A multi-component density functional study on quantum effects of hydrogen nuclei on ground-state and excited-state proton transfer reactions in 7-hydroxyquinoline

The proton transfer (PT) reaction in 7-hydroxyquinoline (7-HQ), mediated by three methanol molecules, has been investigated using time-dependent density functional theory (TD-DFT) and multi-component DFT (MC_TD-DFT) calculations, which can incorporate nuclear quantum effects (NQEs) of protons and deuterons. The NQEs were found to induce the geometrical changes in both the ground-state and excited-state, and alter orbital energies, affecting the HOMO–LUMO energy gap and absorption and fluorescence properties. The MC_DFT calculations predict a Stokes shift of 217 nm, closer to the experimental value (180–200 nm) compared to the conventional DFT (242 nm). For 7-HQ, the NQEs induced red shifts in absorption peaks and blue shifts in fluorescence peaks, aligning the Stokes shift more closely with experimental data. In addition, the MC_DFT calculations revealed that geometrical relaxation induced by the NQEs can be attributed to the shifts in the peaks in the case of 7-HQ. This study highlights the critical role of NQEs in understanding PT mechanisms, absorption and fluorescence properties, and H/D isotope effects, demonstrating the importance of including NQEs for accurate theoretical modeling.