Accurate evaluation of the resonance energies of benzene and pyridine via cyclic reference state

In an attempt to eliminate the stoichiometric arbitrariness in the evaluation of the resonance energy we use benzene and pyridine as an example to illustrate a unique thermodynamic way the resonance energy may and should be evaluated. In doing this, we employ a complete set of possible cyclic reference species and a Tatevski type group additivity model. Three different methods used to evaluate the resonance energies of benzene and pyridine were shown to be identical thus proving the uniqueness of the solution. Based on high level coupled-cluster (estimated CCSD(T)/CBS) computations it was concluded that benzene is more aromatic/stable than pyridine by 2.2 kcal mol⁻¹.