20 research outputs found
The enormous acidifying effect of the supersubstituent á á 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 á á O oxygen atoms by á á 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 á á 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 ÏȘ1 . Substituting the second á á O by the á á NSO 2 CF 3 group leads to an additional acidity increase of 10.7 kcal mol ÏȘ1 ; the total acidity increase is thus 34.3 kcal mol ÏȘ1 (25 powers of ten!). In DMSO solution the total acidity increase is 13 pK a units (17.7 kcal mol ÏȘ1 ). These findings are also supported by computational studies using DFT B3LYP at the 6-31Ï©G* 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