Helium hydride 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, first observed in 1925. [cite journal | author = T. R. Hogness and E. G. Lunn | title = The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis | journal = Physical Review | year = 1925 | volume = 26 | pages = 44–55 | doi = 10.1103/PhysRev.26.44] It is the strongest known acid, with a proton affinity of 177.8 kJ/mol.cite journal | author = Lias, S. G.; Liebman, J. F.; Levin, R. D. | year = 1984 | journal = "J. Phys. Chem. Ref. Data." | volume = 13 | pages = 695] This ion is also called helium-hydride molecular ion. It has been suggested that it should occur naturally in the interstellar medium. [cite journal | author = J. Fernandez; F. Martin | title = Photoionization of the HeH⁺ molecular ion | journal = J. Phys. B: At. Mol. Opt. Phys | year = 2007 | volume = 40 |pages = 2471–2480 | doi = 10.1088/0953-4075/40/12/020] It is the simplest heteronuclear ion, and is comparable with the hydrogen molecular ion, H₂⁺. Unlike H₂⁺, however, it has a permanent dipole moment, which makes the spectroscopic characterization easier. [cite journal | doi = 10.1006/jmsp.1998.7740 | title = Experimental Born–Oppenheimer Potential for theX1Σ+Ground State of HeH+: Comparison with theAb InitioPotential | year = 1999 | author = Coxon, J | journal = Journal of Molecular Spectroscopy | volume = 193 | pages = 306]

Properties

HHe⁺ cannot be prepared in a condensed phase, as it would protonate any anion, molecule or atom with which it were associated. However it is possible to estimate a "hypothetical" aqueous acidity using Hess's law:

A free energy change of dissociation of −360 kJ/mol is equivalent to a p"K"_a of −63.

Other helium hydride ions are known or have been studied theoretically. HeH₂⁺, which has been observed using microwave spectroscopy, [cite journal | author = Alan Carringtona, David I. Gammiea, Andrew M. Shawa, Susie M. Taylora and Jeremy M. Hutson | title = Observation of a microwave spectrum of the long-range He···H₂⁺ complex | journal = Chemical Physics Letters | year = 1996 | volume = 260 | pages = 395–405 | doi = 10.1016/0009-2614(96)00860-3] has a calculated binding energy of 6 kcal/mol, while HeH₃⁺ has a calculated binding energy of 0.1 kcal/mol. [http://asilomar.caltech.edu/abstracts_processed/Pauzat_2z6IeIOR.pdf]

Natural occurrence

Helium hydride ion is formed during the decay of tritium in the HT or tritium molecule T₂. Although excited by the recoil from the beta decay the molecule remains bound together. [F Mannone: "Safety in Tritium Handling Technology" Springer 1993, p 92]

HeH⁺ is believed to be the first kind of molecule formed in the universe after the big bang, forming even before H₂. HeH⁺ in theory is most common where the temperature is 3500K. HeH⁺ is important because of the substances that could be formed in the early metal free universe it has the highest opacity. Its opacity is because of the high dipole moment. [Elodie A. Engel et al: [http://arxiv.org/abs/astro-ph/0411267v1 "Calculated spectra for HeH+ and its effect on the opacity of cool metal poor stars"] 2004] In metal free stars above 4000K H^- dominates the spectrum, and below 3000K H₃⁺ dominates. Spectral lines of helium hydride ion appear for solar abundance He stars, where the density is below 10^-8 gcm^-3. For high percentages of helium, spectral lines can appear at higher densities, and may be found in cool helium rich white dwarfs. For higher temperature stars, the spectrum will be modified by HeH⁺ by causing absorption at temperatures below 6000K. Even at the optimal conditions the ions will only make up about 10^-7 of the total atoms. [G. J. Harris, A. E. Lynas-Gray, S. Miller, J. Tennyson: ["The role of HeH+ in cool helium rich white dwarfs" http://arxiv.org/abs/astro-ph/0411331] 2004]

X. Liu claimed to have observed the HeH⁺ 149.1404805 μm j=1-0 rotational emissionline with the Infrared Space Observatory in NGC 7027. [Liu X, Barlow M.J, Dalgarno A, et al in 1997, MNRAS, 290, L71-L75]

Neutral molecule

Unlike the helium hydride ion, the neutral helium hydride "molecule" is not stable in the ground state. However, it is stable in an excited state as an excimer, and its spectrum was first observed in the mid 1980s. [cite journal | title = Observation of Fluorescence of the HeH Molecule | author = Thomas Möller, Michael Beland, and Georg Zimmerer | journal = Phys. Rev. Lett. | volume = 55 | pages = 2145–2148 | year = 1985 | doi = 10.1103/PhysRevLett.55.2145 ] [http://nobelprize.org/nobel_prizes/physics/laureates/2001/ketterle-autobio.html] [cite journal | title = Emission spectra of bound helium hydride | author = W. Ketterle, H. Figger, and H. Walther | journal = Phys. Rev. Lett. | volume = 55 | pages = 2941–2944 | year = 1985 | doi = 10.1103/PhysRevLett.55.2941 ]

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.