Schleyer-type hyperconjugative aromaticity in CH isomers of diazoles revealed by DFT and NBO analysis

Aromaticity is a central concept in organic chemistry, traditionally evaluated through structural, magnetic, and electronic criteria. In this study, density functional theory (DFT) calculations were performed to systematically investigate the impact of various substituents (-H, -Me, -CH₂-CH₂-, -F, -Cl, -SiH₃, -GeH₃, -NH₂, -OH, -BH₂, -CN) on the aromaticity of CH isomers of diazoles across four positional isomers (A-D). Aromaticity was quantified using the HOMED, BI, NICS(1), and NICS_zz(1) indices, while Natural Bond Orbital (NBO) analysis provided insights into charge distribution and second-order stabilization energies (E⁽²⁾). The results demonstrate that silyl and germyl substituents significantly enhance Schleyer-type hyperconjugative aromaticity, reflected by higher HOMED/BI values, more negative NICS indices, and stronger σ→π/π* delocalization energies in the NBO framework. In contrast, halogen substituents, particularly fluorine, diminish aromaticity, in some cases leading to near-zero or even positive NICS_zz values. The π-donating substituents -NH₂ and -OH exert only modest effects on aromaticity, whereas the π-accepting -CN group generally suppresses aromatic stabilization. In contrast, the -BH₂ substituent behaves as an efficient σ-donor, significantly enhancing aromaticity, comparable to those observed for -SiH₃ and -GeH₃ derivatives. Strong correlations were observed between NICS and NICS_zz, as well as between NICS and E⁽²⁾, confirming the direct relationship between ring currents and hyperconjugative stabilization. This comprehensive multicriteria approach highlights the crucial role of substituents and positional isomers in governing hyperconjugative aromaticity in CH tautomer of diazole scaffolds and provides new insights into the electronic origin of this phenomenon.