Interpretation of Brønsted acidity by triadic paradigm: a G3 study of mineral acids

Deprotonation enthalpies and the gas-phase acidities of 24 inorganic acids are calculated by using composite G3 and G2 methodologies. The computed values are in very good accordance with available measured data. It is found that the experimental DeltaH(acid) values of the FSO(3)H and CF(3)SO(3)H are too high by some 6 and 7 kcal mol(-1), respectively. Furthermore, a new DeltaH(acid) value for HClO(4) of 300 kcal mol(-1) is recommended and suggested as a threshold of superacidicity in the gas phase. The calculated deprotonation enthalpies are interpreted by employing the trichotomy paradigm. Taking into account that the deprotonation enthalpy is a measure of acidity, it can be safely stated that the pronounced acidities of mineral acids are to a very large extent determined by Koopmans' term with very few exceptions, one of them being H(2)S. To put it in another way, acidities are predominantly a consequence of the ability of the conjugate bases to accommodate the excess electron charge, since Koopmans' term in trichotomy analysis is related to conjugate base anion. The final state is decisive in particular for superacids like ClSO(3)H, CF(3)SO(3)H, HClO(4), HBF(4), HPF(6), HAlCl(4), and HAlBr(4). However, in the latter two molecules the bond dissociation energy of the halogen-H bond substantially contributes to their high acidity too. Therefore, acidity of these two most powerful superacids studied here is determined by cooperative influence of both initial and final state effects. It should be emphasized that acidity of hydrogen halides HCl and HBr is a result of concerted action of all three terms included in triadic analysis. A byproduct of the triadic analysis are the first adiabatic ionization energies of the anionic conjugate bases. They are in fair to good agreement with the experimental data, which are unfortunately sparse. A fairly good qualitative correlation is found between the gas-phase deprotonation enthalpies of six mineral O-H acids and available Hammett-Taft sigma(p)- constants of the corresponding substituent groups.