Proton Affinity - Difference From PK_a

Difference From PK_a

Both proton affinity and pK_a are measures of the acidity of a molecule, and so both reflect the thermodynamic gradient between a molecule and the anionic form of that molecule upon removal of a proton from it. Implicit in the definition of pK_a however is that the acceptor of this proton is water, and an equilibrium is being established between the molecule and bulk solution. More broadly, pK_a can be defined with reference to any solvent, and many weak organic acids have measured pK_a values in DMSO. Large discrepancies between pK_a values in water versus DMSO (i.e., the pK_a of water in water is 15.7, but water in DMSO is 32) demonstrate that the solvent is an active partner in the proton equilibrium process, and so pK_a does not represent an intrinsic property of the molecule in isolation. In contrast, proton affinity is an intrinsic property of the molecule, without explicit reference to the solvent.

A second difference arises in noting that pK_a reflects a thermal free energy for the proton transfer process, in which both enthalpic and entropic terms are considered together. Therefore, pK_a is influenced both by the stability of the molecular anion, as well as the entropy associated of forming and mixing new species. Proton affinity, on the other hand, is not a measure of free energy.