Theoretical studies on the effect of substituent in the proton transfer reaction of 4-substituted pyrazoles

Density functional theory (B3LYP and CAM-B3LYP) and MP2 methods have been employed to study of proton transfer in annular tautomerism and double proton transfer reaction of a series of pyrazoles substituted at the 4 position (NH2, OH, CH3, F, H, Cl, CN, NO2, CHO, NO). It was found that activation energy for monomers of pyrazole derivatives via tautomerism process is in the range of 47.8–55.5 kcal/mol. In addition the pyrazole with electron releasing groups have lower activation energy than electron withdrawing ones. Moreover it was found that formation of dimers favored, and corrected interaction energies were found in the range of 11.4–12.0, 10.0–10.6 and 12.2–12.8 kcal/mol using CAM-B3LYP, B3LYP and MP2 methods, using 6-311++G(d,p) basis function, respectively. The double proton transfer reaction was studied for dimers and the obtained results showed that corrected activation energies vary in the range of 13.0–15.0 and 11.4–12.6 kcal/mol at DFT and MP2 levels of theory, respectively. A good correlation between B3LYP and CAM-B3LYP results for prediction of hydrogen bond strength was observed.