Predating these computational techniques is the desire to understand and explain experimental outcomes in organic chemistry with physicochemical descriptors. A prominent early example are Hammett parameters, developed in 1937 , that relate substituent parameters to the equilibrium constant of the deprotonation of a substituted benzoic acid. The derived substituent parameters are used to gain insight into the mechanism of reactions by observing the influence of substituents on a reaction outcome. However, Hammett parameters have shown to not fully describe observed trends. Therefore, complementary representations capturing other properties of a molecule have been derived (vide infra) .