engleski

Interpretation of Broensted 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. However, there are some serious discrepancies, which strongly indicate that the experimental measurement of acidity of CF3SO3H, FSO3H an HClO4 should be repeated. 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 H2S. 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 ClSO3H, CF3SO3H, HClO4, HBF4, HPF6, HAlCl4 and HAlBr4. 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 by-product 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 6 mineral O-H acids and available Hammett-Taft sigma_p– constants of the corresponding substituent groups.

inorganic acids; anions; proton affinity; triadic formula; substituent effects

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