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<p>Proton sponge systems exhibit exceptional basicity due to intramolecular interactions and structural constraints. In the present work, a comparative density functional theory (DFT) investigation of two symmetric proton sponges, 4,5-bis(dimethylamino)-9,10-dihydrophenanthrene (PSS) and 4,5-bis(dimethylamino)phenanthrene (PSU), differing in bridge saturation has been carried out to elucidate the influence of conjugation on structural compression and hydrogen bond strength. The optimized geometry was computed using the DFT/B3LYP method with the 6-311 + + G (2d,2p) basis set. Structural parameters reveal significant N···N compression upon protonation, accompanied by elongation of the N–H bond. Frontier molecular orbital (FMO) analysis suggests improved charge delocalization in the unsaturated derivative (PSU). Electrostatic potential (ESP) analysis reveals that negative electrostatic potential is distributed over the aromatic framework rather than localized on nitrogen atoms, indicating significant π-electron delocalization. Reduced density gradient (RDG) analysis confirms the formation of strong intramolecular N–H···N hydrogen bonding upon protonation, with PSU + H showing stronger attractive interactions consistent with its shorter N···N distance. Quantitative evaluation reveals higher stabilization energy (− 31.70&#xa0;kcal/mol) in PSU + H compared to PSS + H, confirming stronger intramolecular interaction in the conjugated system. Natural Bond Orbital (NBO) analysis highlights significant lone pair to antibonding orbital interactions, particularly LP(N) → σ*(N–H), demonstrating enhanced charge transfer in the protonated systems. Furthermore, Harmonic Oscillator Model of Aromaticity (HOMA) indices indicate that protonation increases aromatic stabilization, especially in PSS, while PSU benefits from extended conjugation. All these results demonstrate that proton sponge behavior arises from a synergistic combination of electronic delocalization, intramolecular hydrogen bonding, and structural compression, with PSU exhibiting superior stabilization due to its conjugated framework.</p> Graphical abstract <p></p>

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Role of conjugation and structural compression in proton sponge systems: A DFT study of intramolecular hydrogen bonding and charge delocalization

  • 期刊论文

  • 发表日期：

    2026年8月5日

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  • Jisna Jose,
  • Thomas V. Mathew

摘要

Proton sponge systems exhibit exceptional basicity due to intramolecular interactions and structural constraints. In the present work, a comparative density functional theory (DFT) investigation of two symmetric proton sponges, 4,5-bis(dimethylamino)-9,10-dihydrophenanthrene (PSS) and 4,5-bis(dimethylamino)phenanthrene (PSU), differing in bridge saturation has been carried out to elucidate the influence of conjugation on structural compression and hydrogen bond strength. The optimized geometry was computed using the DFT/B3LYP method with the 6-311 + + G (2d,2p) basis set. Structural parameters reveal significant N···N compression upon protonation, accompanied by elongation of the N–H bond. Frontier molecular orbital (FMO) analysis suggests improved charge delocalization in the unsaturated derivative (PSU). Electrostatic potential (ESP) analysis reveals that negative electrostatic potential is distributed over the aromatic framework rather than localized on nitrogen atoms, indicating significant π-electron delocalization. Reduced density gradient (RDG) analysis confirms the formation of strong intramolecular N–H···N hydrogen bonding upon protonation, with PSU + H showing stronger attractive interactions consistent with its shorter N···N distance. Quantitative evaluation reveals higher stabilization energy (− 31.70 kcal/mol) in PSU + H compared to PSS + H, confirming stronger intramolecular interaction in the conjugated system. Natural Bond Orbital (NBO) analysis highlights significant lone pair to antibonding orbital interactions, particularly LP(N) → σ*(N–H), demonstrating enhanced charge transfer in the protonated systems. Furthermore, Harmonic Oscillator Model of Aromaticity (HOMA) indices indicate that protonation increases aromatic stabilization, especially in PSS, while PSU benefits from extended conjugation. All these results demonstrate that proton sponge behavior arises from a synergistic combination of electronic delocalization, intramolecular hydrogen bonding, and structural compression, with PSU exhibiting superior stabilization due to its conjugated framework.

Graphical abstract