Hammett constants from density functional calculations: charge transfer and perturbations

Abstract

Thermodynamic and kinetic components of density functional reactivity theory -based stabilization energies between interacting electron acceptors and electron donors are evaluated with and without taking into account perturbative effects on one reactant caused by the other. The values of the two energy components generated through these two approaches are then correlated to the Hammett’s substituent constant $\left( \sigma \right)$ through the relation $\sigma = m\log \frac{{\Delta E_{X} }}{{\Delta E_{H} }} + b$. Here $\Delta E_{X}$ and $\Delta E_{H}$ represent, respectively, energy components (either thermodynamic or kinetic) of the substituted and unsubstituted benzene derivatives. The generated data on six different series of reactions demonstrate that both perturbative and unperturbative approaches are of comparable reliability when either thermodynamic or kinetic energy components are used in the proposed relation, justifying the validity and generality of Hammett’s free energy relation.