Calculated electrostatic potentials and local surface ionization energies of para-substituted anilines as measures of substituent effects

Abstract

Using an abinitio self-consistent-field molecular orbital approach, we computed 3-21G//STO-3G* and STO-5G//STO-3G* electrostatic potentials and average local ionization energies for 17 para-substituted anilines. Our results demonstrate that the most negative potentials (V_min) and the local surface ionization energy minima (Ī_S,_min) associated with the amine nitrogen lone pairs are highly sensitive indicators of the electron-donating and electron-attracting tendencies of the para substituents. We find excellent linear relationships between the 3-21G//STO-3G* amine nitrogen V_min and Ī_S,_min and the σ_p⁰ Hammett constants of the substituent X; the correlation coefficients are 0.99. Correlations of slightly lesser quality are shown to exist between V_min, Ī_S,_min, and σ_p⁻, σ_p, and pK_a. Estimates of previously unknown σ_p⁰ and pK_a values are given. The presence of ring carbon Ī_S,_minmeta to the substituent X also provides a predictive capability for determining σ_m values.