engleski

Acidity of Bronsted CH Acids in DMSO - The Extreme Acidity of Nonacyanocyclononatetraene

The gas phase (GP) acidity of a large variety of CH organic acids is studied by a carefully selected B3LYP/6-311+G(2d, p)//B3LYP/6-31G(d) theoretical model. Acidities are mirrored by the proton affinities of the corresponding conjugate bases. A very good agreement with available experimental data is achieved. It is concluded that the model applied has a high predictive value and that theoretical PA(anion)GP values can be used instead of measured data in cases, where experiments are not feasible or not performed. It is shown, employing the isodensity polarized continuum model (IPCM), that there is a good linear relationship between enthalpies of the proton transfer reactions in DMSO and the observed pKa(DMSO) values. This relation can be used in predicting acidity of strong neutral organic superacids. As an illustrative case, acidity of the nonacyano derivative of cyclononatetraene is considered. It is conclusively shown that this compound should represent a very potent superacid as evidenced by PA(anion)GP = 260.0 kcal mol-1 and pKa = -14.8. The origin of high acidity is identified as a very strong anionic resonance effect in the resulting conjugate base.

CH acidity; polarized continuum model; proton affinity; solvent effect; substituent effect

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