Critical Test of Some Computational Chemistry Methods for Prediction of Gas-Phase Acidities and Basicities

Gas-phase acidities and basicities were calculated for 64 neutral bases (covering the scale from 139.9 kcal/mol to 251.9 kcal/mol) and 53 neutral acids (covering the scale from 299.5 kcal/mol to 411.7 kcal/mol). The following methods were used: AM1, PM3, PM6, PDDG, G2, G2MP2, G3, G3MP2, G4, G4MP2, CBS-QB3, B1B95, B2PLYP, B2PLYPD, B3LYP, B3PW91, B97D, B98, BLYP, BMK, BP86, CAM-B3LYP, HSEh1PBE, M06, M062X, M06HF, M06L, mPW2PLYP, mPW2PLYPD, O3LYP, OLYP, PBE1PBE, PBEPBE, tHCTHhyb, TPSSh, VSXC, X3LYP. The addition of the Grimmes empirical dispersion correction (D) to B2PLYP and mPW2PLYP was evaluated, and it was found that adding this correction gave more-accurate results when considering acidities. Calculations with B3LYP, B97D, BLYP, B2PLYPD, and PBE1PBE methods were carried out with five basis sets (6-311G**, 6-311+G**, TZVP, cc-pVTZ, and aug-cc-pVTZ) to evaluate the effect of basis sets on the accuracy of calculations. It was found that the best basis sets when considering accuracy of results and needed time were 6-311+G** and TZVP. Among semiempirical methods AM1 had the best ability to reproduce experimental acidities and basicities (the mean absolute error (mae) was 7.3 kcal/mol). Among DFT methods the best method considering accuracy, robustness, and computation time was PBE1PBE/6-311+G** (mae = 2.7 kcal/mol). Four Gaussian-type methods (G2, G2MP2, G4, and G4MP2) gave similar results to each other (mae = 2.3 kcal/mol). Gaussian-type methods are quite accurate, but their downside is the relatively long computational time

Superacidity of <i>closo</i>-Dodecaborate-Based Brønsted Acids: a DFT Study

The structures and intrinsic gas-phase acidities (GA) of some dodecaborane acids, the derivatives of YB₁₂H₁₁H (Y <b>=</b> 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 <b>=</b> PF₃, NF₃) GA’s were approximately in the same range as the H₂A acids’. In the case Y <b>=</b> NH₃ or NMe₃ the GA values were significantly higher. Also, the p<i>K</i>_a 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 p<i>K</i>_a 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

Experimental Basicities of Superbasic Phosphonium Ylides and Phosphazenes

Experimental basicities of some of the strongest superbases ever measured (phosphonium ylides) are reported, and by employing these compounds, the experimental self-consistent basicity scale of superbases in THF, reaching a p<i>K</i>_α (estimate of p<i>K</i>_a) of 35 and spanning more than 30 p<i>K</i>_a units, has been compiled. Basicities of 47 compounds (around half of which are newly synthesized) are included. The solution basicity of the well-known <i>t</i>-Bu-NP₄(dma)₉ phosphazene superbase is now rigorously linked to the scale. The compiled scale is a useful tool for further basicity studies in THF as well as in other solvents, in particular, in acetonitrile. A good correlation between basicities in THF and acetonitrile spanning 25 orders of magnitude gives access to experimentally supported very high (p<i>K</i>_a > 40) basicities in acetonitrile, which cannot be directly measured. Analysis of structure–basicity trends is presented