cancellation for simple molecules using various quantum-chemical methods and various basis sets. We analyzed π-electron delocalization for simple σ-π hyperconjugated, n- π conjugated, π - π conjugated, and aromatic compounds including those displaying prototropic tautomerism
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