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]
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.[18][19][20]
The neutral molecule is the first entry in the Gmelin database.[3]
Acidity[edit]
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 O2H+, NH+
4, HSO+
2, H3O+, and HCO+
2 respectively.[21] Other molecules such as nitric oxide, nitrogen dioxide, nitrous oxide, hydrogen sulfide, methane, acetylene, ethylene, ethane, methanol and acetonitrile react but break up due to the large amount of energy produced.[21]
In fact, HeH+ is the strongest known acid, with a proton affinity of 177.8 kJ/mol.[23] The hypothetical aqueous acidity can be estimated using Hess's law:
HeH+(g) → H+(g) + He(g) +178 kJ/mol [23] HeH+(aq) → HeH+(g) +973 kJ/mol (a) H+(g) → H+(aq) −1530 kJ/mol He(g) → He(aq) +19 kJ/mol (b) HeH+(aq) → H+(aq) + He(aq) −360 kJ/mol
(a) Estimated to be same as for Li+(aq) → Li+(g).
(b) Estimated from solubility data.
A free energy change of dissociation of −360 kJ/mol is equivalent to a pKa of −63 at 298 K.
Other helium-hydrogen ions[edit]
Additional helium atoms can attach to HeH+ to form larger clusters such as He2H+, He3H+, He4H+, He5H+ and He6H+.[21]
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.[21]
The hexahelium hydride ion, He6H+, is particularly stable.[21]
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.[24] 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.[25]
Hydridohelium(1+), specifically [4
He1
H]+, 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.[6]
In 1933, K. Bainbridge used mass spectrometry to compare the masses of the ions [4
He1
H]+ (helium hydride ion) and [2
H
21
H]+ (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 [4
He2
H]+(helium deuteride ion) with [2
H
3]+ (trideuterium ion), both with 3 protons and 3 neutrons.[15]
From decay of tritium[edit]
The helium hydride ion is formed during the decay of tritium in the molecule HT or tritium molecule T2. Although excited by the recoil from the beta decay, the molecule remains bound together.[34]
https://en.wikipedia.org/wiki/Helium_hydride_ion
Dihydrogen Cation
The dihydrogen cation or hydrogen molecular ion is a cation (positive ion) with formula H+
2. It consists of two hydrogen nuclei (protons) sharing a single electron. It is the simplest molecular ion.
The ion can be formed from the ionization of a neutral hydrogen molecule H
2. It is commonly formed in molecular clouds in space, by the action of cosmic rays.
The dihydrogen cation is of great historical and theoretical interest because, having only one electron, the equations of quantum mechanics that describe its structure can be solved in a relatively straightforward way. The first such solution was derived by Ø. Burrau in 1927,[1] just one year after the wave theory of quantum mechanics was published.
https://en.wikipedia.org/wiki/Dihydrogen_cation
Tritium (/ˈtrɪtiəm/ or /ˈtrɪʃiəm/, from Ancient Greek τρίτος (trítos) 'third') or hydrogen-3 (symbol T or 3H) is a rare and radioactiveisotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of the common isotope hydrogen-1 (protium) contains just one proton, and that of hydrogen-2 (deuterium) contains one proton and one neutron.
Naturally occurring tritium is extremely rare on Earth. The atmosphere has only trace amounts, formed by the interaction of its gases with cosmic rays. It can be artificially produced by irradiating lithium metal or lithium-bearing ceramic pebbles in a nuclear reactor, and is a low-abundance byproduct in normal operations of nuclear reactors.
Tritium is used as the energy source in radioluminescent lights for watches, gun sights, numerous instruments and tools, and even novelty items such as self-illuminating key chains. It is used in a medical and scientific setting as a radioactive tracer. Tritium is also used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and in hydrogen bombs.
https://en.wikipedia.org/wiki/Tritium
The trihydrogen cation or protonated molecular hydrogen is a cation (positive ion) with formula H+
3, consisting of three hydrogen nuclei (protons) sharing two electrons.
The trihydrogen cation is one of the most abundant ions in the universe. It is stable in the interstellar medium (ISM) due to the low temperature and low density of interstellar space. The role that H+
3 plays in the gas-phase chemistry of the ISM is unparalleled by any other molecular ion.
The trihydrogen cation is the simplest triatomic molecule, because its two electrons are the only valence electrons in the system. It is also the simplest example of a three-center two-electron bond system.
https://en.wikipedia.org/wiki/Trihydrogen_cation
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