The Acidity of Brønsted CH Acids in DMSO − The Extreme Acidity of Nonacyanocyclononatetraene

Abstract

AbstractThe 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 the enthalpies of the proton‐transfer reactions in DMSO and the observed pKa(DMSO) values. This relation can be used in predicting the acidity of strong, neutral organic superacids. As an illustrative case, the acidity of cyclononatetraene‐1,2,3,4,5,6,7,8,9‐nonacarbonitrile (“nonacyanocyclononatetraene”) is considered. It is conclusively shown that this compound should be a very potent superacid, as evidenced by its 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)