Acidity of meta- and para-substituted aromatic acids: a conceptual DFT study

Abstract

Quantum chemical computations with full geometry optimization at the B3LYP/6-31G(d) level in aqueous phase at 298 K are performed to calculate a set of molecular properties for 45 aromatic organic acids such as benzoic acid, phenol, cinnamic acid, benzohydroxamic acid, anilinium ion and their meta- and para-substituted derivatives. In a separate set of calculations the 6-31+G(d) basis set is employed for substituted benzoic and cinnamic acids as representative test cases, to test the effect of diffuse functions. Molecular descriptors such as ionization potential, electron affinity, hardness, chemical potential, global electrophilicity and free energy of deprotonation of these acids are computed. The local descriptors such as Fukui functions and group philicity (ω⁺_g) are calculated with Mulliken Population Analysis (MPA) and Hirschfeld Population Analysis (HPA) schemes. These aromatic acids are reacted with a strong base, OH^-. The computed Gibbs free energy of deprotonation, the fractional number of electrons transferred, ΔN and electrophilicity based charge transfer index (ECT) in acid-base reaction with a strong base, OH^-, ω⁺_g and group charges are correlated with the experimental pKa values of these acids. It is found that this approach is particularly effective in discussing trends of changes in acidity of intimately related molecules. The effect of substituents on these descriptors is also studied. These parameters are correlated with experimental Hammett substituent constants (σ). The ECT, fractional number of electron transfer (ΔN) and group charge correlate strongly with pK_a and σ in separate groups of aromatic acids. Minimum energy, maximum hardness and minimum electrophilicity principles are tested for the acid-base reactions.