Dependence of pKa on solute cavity for diprotic and triprotic acids

• Published in: Physical chemistry chemical physics : PCCP 2011-01, Vol.13 (21), p.10258-10269
• Main Authors: Lee, Tae Bum, McKee, Michael L
• Format: Article
• Language: English
• Subjects: Accuracy; Acids - chemistry; Diffusion; Gas phases; Holes; Inorganic acids; Mathematical models; Models, Chemical; Physical chemistry; Quantum Theory; Solubility; Solvation; Thermodynamics
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c1cp20161a

A systematic study of ΔG(aq)/pK(a) for monoprotic, diprotic, and triprotic acids has been carried out based on DFT/aug-cc-pVTZ combined with CPCM and SMD solvation modeling. All DFT/cavity set combinations considered showed similar accuracy for ΔG(aq)(1)/pK(a1) (70% within ±2.5 kcal mol(-1) of experiment) while only the M05-2X/Pauling cavity combination gave reasonable results for ΔG(aq)(2)/pK(a2) when both pK(a) values are separated by more than three units (70% within ±5.0 kcal mol(-1) of experiment). The choice of experimental data is critical to the interpretation of the calculated accuracy especially for several inorganic acids. For the calculation of ΔG(aq)(3)/pK(a3), the larger experimental uncertainty and an unrealistic orbital population of diffuse function for trianions in the gas phase hinders an evaluation of the predictive performance. We find the M05-2X functional with the Pauling cavity set is the best choice for ΔG(aq)(2)/pK(a2) prediction in aqueous media while all DFT/cavity sets considered were competitive for ΔG(aq)(1)/pK(a1).