Acidity of Strong Acids in Water and Dimethyl Sulfoxide.

Careful analysis and comparison of the available acidity data of HCl, HBr, HI, HClO4, and CF3SO3H in water, dimethyl sulfoxide (DMSO), and gas-phase has been carried out. The data include experimental and computational pKa and gas-phase acidity data from the literature, as well as high-level computations using different approaches (including the W1 theory) carried out in this work. As a result of the analysis, for every acid in every medium, a recommended acidity value is presented. In some cases, the currently accepted pKa values were revised by more than 10 orders of magnitude.

Superacidity of closo-dodecaborate-based Brønsted acids: a DFT study.

The structures and intrinsic gas-phase acidities (GA) of some dodecaborane acids, the derivatives of YB12H11H (Y = PF3, NH3, NF3, NMe3), B12H12H2, and B12H12H(-) (HA, H2A, and HA(-), respectively) have been computationally explored with DFT B3LYP method at the 6-311+G** level of theory as new possible directions of creating superstrong Brønsted acids. Depending on the nature and number of the substituents different protonation geometries were investigated. In general, the GA values of the neutral systems varied according to the substituents in the following order: CF3 < F < Cl and in case of anionic acids: CF3 < Cl < F. The dodecatrifluoromethyl derivative of H2A, B12(CF3)12H1H2, emerges as the strongest among the considered acids and is expected to be in the gas phase at least as strong as the undecatrifluoromethyl carborane, CB11(CF3)11H1H. The GA values of the respective monoanionic forms of the considered acids all, but the (CF3)11 derivative, remained higher than the widely used threshold of superacidity. The HA derivatives' (Y = PF3, NF3) GA's were approximately in the same range as the H2A acids'. In the case Y = NH3 or NMe3 the GA values were significantly higher. Also, the pKa values of B12H12H2, CB11H12H, and their perfluorinated derivatives in 1,2-dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations. The obtained estimates of pKa values of the perfluorinated derivatives are by around 30 units lower than that of trifluoromethylsulfonylimide, making these acids the strongest ever predicted in solution. The derivatives of B12H12H2 are as a rule not significantly weaker acids than the respective derivatives of CB11H12H. This is important for expanding practical applicability of this type of acids and their anions, as they are synthetically much more accessible than the corresponding CB11H12(-) derivatives.

Acidity of anilines: calculations vs experiment.

The gas-phase acidities of ca. 60 monosubstituted anilines (with acidity span of almost 50 kcal mol(-1)) have been calculated using density functional theory (DFT) at the B3LYP/6-311+G** level. At this relatively simple level of theory the calculated (ΔG(calc)) and available experimental (ΔG(exp)) acidities are in reasonable quantitative correlation according to the following equation: ΔG(obs) = a + bΔG(calc), where a=20.79, b=0.942, n=27, R(2)=0.990, and s=0.78 kcal·mol(-1). The slope is not far from its ideal value. Substituent effects on the acidities were dissected separately into those operating in the neutral acid molecule and in its conjugated anion using the isodesmic homodesmotic reactions. All in all, both forms, neutral and anionic, are contributing in combination to make up the gross acidity of anilines. However, the contributions of the anions into the gross substituent effects are much larger than the substituent effects in the neutral anilines. Some of the systems were used in testing a relatively new theoretical model, COSMO-RS (conductor-like screening model for real solvents), using it for the prediction of pK(a) values in DMSO. The method proved to be rather accurate for showing pK(a) trends (R(2)=0.980 in DMSO). However, the predicted absolute pK(a) values were all somewhat lower (rmsd=2.49 kcal·mol(-1)) than the respective experimental values.

Polytrifluoromethylation versus polyfluorination of the isomers of Kekulé benzene and phenol: a theoretical study.

The study of the polyfluorination and polytrifluoromethylation effects on electronic structure and intrinsic acidities has been performed using DFT B3LYP and NBO calculations for the valence isomers of benzene and phenol, i.e., the structures of prismane, benzvalene, Kekulé, and Dewar systems. Also the isodesmic reaction analysis approach to estimate the effects of the substituents on the acidity of the compounds has been used. Although in some systems the additivity of fluorine substituents was more than 100%, the acidifying effects of the fluorine substituents were comparable to that of the CF(3) in only one case. Isodesmic reaction analysis of substituent effects shows that steric effects of poly-CF(3) substitution in the alicyclic cage compounds are significantly smaller. The relative thermodynamic stabilities of the valence isomers of Kekulé benzene were significantly lower than that of the aromatic cycle. The introduction of fluorine substituents often destabilized the compounds even further. Out of the fluorinated hydroxy derivatives of prismane, Dewar benzene, and benzvalene, not all are predicted to be stable enough to be able to undergo a reversible protonation-deprotonation process. In the case of several hydroxy derivatives deprotonation is accompanied by the rupture of a C(α)-C(ß) bond and in some cases by the rearrangement to the corresponding phenolate anion. The isomerization of benzene derivatives resulted in more cases where the acidities increased compared with the respective phenol derivatives. The only hydroxy compound with significantly higher acidity than the respective phenol was pentafluorinated 2-OH-Dewar benzene, which was expected to rival the gas-phase acidity of perfluoro-1-adamantanol, currently the most acidic experimentally measured perfluorinated alcohol.

Gas-phase Brønsted superacidity of some derivatives of monocarba-closo-borates: a computational study.

The structures and gas-phase acidities (GA) of several CB(11)H(12)H-based carborane acid derivatives (HA) have been calculated with DFT B3LYP method using 6-311+G**, 6-311++G** basis sets. In order to verify the obtained GA values, several systems were also studied at G3(MP2) level of theory. Inserted substituents (CF(3), F, Cl, Br, I, CN, CH(3), etc.) followed the "belts" of the monocarborane cage starting from the boron antipodal to the carbon. In general, the predicted intrinsic gas-phase acidities of the systems varied according to the substituents in the following order of decreasing strength: CF(3) > F > Cl > Br > I > CN > CH(3). Nevertheless, some inconsistencies occurred. F and CN derivatives with lower degree of substitution had weaker intrinsic acidities than the respective Cl derivatives, but the situation was reversed in the case of a larger number of substituents. To obtain better understanding how the substituents influence the basicity of the carborane anion, three hypothetical reaction series were investigated, in which the protonation center was fixed on the boron atom (B(12)), antipodal to the carbon (C(1)), and a single substituent replaced the hydrogens at the vertexes of the three remaining positions (C(1), B(2), and B(7)). The intrinsic gas-phase acidities in these series of neutral carborane-based acids CB(11)X(1)H(11)H are found to clearly depend on the field-inductive and resonance effects of the substituent X. Some influence of the polarizability of X on the reaction center (B(12)) could be detected only in the alpha position (B(7)).