Context
In spite of the fact that molecular acidity is a fundamental physicochemical property of molecular systems, the vast majority of theoreticalstudies have focused attention on monoprotic acids and on the prediction of pKa’s. Polyprotic acids, represent a challenge for electronicstructure calculations since the multiple acidic sites result in a vastgroup of species with different conformations and reactivities. In thiswork, Information-theoretic (IT) concepts of localizability, order anduniformity are applied to the Citric Acid and its deprotonated speciesthrough the one-electron density functionals: Shannon entropy (S),Fisher information (I) and disequilibrium (D), respectively. We pursuethe goal of characterizing the acidity of the aforementioned species withthe aim to associate the IT concepts to chemical features such as thepolarizability of the protonated/deprotonated species, the liability ofthe acidic sites, atomic electrostatic potentials, covalent bonding. ITanalyses looks very promising for future studies on the acidity of specificdeprotonation-sites of polyprotic acids.
Methods
Density functional theory (DFT) calculations were performed with Gaussian 09 program. A sensitivity analysis of the IT-measures was performed for the citric acid and the citrate using B3LYP, B3PW91, BPW91, M05-2X, M06-2X and PBEPBE functionals with the 6-311++g(3df,2p), 6-311++g(d,p), 6.311+g(d,p) and aug-cc-pVDZ basis sets. The rest of the analysis was performed with the M05-2X/6-311+G(d,p) level of theory. Additionally, aqueous media was considered by use of the SMD solvent model. The IT-measures were calculated using a suite of programs developed in our laboratory jointly with the DGRID software package.