Theoretical Study On Intramolecular Proton Transfer For 2-substituted Benzazole Compounds

Proton transfer reactions are among the most fundamentally important reactions that occur in chemical or biological systems.These reactions are known to play crucial roles in a wide variety of processes ranging from the simple tautomerization of an organic molecule to the functioning of enzymes in living systems.It has been widely attention in both experimental and theoretical studies.In this dissertation,ground- and excited-state intramolecular proton transfer(GSIPT and ESIPT) have been studied by using quantum chemistry computational for a series of 2-substituted benzazole compounds.The main results are outlined as follows:1.The tautomers(E₁,E₂,E₃,E₄,and K) and the ground state intramolecular proton transfer reaction of 2-(2-mercaptophenyl)benzoxazole were studied at the B3LYP/6-31G(d,p) level.The effect of solvent(water,dimethylsulfoxide,acetonitrile,ethanol,aniline,and cyclohexane) was studied at the B3LYP/6-31G(d,p) level,using the polarizable continuum model.The results of density functional calculations indicate that the enol form E1 is the most stable tautomer at the ground state.In these solvents there is an equilibrium for 2-(2-mercaptophenyl)benzoxazole in the ground state between E1 and K,and the equilibrium shifts toward the tautomer K as the polarity of the solvent increases.E1 is the preferential conformation in cyclohexane,but K is the more stable tautomer in water.2.The ground- and excited-state intramolecular proton transfer(GSIPT and ESIPT) reaction of 2-(3-mercapto-2-pyridyl) benzimidazole(MPyBI) were studied at the B3LYP/6-31G(d,p) and TD B3LYP/6-31++G(d,p)//CIS/6-31G(d,p) level.The effect of solvent(cyclohexane,benzene,chloroform,ethanol,acetonitrile,dimethylsulfoxide,and water) was studied.The ground-state potential energy surface has been studied;the double proton transfer mechanism has been suggested and the stepwise mechanism is the favorable in reaction kinetics.The excited state proton transfer potential energy surface and the photophysical behavior of MPyBI have been studied.The barrierless ESIPT for MPyBI is predicted.As the polarity of the solvent increases,the barrier in the GSIPT is decreases and the ratio of the tautomers are change;and it can sensitive to control the intensity of fluorescence.3.The ground- and excited-state intramolecular proton transfer(GSIPT and ESIPT) for 2-(2-mercaptophenyl)benzoxazole compounds have been studied at B3LYP/6-31G(d,p) and TD B3LYP/6-31++G(d,p)//CIS/6-31G(d,p) level,respectively.The effects of substituents on the intramolecular proton transfer reactions were explored.The calculated results show that the enol form was the normal form in the ground state.The GSIPT(enol form→keto form) barrier increase when the substituent is electron donating and the barrier decrease when the substituent is electron withdrawing.The electron withdrawing substituent is conducive to the GSIPT and keto form stable.In the excited state,the keto form was the normal form.There is no barrier or a low-barrier(≤1.5 kJ/mol) ESIPT curves for these compounds.The electron donating substituent in favor of ESIPT and the electron donating substituent make against of ESIPT.4.Proton transfer processes of 15 benzimidazole compounds were stadied by the DFT methods and the natural orbital energy index(NOEl) was introduced.NOEI and nucleus independent chemical shift(NICS) were applied to estimate the impact of theÏ€-electron localization and delocalization to the proton transfer.The proton transfer potential energy surfaces were calculated to explore these processes.The calculated results indicated that the changes of theÏ€-electron delocalization of the phenyl(pyridyl) were found to be main factors of stabilizing keto forms and decreasing the barrier by decreasing of these changes.There is high correlation between theÏ€-electron delocalization and the proton transfer barrier.When considering theÏ€-electron localization,the regression increases the correlation coefficient, from 0.9663 to 0.9864.The NOEI can differentiate changes inÏ€-electron localization sensitively;it is an effective complementarity to NICS.5.The ground- and excited-state intramolecular proton transfer process of 2-(2-R(R=OH, NH₂,SH) phenyl(or pyridyl)) benzoxazoles(or benzothiazoles) was investigated by the DFT methods.The calculated results indicated in the ground-state there is;high correlation (R=0.9943) between the intramolecular hydrogen bond(IMHB) strength and the proton transfer barrier through it.The barrier and the energies difference between the tautomers increase by the changes of IMHB strength.In the process,the proton is along the pathway that the change of IMHB angle is as small as it could.In the excited-state,the relationship between the IMHB and the barrier is apparent.