Study On Intramolecular Proton Transfer For 2-Substituted Phenolic Compounds

Proton transfer reactions play an important role in chemical and biochemical processes, received extensive attention of many scholars. Research on proton transfer has a positive meaning. This paper studies the electronic effect of substituents on the intromolecular proton transfer of 2-substituted phenolic compounds by quantum chemical calculation method. Furthermore, We also study the proton transfer process of 2-(1,4,5,6-tetrahydropyrimidin-2-yl) phenol under the supramolecular interaction with cucurbit[7]uril by theoretical and experimental methods. The main results are as follows:1 、 The intramolecular proton transfer reactions of 2-(1H-imidazol-2-yl) benzenethiol(1d), 2-(oxazol-2-yl) benzenethiol(2d), 2-(thiazol-2-yl) benzenethiol(3d) and their subsituent derivatives have been studied at B3LYP/6-311+G(d, p) level. The electronic effects of substituents on the intramolecular proton transfer reactions have been explored. The results show that when electron-withdrawing substituents are introduced, electron density of the whole molecular plane decreases, N2-H1 spacing decreases, intramolecular hydrogen bonding is enhanced, the proton transfer barrier from enol to keto decreases. When electron-donating substituents are introduced, electron density of the whole molecular plane increases, N2-H1 spacing increases, intramolecular hydrogen bonding is weakened, the proton transfer barrier from enol to keto increases. LOL(Localized orbital locator) function analysis reveals that substituents have significant effect on the bonding properties of atom N1. The energy barrier order of the three is 1d<3d<2d, it remains the same when substituents are introduced. In the excited state, Absorption wavelength of the three compounds is less affected by solvent polarity. Fluorescence emission wavelength of the three compounds increases as the polarity of the solvent decreases, but the difference is: 2-(thiazol-2-yl) thiophenol and 2-(oxazol-2-yl)benzenethiol have large red shift with respect to 2-(1H-imidazol-2-yl) thiophenol.2、With quantum chemical calculation methods, we study the intramolecular proton transfer progress of 2-(1,4,5,6-tetrahydropyrimidin-2-yl) phenol(H2thp) and their subsituent derivatives at B3LYP/6-311+G(d, p) level. The electronic effects of substituents on the intramolecular proton transfer reactions have been explored. The results show that when electron-donating substituents are introduced, electron density of the whole molecular plane increases, N2-H1 spacing increases, intramolecular hydrogen bonding is weakened, the proton transfer barrier from enol to keto increases. when electron-withdrawing substituents are introduced, electron density of the whole molecular plane decreases, N2-H1 spacing decreases, intramolecular hydrogen bonding is enhanced, the proton transfer barrier from enol to keto decreases. The keto form becomes the stable isomer when the subsituent is –NO2. LOL(Localized orbital locator) function analysis reveals that substituents have significant effect on the bonding properties of atom N1. Excited state research shows that electron-donating substituents decrease the vertical absorption energy of the first excited energy(S1). –Cl and –CN perform like electron-donating substituents because they can be conjugated with the benzene ring. And –NO2 hinders the first excited state transition.3、The interaction of 2-(1,4,5,6-tetrahydropyrimidin-2-yl) phenol(H2thp) with cucurbit[7]uril(CB7) and the effect of the interaction on proton transfer of H2 thp were studied by using fluorescence, UV absorption spectroscopy, steady-state fluorescence spectra and theoretical calculation. The fluorescence spectrum shows that H2 thp displays single fluorescence emission in all tested solvents. Theoretical calculation shows that the emission is consistent with the emission of the keto form. Fluorescence emission and UV absorption spectroscopy show that significant interaction has occurred between H2 thp and CB7. The proton transfer of H2 thp was accelerated by CB7 in tetrahydrofuran and hindered by CB7 in water. Steady-state fluorescence spectra shows that quantum yield of H2 thp increases when CB7 was titrated in tetrahydrofuran. Furthermore, the absorption and fluorescence data show that the H2 thp molecules were complexed with CB7 forming 1:1 complex both in tetrahydrofuran and water.