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Helium hydride ion

The meaning of «helium hydride ion»

The helium hydride ion or hydridohelium(1+) ion or helonium is a cation (positively charged ion) with chemical formula HeH+. It consists of a helium atom bonded to a hydrogen atom, with one electron removed. It can also be viewed as protonated helium. It is the lightest heteronuclear ion, and is believed to be the first compound formed in the Universe after the Big Bang.[2]

The ion was first produced in a laboratory in 1925. It is stable in isolation, but extremely reactive, and cannot be prepared in bulk, because it would react with any other molecule with which it came into contact. Noted as the strongest known acid, its occurrence in the interstellar medium had been conjectured since the 1970s,[3] and it was finally detected in April 2019 using the airborne SOFIA telescope.[4][5]

The helium hydrogen ion is isoelectronic with molecular hydrogen (H2).[6]

Unlike the dihydrogen ion H+2, the helium hydride ion has a permanent dipole moment, which makes its spectroscopic characterization easier.[7] The calculated dipole moment of HeH+ is 2.26 or 2.84 D.[8] The electron density in the ion is higher around the helium nucleus than the hydrogen. 80% of the electron charge is closer to the helium nucleus than to the hydrogen nucleus.[9]

Spectroscopic detection is hampered, because one of its most prominent spectral lines, at 149.14 μm, coincides with a doublet of spectral lines belonging to the methylidyne radical CH.[2]

The length of the covalent bond in the ion is 0.772 Å.[10]

The helium hydride ion has six relatively stable isotopologues, that differ in the isotopes of the two elements, and hence in the total atomic mass number (A) and the total number of neutrons (N) in the two nuclei:

They all have three protons and two electrons. The first three are generated by radioactive decay of tritium in the molecules HT = 1H3H, DT = 2H3H, and T2 = 3H2, respectively. The last three can be generated by ionizing the appropriate isotopologue of H2 in the presence of helium-4.[6]

The following isotopologues of the helium hydride ion, of the dihydrogen ion H+2, and of the trihydrogen ion H+3 have the same total atomic mass number A:

The masses in each row above are not equal, though, because the binding energies in the nuclei are different.[15]

Unlike the helium hydride ion, the neutral helium hydride molecule HeH is not stable in the ground state. However, it does exist in an excited state as an excimer (HeH*), and its spectrum was first observed in the mid-1980s.[18][19][20]

The neutral molecule is the first entry in the Gmelin database.[3]

Since HeH+ cannot be stored in any usable form, its chemistry must be studied by forming it in situ.

Reactions with organic substances, for example, can be studied by creating a tritium derivative of the desired organic compound. Decay of tritium to 3He+ followed by its extraction of a hydrogen atom yields 3HeH+ which is then surrounded by the organic material and will in turn react.[21][22]

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