 | |||
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| Names | |||
|---|---|---|---|
| Systematic IUPAC name Hydridohelium(1+)[1] | |||
| Other names Helonium Helium hydride | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEBI |
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| ChemSpider |
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| 2 | |||
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| Properties | |||
| HeH+ | |||
| Molar mass | 5.01054 g·mol−1 | ||
| Conjugate base | Helium | ||
| Related compounds | |||
Related compounds | Neonium,Argonium,Kryptonium,Xenonium | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Thehelium hydride ion,hydridohelium(1+) ion, orhelonium is acation (positively charged ion) withchemical formula HeH+. It consists of ahelium atombonded to ahydrogen atom, with oneelectron removed. It can also be viewed asprotonated helium. It is the lightestheteronuclear ion, and is believed to be the first compound formed in theUniverse after theBig Bang.[3]
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 knownacid—stronger than evenfluoroantimonic acid—its occurrence in theinterstellar medium had been conjectured since the 1970s,[4] and it was finally detected in April 2019 using the airborneSOFIA telescope.[5][6]
The helium hydrogen ion isisoelectronic with molecularhydrogen (H
2).[7]
Unlike thedihydrogen ionH+
2, the helium hydride ion has a permanentdipole moment, which makes its spectroscopic characterization easier.[8] The calculated dipole moment of HeH+ is 2.26 or 2.84 D.[9] 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.[10]
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 themethylidyne radical⫶CH.[3]
The length of thecovalent bond in the ion is 0.772 Å[11] or 77.2pm.
The helium hydride ion has six relatively stableisotopologues, that differ in theisotopes of the two elements, and hence in the total atomicmass number (A) and the total number ofneutrons (N) in the two nuclei:
They all have three protons and two electrons. The first three are generated by radioactive decay oftritium in the molecules HT =1H3H, DT =2H3H, andT2 =3H2, respectively. The last three can be generated by ionizing the appropriate isotopologue ofH2 in the presence of helium-4.[7]
The following isotopologues of the helium hydride ion, of the dihydrogen ionH+2, and of thetrihydrogen ionH+3 have the same total atomic mass numberA:
The masses in each row above are not equal, though, because the binding energies in the nuclei are different.[16]
Unlike the helium hydride ion, the neutral helium hydridemolecule HeH is not stable in the ground state. However, it does exist in an excited state as anexcimer (HeH*), and its spectrum was first observed in the mid-1980s.[19][20][21]
The neutral molecule is the first entry in theGmelin database.[4]
Since HeH+ reacts with every substance, it cannot be stored in any container. As a result, its chemistry must be studied by creating itin situ.
Reactions with organic substances can be studied by substituting hydrogen in the desired organic compound withtritium. The decay of tritium to3He+ followed by its extraction of a hydrogen atom from the compound yields3HeH+, which is then surrounded by the organic material and will in turn react.[22][23]
HeH+ cannot be prepared in acondensed phase, as it woulddonate a proton to anyanion, molecule or atom that it came in contact with. It has been shown to protonateO2,NH3,SO2,H2O, andCO2, givingHO+
2,NH+
4,HSO+
2,H3O+, andHCO+
2 respectively.[22] Other molecules such asnitric oxide,nitrogen dioxide,nitrous oxide,hydrogen sulfide,methane,acetylene,ethylene,ethane,methanol andacetonitrile react, but subsequently break up due to the large amount of energy produced.[22]
In fact, HeH+ is the strongest knownacid, with aproton affinity of 177.8 kJ/mol, or apKa of −63.[24]
Additional helium atoms can attach to HeH+ to form larger clusters such as He2H+, He3H+, He4H+, He5H+ and He6H+.[22]
The dihelium hydride cation, He2H+, is formed by the reaction ofdihelium cation with molecular hydrogen:
It is a linear ion with hydrogen in the centre.[22]
The hexahelium hydride ion, He6H+, is particularly stable.[22]
Other helium hydride ions are known or have been studied theoretically. Helium dihydride ion, ordihydridohelium(1+),HeH+
2, has been observed using microwave spectroscopy.[25] It has a calculated binding energy of 25.1 kJ/mol, whiletrihydridohelium(1+),HeH+
3, has a calculated binding energy of 0.42 kJ/mol.[26]
Hydridohelium(1+), specifically[4He1H]+, was first detected indirectly in 1925 byT. 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 likeH+
,H+
2 andH+
3. They observed thatH+
3 appeared at the same beam energy (16eV) asH+
2, and its concentration increased with pressure much more than that of the other two ions. From these data, they concluded that theH+
2 ions were transferring a proton to molecules that they collided with, including helium.[7]
In 1933,K. Bainbridge usedmass 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.[16]
The first attempt to compute the structure of the HeH+ ion (specifically,[4He1H]+) by quantum mechanical theory was made by J. Beach in 1936.[27] Improved computations were sporadically published over the next decades.[28][29]
H. Schwartz observed in 1955 that the decay of the tritium moleculeT2 =3H2 should generate the helium hydride ion[3HeT]+ with high probability.
