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 p 0 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 p 0 and pK a values are given. The presence of ring carbon Ī S , min meta to the substituent X also provides a predictive capability for determining σ m values.