The proton affinity (PA, Epa) of an anion or of a neutral atom or molecule is the negative of the enthalpy change in the reaction between the chemical species concerned and a proton in the gas phase:"Proton affinity." Compendium of Chemical Terminology.
A- + H+ -> HA B + H+ -> BH+
These reactions are always exothermic in the gas phase, i.e. energy is released (enthalpy is negative) when the reaction advances in the direction shown above, while the proton affinity is positive. This is the same sign convention used for electron affinity. The property related to the proton affinity is the gas-phase basicity, which is the negative of the Gibbs energy for above reactions,"Gas-phase basicity." Compendium of Chemical Terminology. i.e. the gas-phase basicity includes entropic terms in contrast to the proton affinity.
Acid/base chemistry The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase. The (reportedly) strongest known base is the ortho-diethynylbenzene dianion (Epa = 1843 kJ/mol), followed by the methanide anion (Epa = 1743 kJ/mol) and the hydride ion (Epa = 1675 kJ/mol), making methane the weakest proton acidThe term "proton acid" is used to distinguish these acids from Lewis acids. It is the gas-phase equivalent of the term Brønsted acid. in the gas phase, followed by dihydrogen. The weakest known base is the helium atom (Epa = 177.8 kJ/mol),Lias, S. G.; Liebman, J. F.; Levin, R. D. (1984). Title J. Phys. Chem. Ref. Data. 13':695. making the hydrohelium(1+) ion the strongest known proton acid.
Hydration Proton affinities illustrate the role of hydration in aqueous-phase Brønsted acidity. Hydrofluoric acid is a weak acid in aqueous solution (pKa = 3.15)Jolly, William L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. . but a very weak acid in the gas phase (Epa (F−) = 1554 kJ/mol): the fluoride ion is as strong a base as SiH3− in the gas phase, but its basicity is reduced in aqueous solution because it is strongly hydrated, and therefore stabilized. The contrast is even more marked for the hydroxide ion (Epa = 1635 kJ/mol), one of the strongest known proton acceptors in the gas phase. Suspensions of potassium hydroxide in dimethyl sulfoxide (which does not solvate the hydroxide ion as strongly as water) are markedly more basic than aqueous solutions, and are capable of deprotonating such weak acids as triphenylmethane (pKa = ca. 30).
To a first approximation, the proton affinity of a base in the gas phase can be seen as offsetting (usually only partially) the extremely favorable hydration energy of the gaseous proton (ΔE = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity:
Proton affinity HHe+(g) → H+(g) + He(g) +178 kJ/mol HF(g) → H+(g) + F−(g) +1554 kJ/mol H2(g) → H+(g) + H−(g) +1675 kJ/mol Hydration of acid HHe+(aq) → HHe+(g) +973 kJ/mol Estimated to be the same as for Li+(aq) → Li+(g). HF(aq) → HF(g) +23 kJ/mol H2(aq) → H2(g) −18 kJ/mol Estimated from solubility data. Hydration of proton H+(g) → H+(aq) −1530 kJ/mol H+(g) → H+(aq) −1530 kJ/mol H+(g) → H+(aq) −1530 kJ/mol Hydration of base He(g) → He(aq) +19 kJ/mol F−(g) → F−(aq) −13 kJ/mol H−(g) → H−(aq) +79 kJ/mol Dissociation equilibrium HHe+(aq) → H+(aq) + He(aq) −360 kJ/mol HF(aq) → H+(aq) + F−(aq) +34 kJ/mol H2(aq) → H+(aq) + H−(aq) +206 kJ/mol Estimated pKa −63 +6 +36
These estimates suffer from the fact the free energy change of dissociation is in effect the small difference of two large numbers. However, hydrofluoric acid is correctly predicted to be a weak acid in aqueous solution and the estimated value for the pKa of dihydrogen is in agreement with the behaviour of saline hydrides (e.g., sodium hydride) when used in organic synthesis.
