The enormous acidifying effect of the supersubstituent ᎐ ᎐ NSO 2 CF 3 on the acidity of derivatives of benzenesulfonamide and toluene-p- sulfonamide in the gas phase and in dimethyl sulfoxide

The effect of stepwise replacement of ᎐ ᎐ O oxygen atoms by ᎐ ᎐ NSO 2 CF 3 fragments in the sulfonyl group of toluenep-sulfonamide and benzenesulfonamide on their acidity has been studied in the gas phase and dimethyl sulfoxide (DMSO). Incorporation of the first ᎐ ᎐ NSO 2 CF 3 group into 4-MeC 6 H 4 SO 2 NH 2 increases its gas-phase acidity by 23.6 kcal mol Ϫ1 . Substituting the second ᎐ ᎐ O by the ᎐ ᎐ NSO 2 CF 3 group leads to an additional acidity increase of 10.7 kcal mol Ϫ1 ; the total acidity increase is thus 34.3 kcal mol Ϫ1 (25 powers of ten!). In DMSO solution the total acidity increase is 13 pK a units (17.7 kcal mol Ϫ1 ). These findings are also supported by computational studies using DFT B3LYP at the 6-31ϩG* level and the semiempirical PM3 method. The results of this work have potentially important implications for the design of new strongly acidic catalytic materials

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