The helium hydride ion, 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. 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—stronger than even fluoroantimonic acid—its occurrence in the interstellar medium had been conjectured since the 1970s, and it was finally detected in April 2019 using the airborne SOFIA telescope.
Physical properties The helium hydrogen ion is isoelectronic with molecular hydrogen (H2). Unlike the dihydrogen ion H+2, the helium hydride ion has a permanent dipole moment, which makes its spectroscopic characterization easier. The calculated dipole moment of HeH+ is 2.26 or 2.84 D. 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. 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. The length of the covalent bond in the ion is 0.772 Å or 77.2 pm.
Isotopologues 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:
[3He1H]+ or [3HeH]+ (A = 4, N = 1) [3He2H]+ or [3HeD]+ (A = 5, N = 2) [3He3H]+ or [3HeT]+ (A = 6, N = 3; radioactive) [4He1H]+ or [4HeH]+ (A = 5, N = 2) [4He2H]+ or [4HeD]+ (A = 6, N = 3) [4He3H]+ or [4HeT]+ (A = 7, N = 4; radioactive) 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. 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:
[3HeH]+, [D2]+, [TH]+, [DH2]+ (A = 4) [3HeD]+, [4HeH]+, [DT]+, [TH2]+, [D2H]+ (A = 5) [3HeT]+, [4HeD]+, [T2]+, [TDH]+, [D3]+ (A = 6) [4HeT]+, [TD2]+, [T2H]+ (A = 7) The masses in each row above are not equal, though, because the binding energies in the nuclei are different.
Neutral molecule 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. The neutral molecule is the first entry in the Gmelin database.
Chemical properties and reactions
Preparation Since HeH+ reacts with every substance, it cannot be stored in any container. As a result, its chemistry must be studied by creating it in situ. Reactions with organic substances can be studied by substituting hydrogen in the desired organic compound with tritium. The decay of tritium to 3He+ followed by its extraction of a hydrogen atom from the compound yields 3HeH+, which is then surrounded by the organic material and will in turn react.
TR → 3He+ + R• (beta decay) 3He+ + HR → 3HeH+ + R• (hydrogen abstraction)
Acidity HeH+ cannot be prepared in a condensed phase, as it would donate a proton to any anion, molecule or atom that it came in contact with. It has been shown to protonate O2, NH3, SO2, H2O, and CO2, giving HO+2, NH+4, HSO+2, H3O+, and HCO+2 respectively. Other molecules such as nitric oxide, nitrogen dioxide, nitrous oxide, hydrogen sulfide, methane, acetylene, ethylene, ethane, methanol and acetonitrile react, but subsequently break up due to the large amount of energy produced. In fact, HeH+ is the strongest known acid, with a proton affinity of 177.8 kJ/mol, or a pKa of −63.
Other helium-hydrogen ions Additional helium atoms can attach to HeH+ to form larger clusters such as He2H+, He3H+, He4H+, He5H+ and He6H+. The dihelium hydride cation, He2H+, is formed by the reaction of dihelium cation with molecular hydrogen:
He+2 + H2 → He2H+ + H It is a linear ion with hydrogen in the centre. The hexahelium hydride ion, He6H+, is particularly stable. Other helium hydride ions are known or have been studied theoretically. Helium dihydride ion, or dihydridohelium(1+), HeH+2, has been observed using microwave spectroscopy. It has a calculated binding energy of 25.1 kJ/mol, while trihydridohelium(1+), HeH+3, has a calculated binding energy of 0.42 kJ/mol.
History
Discovery in ionization experiments Hydridohelium(1+), specifically [4He1H]+, was first detected indirectly in 1925 by T. R. Hogness and E. G. Lunn. They were injecting protons of known energy into a rarefied mixture of hydrogen and helium, in order to study the formation of hydrogen ions like H+, H+2 and H+3. They observed that H+3 appeared at the same beam energy (16 eV) as H+2, and its concentration increased with pressure much more than that of the other two ions. From these data, they concluded that the H+2 ions were transferring a proton to molecules that they collided with, including helium. In 1933, K. Bainbridge used mass spectrometry to compare the masses of the ions [4He1H]+ (helium hydride ion) and [2H21H]+ (twice-deuterated trihydrogen ion) in order to obtain an accurate measurement of the atomic mass of deuterium relative to that of helium. Both ions have 3 protons, 2 neutrons, and 2 electrons. He also compared [4He2H]+ (helium deuteride ion) with [2H3]+ (trideuterium ion), both with 3 protons and 3 neutrons.
Early theoretical studies The first attempt to compute the structure of the HeH+ ion (specifically, [4He1H]+) by quantum mechanical theory was made by J. Beach in 1936. Improved computations were sporadically published over the next decades.
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