Hydrogen-bonded channel-dependent mechanism of long-range proton transfer in the excited-state tautomerization of 7-hydroxyquinoline: a theoretical study

The first excited-state proton transfer (ESPT) in 7-hydroxyquinoline (7HQ) mediated by water and methanol molecules in a nonpolar solution was studied by quantum mechanical calculations. The multiple proton transfer through a hydrogen-bonded channel consisting of two or three hydroxyl molecules occurs in a concerted and asynchronous pathway. When the H-bonded channel molecules are water or methanol, proton transfer takes place in a solvolytic or protolytic fashion, respectively. The barrier height of the ESPT depends on the basicity of the H-bonded channel. The predicted barrier height is 2.97 kcal/mol lower with two CH3OH than with two H2O since monomeric methanol is more basic than monomeric water in heptane and DPE. The ESPT barrier heights are 3–4 kcal/mol lower with three H-bonded molecules than with two. The rate constants and KIEs for the ESPT of 7HQ-(ROH)2 (ROH = H2O, CH3OH) were calculated and compared to the experimental values.