Interpretation of Brønsted Acidity by Triadic Paradigm:  A G3 Study of Mineral Acids

Abstract

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.

Design of Exceptionally Strong Organic Superbases Based on Aromatic Pnictogen Oxides: Computational DFT Analysis of the Oxygen Basicity in the Gas Phase and Acetonitrile Solution

“…A completely opposite situation is observed with 8As approaches the gas-phase proton affinity of 300 kcal mol −1 , proposed as a borderline between superbases and hyperbases. 55 In analogy with classical proton sponges, the pronounced basicity of 5X−8X (X = N, P, As) should be a consequence of an interplay of two contributions: (a) strong electron repulsions between two neighboring negatively charged oxygen atoms, which destabilize the initial base, and (b) the formation of a favorable [O−H•••O] − hydrogen bonding, which relieves steric strain and stabilizes the conjugate acid. Both contributions could be quantitatively estimated by the following two homodesmotic reactions: 56,57 Here, E STRAIN (n) gives the steric interference of two basic pnictogen oxide groups (X−O) in neutral bases with positive values, indicating the existence of strain, which contributes to basicity, whereas E HB (nH + ) denotes the energy of the intramolecular hydrogen bond in conjugate acids with negative values pointing to a stabilizing hydrogen bonding that also improves the basicity.…”

Rees polycyanated hydrocarbons and related compounds are extremely powerful Brønsted superacids in the gas-phase and DMSO—a density functional B3LYP study

“…Another operational definition of the superacidity threshold, which would be perhaps more practical in the gas-phase, should be the deprotonation enthalpy of HClO 4 , which is 300 kcal mol À1 . 79 If this criterion is accepted, then the most stable cyano system 1a CN is by 28 orders of magnitude more acidic than the threshold of superacidity. It should be kept in mind that it is the NH acid.…”

Toward a rational control of solid acid catalysis for green synthesis and biomass conversion

“…The catalytic performances are compared as a function of deprotonation energy (DPE), which rigorously reflects the energy required to remove a proton from a Brønsted acid as a probe-independent intrinsic property of an acid. [125][126][127][128][129] Generally, TRS yield, cellulose conversion, and TRS selectivity decreased as DPE increased. Since the lower energy requirement for removing a proton from a Brønsted acid means higher protonic acidity, the figure indicates that stronger Brønsted acid is more favourable for the hydrolysis of b-1,4 glycosidic bonds in cellulose.…”