Hydrohelium(1+) ion

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The hydrohelium(1+) cation, HHe⁺, is a positively-charged ion formed by the reaction of a proton with a helium atom in the gas phase. It is the strongest known acid, with a proton affinity of 177.8 kJ/mol.^[1] This ion is also called "Helium-Hydride Molecular Ion". The ion was first studied by Wolfgang Ketterle in 1986. It has been found to occur naturally in the interstellar medium.

HHe⁺ cannot be prepared in a condensed phase, as it would protonate any anion with which it were associated. However it is possible to estimate a hypothetical aqueous acidity:

HHe+(g)	→	H+(g)	+ He(g)	+178 kJ/mol	[1]
H+(g)	→	H+(aq)		−1530 kJ/mol
He(g)	→	He(aq)		+19 kJ/mol	[2]
HHe+(aq)	→	H+(aq)	+ He(aq)	−360 kJ/mol

A free energy change of dissociation of −360 kJ/mol is equivalent to a pK_a of −63.

[edit] References and notes

Unless otherwise stated, numerical data are taken from Weast, R. C. (Ed.) (1981). CRC Handbook of Chemistry and Physics (62nd Edn.). Boca Raton, FL: CRC Press. ISBN 0-8493-0462-8.

1. ^ ^{a b} Lias, S. G.; Liebman, J. F.; Levin, R. D. (1984). J. Phys. Chem. Ref. Data. 13':695.
2. ^ Estimated from solubility data.