Helium hydride

What’s that you say? How can this molecule possibly exist? Surprise—it was produced in a lab almost 100 years ago.

Helium hydride is actually a molecular cation, HeH⁺. In 1925, chemists Thorfin R. Hogness and E. G. Lunn at the University of California, Berkeley, bombarded a hydrogen–helium mixture at low pressure and observed the presence of HeH⁺ and possibly HeH₂⁺.

Not much was thought of this discovery until the 1970s, when astronomers conjectured that HeH⁺ might exist in interstellar space. Models predicted that it should be formed, but last month it was finally reported.

Using a NASA airplane-based radio telescope, Rolf Güsten and co-workers at the Max Planck Institute for Radio Astronomy (Bonn, Germany) found a spectral line in the planetary nebula NGC 7027 that corresponded to one expected for HeH⁺. The discovery has historical significance.

Astronomers believe that by ≈380,000 years after the Big Bang, the universe was cool enough for atomic particles to combine. They say that helium, the first atom, combined with protons to produce HeH⁺. The work by Güsten and his colleagues helps confirm the models for the early years of the universe.

For obvious reasons, few physical and chemical data and no hazard information are available for HeH⁺. But chemists believe that if it could be corralled, HeH⁺ would be the superacid to end all superacids. No one will ever know how strong an acid it is because the cation would protonate anything it touches.