In 1963,F. Cacace at theSapienza University of Rome conceived thedecay technique for preparing and studying organicradicals andcarbenium ions.[30] In a variant of that technique, exotic species likemethanium are produced by reacting organic compounds with the[3HeT]+ that is produced by the decay ofT2 that is mixed with the desired reagents. Much of what we know about the chemistry of[HeH]+ came through this technique.[31]
In 1980, V. Lubimov (Lyubimov) at theITEP laboratory in Moscow claimed to have detected a mildly significant rest mass (30 ± 16) eV for theneutrino, by analyzing the energy spectrum of the β decay of tritium.[32] The claim was disputed, and several other groups set out to check it by studying the decay of molecular tritiumT
2. It was known that some of the energy released by that decay would be diverted to the excitation of the decay products, including[3HeT]+; and this phenomenon could be a significant source of error in that experiment. This observation motivated numerous efforts to precisely compute the expected energy states of that ion in order to reduce the uncertainty of those measurements.[citation needed] Many have improved the computations since then, and now there is quite good agreement between computed and experimental properties; including for the isotopologues[4He2H]+,[3He1H]+, and[3He2H]+.[18][13]
In 1956, M. Cantwell predicted theoretically that the spectrum of vibrations of that ion should be observable in the infrared; and the spectra of the deuterium and common hydrogen isotopologues ([3HeD]+ and[3He1H]+) should lie closer to visible light and hence easier to observe.[12] The first detection of the spectrum of[4He1H]+ was made by D. Tolliver and others in 1979, at wavenumbers between 1,700 and 1,900 cm−1.[33] In 1982, P. Bernath and T. Amano detected nine infrared lines between 2,164 and 3,158 waves per cm.[17]
HeH+ has been conjectured since the 1970s to exist in theinterstellar medium.[34] Its first detection, in the nebulaNGC 7027, was reported in an article published in the journalNature in April 2019.[5]
The helium hydride ion is formed during the decay oftritium in the molecule HT or tritium molecule T2. Although excited by the recoil from the beta decay, the molecule remains bound together.[35]
It is believed to be the first compound to have formed in the universe,[3] and is of fundamental importance in understanding the chemistry of the early universe.[36] This is because hydrogen and helium were almost the only types of atoms formed inBig Bang nucleosynthesis. Stars formed from the primordial material should contain HeH+, which could influence their formation and subsequent evolution. In particular, its strongdipole moment makes it relevant to the opacity ofzero-metallicity stars.[3] HeH+ is also thought to be an important constituent of the atmospheres of helium-rich white dwarfs, where it increases the opacity of the gas and causes the star to cool more slowly.[37]
HeH+ could be formed in the cooling gas behind dissociative shocks in dense interstellar clouds, such as the shocks caused bystellar winds,supernovae and outflowing material from young stars. If the speed of the shock is greater than about 90 kilometres per second (56 mi/s), quantities large enough to detect might be formed. If detected, the emissions from HeH+ would then be useful tracers of the shock.[38]
Several locations had been suggested as possible places HeH+ might be detected. These included coolhelium stars,[3]H II regions,[39] and denseplanetary nebulae,[39] likeNGC 7027,[36] where, in April 2019, HeH+ was reported to have been detected.[5]
