A G4 approach to computing the Hammett substituent constants σp, σm, σ−, σ+, and σ+m

Abstract

By performing appropriate empirical scaling of G4 energy differences, it is possible to calculate Hammett substituent constants σp, σm, σ−, σ+, and σ+m with a typical accuracy (mean absolute error) of ±0.1. The procedure is straightforward and easily carried out by non-experts using the Gaussian software package. Values are provided for 41 substituents of widely varying electronic character, including some reactive species for which experimental values are unavailable. Additionally, comparison of calculated and experimental values suggests that some previously published numbers might be significantly in error, particularly for some ionic, acidic, and/or basic substituents. Values are also provided for a newly defined substituent constant, σ−m.