Computational analysis of substituent effects and Hammett constants for the ionization of gas phase acids

A Hammett plot is a useful tool in physical organic chemistry for determining the effect of substituents on reaction rates and equilibria. In this work, gas phase energetics calculated at the relatively low computational cost B3LYP/6-311++G** level of theory for the ionization of benzoic acids and phenols have been shown to be in excellent agreement with experimental values. These data were used to generate theoretical Hammett plots for the gas phase ionization of substituted phenylacetic acids and phenols, in which good correlation was shown. To determine the effect of solvation on ionization, the calculated energetics for the gas phase ionization of benzoic acids and phenols were compared to experimental solution phase energetics. Resulting ρ values obtained demonstrated the pronounced effect of solvent in damping substituent effects. Overall, a good correlation between experimental solution phase and computational gas phase energetics was shown.