Similarity in physical organic chemistry: Substituent effects on the intrinsic basicity of 4‐substituted pyrazoles

The gas‐phase basicities of eight pyrazoles substituted only at position 4 (R4 = H, NO2, F, Cl, CO2C2H5, CH3, NH2, 1‐adamantyl) were measured by Fourier transform ion cyclotron resonance. The experimental values were treated in two ways, first by comparing these values with the AM1‐calculated proton affinities. Since the correlation was reasonably good [PA(calc.) = −11·3 + 1·063PA(exp.), n = 8, r = 0·984], a set of 17 further 4‐substituted pyrazoles and their cations were calculated using the AM1 approximation and their gas‐phase basicities were estimated. Second, both the experimental and the AM1‐calculated values were considered within the framework of the Taft–Topsom analysis of substituent effects. Comparison of the analyses for pyrazoles and pyridines led to the unexpected result that, in spite of differences in ring size and number of heteroatoms, both systems behave remarkably alike.