pK_a of Acetate in Water:  A Computational Study

Abstract

Several computational methods including the conductor-like polarizable continuum model, CPCM with both UAKS and UAHF cavities, Cramer and Truhlar's generalized Born solvation model, SM5.4(AM1), SM5.4(PM3), and SM5.43R(mPW1PW91/6-31+G(d)), and mixed QM/MM-Ewald simulations were used to calculate the pK(a) values of acetate and bicarbonate anions in aqueous solution. This work provided a critical and comprehensive assessment of the quality of these theoretical models in the calculation of aqueous solvation free energies for the singly charged acetate and bicarbonate ions, as well as the doubly charged acetate dianion and carbonate dianion. It was shown that QM/MM-Ewald simulations could give an accurate and consistent evaluation of the pK(a) values of acetate and bicarbonate based on both the relative and absolute pK(a) formulas, while other methods could yield satisfactory results only for certain calculations. However, this does not mean that the current QM/MM-Ewald protocol is superior to other methods. The useful information obtained in this investigation is that both the absolute and relative pK(a) formulas should better be tested in accurate calculations of pK(a) values based on any methods.

A New Class of Chiral Phosphazene Bases: Synthesis and Characterization

“…[26,27] As a result of fortuitous error cancellations, relative pK a calculations, also at lower levels such as those presented here, are usually quite reliable. [27] The NMR experiments and DFT calculations clearly indicate that the corresponding protonated form of 2a has a pK a value of at least 28 in acetonitrile, since that represents the pK a value of nitromethane in acetonitrile.…”

Acid/base and hydrogen bonding effects on the proton-coupled electron transfer of quinones and hydroquinones in acetonitrile: Mechanistic investigation by voltammetry, 1H NMR and computation

“…Open-shell species were treated using the unrestricted formalism of the aforementioned levels of theory, UHF and UB3LYP, respectively. pK a s were determined using a thermodynamic cycle as reported previously [55][56][57][58]. From this scheme the pK a of QH 2 (or relative acid/conjugate base couple) in acetonitrile is related to the acidity of the first deprotonation state, QH − ; gas-phase acidity, G gas ; and acetonitrile solvation Gibbs energies for QH 2 , QH − , and H + , G sol .…”

DFT/TDDFT investigation of the stepwise deprotonation in tetracycline: pKa assignment and UV–vis spectroscopy

“…However, for negatively charged species, the description of the solvation energy using only a continuum model can introduce larger errors [76]. For these species, the solvation energy calculations could be improved with a combination of a continuum and an explicit solvation model [75,77].…”