Proton Changes the Rules of the Game: Insight into the Nature of "Proton Sponges"

There are cases when a small change, such as capture of a simple particle, has a significant impact on the molecular properties. This indeed happens with the "proton sponges". It is shown how the substituent effect (SE) and the presence of an intramolecular hydrogen bond modulate the properties of 1,8-bis(dimethylamino)naphthalene (DMAN), its protonated form (DMANH⁺) and their 4-X substituted derivatives, based on the geometric, electronic structure and energetic parameters. For this purpose, the following substituents have been chosen: X=H, NO, NO₂, CN, CHO, Cl, F, CH₃, OCH₃, OH and NH₂, placed in the para position (C4 carbon atom) relative to the N(CH₃)₂ group, thus breaking the symmetry of the molecules. The study was performed using Density Functional Theory (DFT) at the B3LYP/6-311++G(d,p) level of theory. The Harmonic Oscillator Model of Aromaticity (HOMA) index was applied to study changes in the aromaticity of the naphthalene moiety. The strength of the intramolecular hydrogen bond (HB) was estimated by Espinosa-Molins-Lecomte (EML) equation. The Quantum Theory of Atoms in Molecules (QTAIM) and Interaction Region Indicator (IRI) were employed to investigate differences in electronic structure and non-covalent interactions caused by substitution. Finally, the Substituent Effect Stabilization Energy (SESE) and Charge of the Substituent Active Region (cSAR) were computed to evaluate the substituent effect characteristics. Natural Bond Orbital (NBO) analysis was carried out to evaluate the SE influence on the lone pair of nitrogen atoms (N1, N2). We have shown that the presence of a strong intramolecular HB significantly weakens the substituent effect. For electron-donating (ED) substituents, the SE effect was found to have a greater influence than hydrogen bonding in the DMANH⁺ series. The opposite result was obtained for electron-withdrawing (EW) substituents - the SE was estimated to be twice weaker than in the ED case.