SIMILARITY IN PHYSICAL ORGANIC-CHEMISTRY - SUBSTITUENT EFFECTS ON THEINTRINSIC BASICITY OF 4-SUBSTITUTED PYRAZOLES

The gas-phase basicities of eight pyrazoles substituted only at positi on 4 (R(4) = H, NO2, F, Cl, CO2C2H5, CH3, NH2, 1-adamantyl) were measu red by Fourier transform ion cyclotron resonance, The experimental val ues were treated in two ways, first by comparing these values with the AM1-calculated proton affinities. Since the correlation was reasonabl y 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 estima ted, Second, both the experimental and the AM1-calculated values were considered within the framework of the Taft-Topsom analysis of substit uent effects, Comparison of the analyses for pyrazoles and pyridines l ed to the unexpected result that, in spite of differences in ring size and number of heteroatoms, both systems behave remarkably alike.