Absolute pK_aDeterminations for Substituted Phenols

Abstract

The CBS-QB3 method was used to calculate the gas-phase free energy difference between 20 phenols and their respective anions, and the CPCM continuum solvation method was applied to calculate the free energy differences of solvation for the phenols and their anions. The CPCM solvation calculations were performed on both gas-phase and solvent-phase optimized structures. Absolute pK(a) calculations with solvated phase optimized structures for the CPCM calculations yielded standard deviations and root-mean-square errors of less than 0.4 pK(a) unit. This study is the most accurate absolute determination of the pK(a) values of phenols, and is among the most accurate of any such calculations for any group of compounds. The ability to make accurate predictions of pK(a) values using a coherent, well-defined approach, without external approximations or fitting to experimental data, is of general importance to the chemical community. The solvated phase optimized structures of the anions are absolutely critical to obtain this level of accuracy, and yield a more realistic charge separation between the negatively charged oxygen and the ring system of the phenoxide anions.

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

“…Table 1 lists the pKa values of the hydroxylic and carboxylic additives used here. [25][26][27] Typical voltammograms are shown in Figs. 1 -3.…”

A Density Functional Theory- and Atoms in Molecules-based Study of NiNTA and NiNTPA Complexes toward Physical Properties Controlling their Stability. A New Method of Computing a Formation Constant

“…2,3 Experimental data do not, however, provide sufficient theoretical insight on physical parameters controlling the affinity between metal ions and donor atoms as well as physical properties (on a fundamental level) of compounds (either ligands or metal complexes). Because of that, theoretical prediction of dissociation constants by use of computational techniques is being explored recently extensively [4][5][6][7][8][9][10][11] 6 ] n+ ) in the gas phase with a single NH 3 ligand (formation of an ML complex, [M(H 2 O) 5 NH 3 ] n+ ) has been reported recently. 12,13 The DFT-computed ∆G(g) values for a number of complex formation reactions, ( 2 O, have been used to generate the LFER (linear free-energy relationship) for divalent metal ions involving reported experimental values of ∆G(aq) for the relevant reaction in aqueous solution (or, equivalently, the log K 1 value).…”

Calculation of the Solvation Free Energy of the Proton in Methanol

“…1 Examples 1-24 include carboxylic acids, [7][8][9]11,14,[17][18][19] alcohols, 11 thiols, 11 phenols, 21 pyrone derivatives, 12 amines, 3 imidazoles, 5,24 hydrated transition metal cations, 2 transition metal complexes, 16 phosphoranes, 22 and DNA (RNA) bases. 6,15,23 Recent applications are extended to the evaluation of pK a values of 1) intermediate species that are not easily measured experimentally, 22 2) weak organic acids with high pK a values, 13 which cannot be measured experimentally in aqueous phase, and 3) molecules having multiple protonation sites.…”