Additivity of substituent effects on the proton affinity and gas-phase basicity of pyridines

The change in the proton affinity (PA) and basicity (GB) of pyridine with substituents have been considered by quantum mechanical methods at the B3LYP/6-311++G(d,p) level of theory. The PA and GB values increase by the electron-donating substituents and decrease by the electron-withdrawing substituents. The effects of substituents on the PA and GB are approximately additive. The deviations of changes that are predicted from the additivity of substituent effects are generally lower than 30% from the calculated changes. Linear relationships are observed between the calculated PA values of substituted pyridines and the topological properties of electron density, the molecular electrostatic potentials (MEP), and the N-H bond lengths. In addition, well-defined relations are established between the calculated PA values and the Hammett constants, and the reaction constant (ρ) has been calculated for the protonation reaction. With some exceptions, the effect of substituents are also additive on the electron density and its Laplacian calculated at N-H BCP, and the MEP values calculated around the N atom.