Theoretical Calculation of pK_a Using the Cluster−Continuum Model

Abstract

The pK a's of 17 species from −10 to 50 were calculated using the ab initio MP2/6-311+G(2df,2p) level of theory and inclusion of solvent effects by the cluster−continuum model, a hybrid approach that combines gas-phase clustering by explicit solvent molecules and solvation of the cluster by the dielectric continuum. In addition, the pure continuum methods SM5.42R and PCM were also used for comparison purposes. Species such as alcohols, carboxylic acids, phenol, acetaldehyde and its hydrate, thiols, hydrochloric acid, amines, and ethane were included. Our results show that the cluster−continuum model yields much better agreement with experiment than do the above-mentioned pure continuum methods, with a rms error of 2.2 pK a units as opposed to 7 pK a units for the SM5.42R and PCM methods. The good performance of the cluster−continuum model can be attributed to the introduction of strong and specific solute−solvent interactions with the molecules in the first solvation shell of ions. This feature decreases the dielectric continuum contribution to the difference in the solvation free energy between ions, making the method less susceptible to error because of the continuum contribution to solvation. Because the method is not based on extensive parametrizations and it is shown to fare well for several functional groups, the present results suggest that this method could be used as a general approach for predicting reliable pK a values.

Mechanism of pH-Dependent Decomposition of Monoalkylamine Diazeniumdiolates to Form HNO and NO, Deduced from the Model Compound Methylamine Diazeniumdiolate, Density Functional Theory, and CBS-QB3 Calculations

“…Absolute pK a values were computed utilizing the method developed by Riveros and co-workers and B3LYP/ 6-311+G(d) energies with CPCM aqueous solvation energies. 26 Relative pK a values were predicted using the relationship between ΔΔG aq and pK a , ΔΔG = 1.36(ΔpK a ) at 25 °C. The most basic computed pK a was set equal to the experimentally measured pK a value of 4.5.…”

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).…”

How Accurate is the SMD Model for Predicting Free Energy Barriers for Nucleophilic Substitution Reactions in Polar Protic and Dipolar Aprotic Solvents?

“…These results are unexpected considering that it is well documented that continuum models work better for anions in aprotic solvents than in protic solvents. 51,52 In fact, several theoretical studies of anion-molecule S N 2 reactions in DMSO using the PCM method and the Pliego and Riveros 45 atomic cavities have led to reliable activation barriers. 24,53,54 In part, the reason for this high deviation can be attributed to reaction of the PhS -nucleophile in DMF, with a deviation of 8.3 kcal mol -1 for TS4.…”

“…Yet another possibility before going to full explicit solvation method is the use of hybrid cluster-continuum approaches. 23,52,[63][64][65] Our group has successfully applied this model to some interesting ion-molecule reactions. …”