Ab initio study of the triple-proton-transfer reactions of ground and excited states of 7-hydroxyquinoline in methanol solution

The triple-proton-transfer reactions in the ground and first excited states of 7-hydroxyquinoline (7HQ) in methanol solution have been investigated by employing different ab initio quantum mechanical methods. The solvent effects on the structures and potential energy profiles of the reactions were included with the use of the Onsager self-consistent reaction field model. The potential energy profiles governing multiproton transfers were optimized at the HF and MP2 levels for the ground state and at the CIS and CASSCF levels for the first excited state. A conclusive description of the photoinduced proton-transfer cycle is given on the basis of the calculated potential energy profiles and transition energies, together with the experimental findings. The main features of the absorption and emission spectra concerning the ground and the first excited states of 7HQ were elucidated by analyzing the CIS and CASSCF calculated results. Electron correlation plays an important role in the description of the geometric features of 7HQ and the proton-transfer reactions in the complex, and this has been discussed in detail.