The helium hydride ion

20 April 2019 - Chemical Zoo

In the news this week: helium hydride or more precise the helium hydride cation. And in the news why? The first ever detection of the molecule in space by an international group of radio astronomers. (DOI) The observation was made inside SOFIA (a plane with an on-board telescope operating at an altitude of 12,000 km!) , the target was NGC 7027 and selected for it being very young (600 years) and having a central star with a luminosity 10,000 times that of the sun. It has active Strömgren spheres, one containing ionized hydrogen and one containing ionized helium. The overlapping region should contain helium hydride which was indeed detected by a rotational spectroscopic transition at 149.1 micrometer. Main problem to tackle was the interference of another interstellar species, the methylidyne radical which has a transition in the same region. The 149.1 figure is not at all apparent from the figure 1 in the research article which we have to take as exhibit 1. Not a intensity - wavelength plot as you might think but a temperature - velocity plot with velocity in kilometers per second. With any background information lacking, do only radio astronomers read Nature? In any event, the astronomers also estimated reaction rates for helium hydride forming reactions and estimated the abundance of helium hydride with a clear peak in the overlap region.

Helium hydride is a fascinating species. in the early universe (Big Bang nucleosynthesis) with just hydrogen and helium created, the He-H bond was the first molecular bond ever formed and helium hydride the first ion. Naked helium and hydrogen first recombine with electrons (also created in the Big Bang by association of two photons) and then He+ and H combine. In a second reaction HeH+ and H can combine to form molecular hydrogen ions.

Helium hydride is isoelectronic with dihydrogen with both atoms sharing two electrons. Unlike dihydrogen the ion is very reactive and known as the strongest ever acid. (Link) Also the reason why it is only found in outer space. But why is it called a hydride when it is a cation? A hydride is a hydrogen anion as in lithium hydride and therefore the shorthand HeH+ seems out of order. In a more general definition the hydrogen atom in a hydride is bonded to a more electropositive atom. This definition does also not fit. In other literature (example here) the alternative term hydrohelium ion is used as in hydrochloric acid.