Ionization of Organic Acids in Dimethyl Sulfoxide Solution:  A Theoretical Ab Initio Calculation of the pK_a Using a New Parametrization of the Polarizable Continuum Model

Abstract

The pK a values of over 41 organic acids in dimethyl sulfoxide (DMSO) solution were calculated using ab initio electronic structure methods at MP2 and MP4 levels of electron correlation and including basis set of 6-31+G(d) and 6-311+G(2df,2p) quality. The solvation was included through the polarizable continuum model (PCM), using the recent parametrization of Pliego and Riveros. The root-mean-square (RMS) error over this set of molecules having different functional groups is only 2.2 units. A linear fit on this data set decreases this error by only 0.2 units, indicating that this empirical correction is not necessary. The major error in the calculated pK a value was −5.3 units for the CH3SO3H solute. Halogenated carboxylic acids have also presented a high deviation (∼4 units). An explanation for these high deviations is the possibility of strong hydrogen-bond formation involving the neutral acid molecule and DMSO. The pK a values were also calculated using a combination of theoretical solvation data with experimental gas-phase data. In this case, the RMS error increased to 2.3 units for a set of 36 acids. Our results show that the performance of the PCM model with a fixed atomic radius in DMSO solution is very superior to its performance in aqueous solution, which is a behavior that can be attributed to the presence of strong and specific solute−solvent interactions of ionic solutes with water molecules. In addition, no extensive parametrization of the PCM model is needed to describe the solvation of anions in DMSO solution.

Prediction of acidity in acetonitrile solution with COSMO‐RS

“…22 Looking in detail into the DCSM studies, [10][11][12][13][14][15][16][17] in the regression of pK a values versus the calculated dissociation free energy DG diss the studies report slopes that are significantly lower than the theoretically expected value of 1/RT ln (10). Such a behavior has been reported for aqueous [10][11][12] and nonaqueous acids 10,[13][14][15]23 as well as for bases. 16,17,24 This drawback is common to all simple DCSMs unless considerable effort is taken in the (often physically hardly justifiable) adjustment of numerous additional and often physically doubtful parameters of the DCSM.…”

“…Atom type or hybridization specific cavity radii and cavity definitions that depend on the charge of the molecule are examples of such parameters. 25 Although such models became quite popular and successful applications for nonaqueous solvents have been reported, [26][27][28] it remains doubtful if the predictive power of such empirical adjustments persists for more complex chemically multifunctional solutes or for solutes such as free radicals, zwitterions or excited states. 23 Quite some effort has been devoted to the computational prediction of pKa values in Acetonitrile.…”

Modelos contínuos do solvente: fundamentos

“…Para obtermos a energia livre de solvatação, precisamos agora obter o potencial eletrostático gerado por estas cargas na origem do sistema de coordenadas, onde se localiza a carga pontual. Usando a Equação 33, este potencial será: (52) Por fim, aplicando a Equação 52 na Equação 47, chegamos à famosa fórmula de Born: (53) Esta fórmula estabelece uma relação simples entre a energia livre de solvatação com o raio da cavidade, a dimensão da carga e a constante dielétrica do solvente. Isto nos permite algumas discussões qualitativas sobre observações experimentais.…”

“…Atualmente, em um grande número de trabalhos publicados, esta abordagem vem sendo o método escolhido. Além disso, continuam os esforços de pesquisa envolvendo tanto o desenvolvimento do algoritmo computacional como a parametrização destes modelos para diferentes solventes 13,17,28,[52][53][54][55][56][57] .Entretanto, apesar da popularidade dos modelos contínuos, não é facil encontrar na literatura um texto didático, datalhado e crítico dos fundamentos teóricos desta abordagem. Este trabalho visa justamente preencher esta lacuna.…”

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.…”