DFT insights into proton–π interactions and their role in enhancing the basicity of benzo[h]quinoline amines

In this study, the nature and strength of intramolecular proton–π (H⁺…π) interactions in a series of benzo [h]quinoline derivatives bearing an aryl group at carbon-6 were investigated using quantum chemical methods. The primary goal was to clarify the role of these interactions in stabilizing conjugate acids and their effects on the aromaticity of the involved rings. Density Functional Theory calculations were performed at the B3LYP/6–311 + G(d,p) level in the gas phase and solvent environments using the CPCM model. Thermodynamic analysis showed that proton affinity and gas-phase basicity significantly increased in the presence of electron-donating substituents on the aryl ring, such as amino group with proton affinity exceeding 1000 kJ/mol for compound 5. In contrast, electron-withdrawing groups (e.g., nitro group) or heteroaromatic rings with low electron density notably decreased these values. Topological analysis based on the Atoms in Molecules framework confirmed bond critical points between the proton and aromatic carbon atoms, with electron density ρ(r) and potential energy density V(r) values indicating medium to strong H⁺…π interactions (20–26 kJ/mol). The strongest interactions were observed in the conjugate acids [4 + H]⁺ and [5 + H]⁺. Aromaticity indices-including HOMA, BI, PLR, PDI, and SA-consistently demonstrated a significant decrease in aromaticity and increased ring distortion upon protonation compared to neutral species. Finally, solvent effects showed that protonation tendency and conjugate acid stability increased in polar solvents such as water, while changes were less pronounced in nonpolar media. Overall, these results provide strong evidence for the presence and importance of intramolecular H⁺…π interactions in aromatic systems, highlighting their crucial role in conjugate acid stabilization, aromaticity reduction, and electronic density redistribution.