Theoretical studies on proton transfer reaction of 3(5)-substituted pyrazoles

Abstract

The inter and intra molecular proton transfer reactions of a series of pyrazole derivatives have been studied by using density functional theory (DFT) and MP2 methods implementing 6-311++G(d,p) atomic basis set. The substituents have been selected to cover a wide range of electronic effects. Proton transfer process was studied for mechanisms including single proton transfer, double proton transfer and proton transfer assisted by a water or ammonia molecule. The results showed single proton transfer reactions for interconversion pyrazole derivatives need highest activation energies in the range of 45.7 − 51.59 and 49.4 − 53.96 kcal/mol at B3LYP and MP2 levels, respectively. It was found that for the 3–substituted pyrazoles, electron withdrawing groups form stronger dimers but in the 5-substituted tautomers electron donating groups form stronger hydrogen bond. The double proton transfer reactions between dimers were studied and transition states calculated. The ranges of activation energies were found to be 17.51 − 19.36 and 17.02 − 17.80 kcal/mol for the C→E and D→D reactions respectively. In addition, the activation energies for the proton transfer reaction assisted by water or ammonia molecules were found to be in the range of 26.62−31.78 and 17.25 − 22.46 kcal/mol, respectively, calculated at MP2/6-311++G(d,p) level of theory. DFT and MP2 methods have been employed to study of proton transfer reaction of substituted pyrazoles. Activation energy of monomers are in the range of 45.7–51.59 kcal/mol at MP2 levels. The double proton transfer reaction was studied for all dimers and activation energies were found to be in the range of 17.51–19.36 kcal/mol.