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The proton affinity, ''E''pa, of an anion or of a neutral atom or molecule is a measure of its gas-phase basicity. It is the energy released in the following reactions:〔"(Proton affinity. )" ''Compendium of Chemical Terminology''.〕 ::A− + H+ → HA ::B + H+ → BH+ These reactions are always exergonic in the gas phase, i.e. energy is released when the reaction advances in the direction shown. However, proton affinities are conventionally quoted with the opposite sign convention from most other thermodynamic properties, a positive ''E''pa indicating a release of energy by the system. This is the same sign convention as is used for electron affinity. ==Acid/base chemistry== The higher the proton affinity, the stronger the base and the weaker the conjugate acid ''in the gas phase''. The strongest known base is the methanide anion (''E''pa = 1743 kJ/mol), slightly stronger than the hydride ion (''E''pa = 1675 kJ/mol),〔Bartmess, J. E.; Scott, J. A.; McIver, R. T. (1979). ''J. Am. Chem. Soc.'' 101:6046.〕 making methane the weakest proton acid〔The 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 (''E''pa = 177.8 kJ/mol),〔Lias, S. G.; Liebman, J. F.; Levin, R. D. (1984). ''J. Phys. Chem. Ref. Data.'' 13':695.〕 making the hydrohelium(1+) ion the strongest known proton acid. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「proton affinity」の詳細全文を読む スポンサード リンク
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