Theoretical Study Of Solvent Effects On Tautomerism And Electronic Absorption Spectra Of Two Pyridinol Derivatives

Tautomeric equilibria reaction is one of the most fundamental chemical reactions. Such reaction is usually occurred in solution. Solvent effects on solute electronic structure and their spectroscopic signature are important for solution chemistry. Keto-enol or thione-thiol tautomeric equilibria of pyridinol derivatives have attracted great experimental and theoretical interests due to their significant importance to chemistry and biochemistry. In this work, we have studies two important pyridinol deviatives: 3-hydroxy-2-mercaptopyridine (HMP) and 2,3-dihydroxypyridine (DHP). The tautomeric equilibria of 3-hydroxy-2-mercaptopyridine (HMP) and 2,3-dihydroxypyridine (DHP) in vacuum and in ethanol solution have been studied using the density functional theory (DFT) at B3LYP/6-31G(d) level. Solvent effects were modeled by using the PCM method. The results indicate that the thione form of HMP and the keto form of DHP are the most stable tautomers in the equilibrium. And geometry optimized structures of DHP(HMP) tautomers-ethanol clusters and the related transition states (TS) of the proton transfer process have been obtained at B3LYP/6-31G(d) level. The tautomeric energies of DHP(HMP)-ethanol clusters is close to that of the isolated solute molecule. It indicates that the solvent molecule have not changed the tautomeric equilibrium. The activation energy barrier of HMP for the thiol–thione proton transfer decreases from 23.8kcal/mol to 2.9kcal/mol and the energy barrier of DHP for enol–keto proton transfer decreases from 40.7kcal/mol to 8.1kcal/mol when the tautomeric process is mediated by a specific ethanol molecule in solution. Proton transfer reaction could occur easily due to the decrease of activation energies. The time-dependent density functional theory-polarizable continuum model (TDDFT-PCM) calculations on all tautomers of HMP and DHP in vacuo and in ethanol have assigned the lowest π→π* excitations of thione and keto tautomers to the observed absorption bands of HMP and DHP in ethanol solutions. However, the lowest π→π* excitations of thione calculated at 3.50eV is missed from th experimental spectrum in dioxane solution, as the lowest π→π* excitations was observed at 351nm(3.54eV)in ethanol. The solvent shifts of HMP for the observed π→π* excitations are predicted to be 0.12-0.18eV from the B3LYP calculations and 0.08-0.13eV from the BP86 calculations, respectively. The solvent shifts of DHP are calculated to be 0.04-0.13eV at the B3LYP level and 0.05-0.14eV at the BP86 level. From the vertical excitation energies on HMP-ethanol and DHP-ethanol tautomers in vacuo and in ethanol, the contributions of hydrogen bond are less than 0.10eV for π→π* excitations and 0.07-025eV for n→π* excitations.