Exploring molecular structures, orbital interactions, intramolecular proton-transfer reaction kinetics, electronic transitions and complexation of 3-hydroxycoumarin species using DFT methods

• Electronic behaviors of 3-hydroxycoumarin species were theoretically examined.
• Activation energy of intramolecular proton transfer decreases upon excitation.
• Absorption of most species is mainly due to the HOMO–LUMO transition (π → π*).
• Complexation between 3-hydroxycoumarin and transition metal ions was studied.
• The most preferable complexation scheme was proposed for each type of metal ion.

Optimal structures and electronic properties of various species of 3-hydroxycoumarin (3-HCou) have been explored using density functional theory (DFT) methods under polarizable continuum model (PCM) of solvation. Electron transfer from pyrone to benzene moieties is enhanced upon deprotonation. Anionic and radical species have similar orbital-interaction characteristics but the charges in the former are distributed more uniformly. The rate of intramolecular proton transfer for the neutral species increases many folds upon excitation. The HOMO–LUMO transition with π → π* character mainly accounts for the UV absorption of most 3-HCou species in solution. The wavelengths of maximal absorption predicted using TD-DFT method are in agreement with the previous experiment. For the charged species, calculations with the range-corrected functional yield better agreement with the previous experiment. Anionic 3-HCou species shows high degrees of complexation with chromium(III) and copper(II) compared with oxovanadium(IV) and zinc(II). Either oxovanadium(IV) or zinc(II) prefers forming two isomeric complexes with comparable degrees of formation.

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