Difference from pKa Both proton affinity and pKa are measures of the acidity of a molecule, and so both reflect the thermodynamic gradient between a molecule and the deprotonated form of that molecule. While pKa values often refer to aqueous solutions, pKa can be defined with reference to any solvent, and many weak organic acids have measured pKa values in DMSO. Large discrepancies between pKa values in water versus DMSO (i.e., the pKa of water in water is 14, but water in DMSO is 32) demonstrate that the solvent is an active partner in the proton equilibrium process, and so pKa 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 pKa reflects a thermal free energy for the proton transfer process, in which both enthalpic and entropic terms are considered together. Therefore, pKa 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.
List of compound affinities Proton affinities are quoted in kJ/mol, in increasing order of gas-phase basicity of the base.
Proton affinityJolly, William L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. Base Affinity Neutral molecules Helium 178 Neon 201 Argon 371 Dioxygen 422 Dihydrogen 424 Krypton 425 Hydrogen fluoride 490 Dinitrogen 495 Xenon 496 Nitric oxide 531 Carbon dioxide 548 Methane 552 Hydrogen chloride 564 Hydrogen bromide 569 Nitrous oxide 571 Sulfur trioxide 589 Carbon monoxide 594 Ethane 601 Nitrogen trifluoride 602 Hydrogen iodide 628 Carbonyl sulfide 632 Acetylene 641 Arsenic trifluoride 649 Silane 649 Sulfur dioxide 676 Hydrogen peroxide 678 Ethylene 680 Phosphorus trifluoride 697 Water 697 Carbon disulfide 699 Phosphoryl trifluoride 702 2,4-Dicarba-closo-heptaborane(7) 703 Hydrogen sulfide 712 Hydrogen selenide 717 Hydrogen cyanide 717 Formaldehyde 718 Carbon monosulfide 732 Cyanogen chloride 735 Arsine 750 Benzene 759 Methanol 761 Methanethiol 784 Ethanol 788 Acetonitrile 788 Phosphine 789 Nitrogen trichloride 791 Ethanethiol 798 Propanol 798 Propane-1-thiol 802 Hydroxylamine 803 Dimethyl ether 804 Glyceryl phosphite 812 Borazine 812 Acetone 823 Diethyl ether 838 Dimethyl sulfide 839 Iron pentacarbonyl 845 Ammonia 854 Methylphosphine 854 Hydrazine 856 Diethyl sulfide 858 1,6-Dicarba-closo-hexaborane(6) 866 Aniline 877 P(OCH2)3CCH3 877 Ferrocene 877 Dimethyl sulfoxide 884 Dimethyl formamide 884 Trimethyl phosphate 887 Trimethylarsine 893 Methylamine 896 Tri-O-methyl thiophosphate 897 Dimethylphosphine 905 Trimethyl phosphite 923 Dimethylamine 923 Pyridine 924 Trimethylamine 942 Trimethylphosphine 950 Triethylphosphine 969 Triethylamine 972 Lithium hydroxide 1008 Sodium hydroxide 1038 Potassium hydroxide 1100 Caesium hydroxide 1125 Anions Trioxophosphate(1−) 1301 Iodide 1315 Pentacarbonylmanganate(1−) 1326 Trifluoroacetate 1350 Bromide 1354 Nitrate 1358 Pentacarbonylrhenate(1−) 1389 Chloride 1395 Nitrite 1415 Hydroselenide 1417 Formate 1444 Acetate 1458 Phenoxide 1470 Cyanide 1477 Hydrosulfide 1477 Cyclopentadienide 1490 Ethanethiolate 1495 Nitromethanide 1501 Arsinide 1502 Methanethiolate 1502 Germanide 1509 Trichloromethanide 1515 Formylmethanide 1533 Methylsulfonylmethanide 1534 Anilide 1536 Acetonide 1543 Phosphinide 1550 Silanide 1554 Fluoride 1554 Cyanomethanide 1557 Propoxide 1568 Acetylide 1571 Trifluoromethanide 1572 Ethoxide 1574 Phenylmethanide 1586 Methoxide 1587 Hydroxide 1635 Amide 1672 Hydride 1675 Methanide 1743
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