Hydrohelium(1+) ion

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. 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:

A free energy change of dissociation of &minus;360 kJ/mol is equivalent to a pKa of &minus;63.

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.

Hydrohelium