A Computational Review for Substituent Effect on the Fused Pyrrole Remote Hammick N-Heterocyclic Germylenes (RHNHGes) via the Adsorption Process to NCBC17 Heterofullerenic Isomers, at DFT

This study presents a comprehensive density functional theory (DFT) investigation into the substituent effects on the fused pyrrole remote Hammick N-heterocyclic germylenes (RHNHGes), focusing on their singlet and triplet states and their interactions with NCBC₁₇ heterofullerenic isomers. The research examines a series of benzogermapyridine-4-ylidene derivatives (I_s and I_t) and fused furan analogues (II_X-s, and II_X-t, X = CH₂, SiH₂, GeH₂, NH, PH, AsH, O, S, and Se), evaluating their thermodynamic and kinetic stability, electronic properties, and reactivity descriptors. The greatest stabilization is supported by the NH substituent, and the least stabilization is provided by Se in terms of singlet-triplet energy differences (ΔE_s-t = E_t–E_s) exhibiting the highest ΔE_s-t of 57.78 kcal/mol and the lowest ΔE_s-t of 46.60 kcal/mol, correspondingly, and suggesting singlet II_NH and II_Se as the most and least stable species. All singlet RHNHGes are established as ground state (GS) with positive ΔE_s-t, higher band gaps, and greater thermodynamic and kinetic stability compared to their triplet counterparts. In addition, singlet II_NH and II_Se are demonstrated the polarity of 1.78 and 2.78 Debye; polarizability of 114.68 and 130.50 a.u.; and the smallest vibrational frequency (υ_min) of 229.19 and 182.97 cm⁻¹, respectively. Nemirowski and Schreiner realized that the classical π-donor/σ-acceptor amino substituent simultaneously stabilizes singlet NHCs and destabilizes the corresponding triplet ones. In contrast to this statement, in this work, all substituents were stabilized not only singlet RHNHGes but also triplet RHNHGes. Substituent effects are rationalized by electronegativity and π-electron delocalization involving the germanic center (GC), mediated through inductive and mesomeric effects. The detailed analyses reveal that the singlet RHNHGes possess more charge, lower nucleophilicity, higher electrophilicity, more chemical potential, more hardness, and less softness than its triplet state. Stabilization is anticipated dependent on the substituent's electronegativity in the fused pyrrole ring and π-electron delocalization with the empty 4p-orbital of the GC through either inductive effect or mesomeric effect. Adsorption studies with NCBC₁₇ heterofullerenic isomers further elucidate how substituent identity modulates RHNHGe stability and interaction energies. These findings provide valuable insights into the design and stabilization of novel divalent germylene species, with implications for their reactivity and potential applications in organometallic and materials chemistry.