Electron localization-delocalization matrices in the prediction of pK a's and UV-wavelengths of maximum absorbance of p-benzoic acids and the definition of super-atoms in molecules

An integration of chemical graph theory and the quantum theory of atoms in molecules (QTAIM) is proposed. QTAIM localization and delocalization indices organized as matrices are shown useful in the modeling of ground and excited state molecular properties. The pK a's of 14 para-substituted benzoic acids and the λ max's of a subset of those are predicted with accuracy (pK a: r 2 =0.986, q 2 =0.982; λ max: r 2 =0.973, q 2 =0.944). ‘Super-atoms’ obtained by pruning the branches of the substituents’ graphs equalize the matrices of all molecules of the set. The pK a of p-dimethylaminobenzoic acid is found to be 5.02 rather than the reported value of 6.03.