The proton affinity (PA, ''E''_pa) of an

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

or of a neutral

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

is the negative of the

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

change in the reaction between the chemical species concerned and a

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

in the gas phase: ::: A- + H+ -> HA ::: B + H+ -> BH+ These reactions are always

exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...

in the gas phase, i.e. energy is released (

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 electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy Note that this is ...

. The property related to the proton affinity is the gas-phase basicity, which is the negative of the

Gibbs energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work (physics), work that may be performed by a closed system, thermodynamically closed system a ...

for above reactions, i.e. the gas-phase basicity includes

entropic Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

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 In organic chemistry, a diethynylbenzene dianion is an anion consisting of two ethynyl anions as substituents on a benzene ring. With the chemical formula , three positional isomers are possible, differing in the relative positions of the two ...

(''E''_pa = 1843 kJ/mol), followed by the methanide anion (''E''_pa = 1743 kJ/mol) and the

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

ion (''E''_pa = 1675 kJ/mol), making

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

the weakest proton acid in the gas phase, followed by

dihydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, and ...

. The weakest known base is the

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

atom (''E''_pa = 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 Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...

in aqueous-phase Brønsted acidity.

Hydrofluoric acid Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...

is a weak acid in aqueous solution (p''K''_a = 3.15)Jolly, William L. (1991). ''Modern Inorganic Chemistry'' (2nd Edn.). New York: McGraw-Hill. . but a ''very weak'' acid in the gas phase (''E''_pa (F⁻) = 1554 kJ/mol): the

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typ ...

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 Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

ion (''E''_pa = 1635 kJ/mol), one of the strongest known proton acceptors in the gas phase. Suspensions of

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

dimethyl sulfoxide Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds a ...

(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 Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C6H5)3CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane is the basic skeleton of many synthetic dyes called triarylmetha ...

(p''K''_a = ''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 (Δ''E'' = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity: 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 p''K''_a of dihydrogen is in agreement with the behaviour of saline hydrides (e.g.,

sodium hydride Sodium hydride is the chemical compound with the empirical formula Na H. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H− ions, in co ...

) when used in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

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 14, 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.

List of compound affinities

Proton affinities are quoted in kJ/mol, in increasing order of gas-phase basicity of the base.

References

{{Reflist Chemical properties

Acid/base chemistry

Hydration

Difference from pKa

List of compound affinities

References

Difference from pK_a