A molecular balance to measure the strength of N–H⋯π hydrogen bonds based on the tautomeric equilibria of C-benzylphenyl substituted NH-pyrazoles

A theoretical (B3LYP/6-31G∗∗) study of 30 pyrazoles, most of them existing in two tautomeric forms, has been carried out. 3(5)-(2-Benzylphenyl)-5(3)-methyl-1H-pyrazole (11) and 3(5)-(2-benzylbenzylphenyl)-5(3)-phenyl-1H-pyrazole (20) were synthesized from 2-benzoylacetophenone, and their annular tautomeric equilibrium determined. The substituent effects were statistically analyzed and discussed with the help of Hammett substituent constants. In the case of the 5-(2-benzylphenyl) groups, the strength of the N–H⋯π hydrogen bond depends on the electronic effect of the substituent at position 3.