Helium hydride ion

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Helium hydride ion
Helonium 2D labelled.png
Helium-hydride-cation-3D-SF.png
Helium-hydride-cation-3D-balls.png
Names
Systematic IUPAC name
Hydridohelium(1+) [1]
Other names
Helonium
Helium hydride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
2
  • InChI=1S/HHe/h1H/q+1 Yes check.svgY
    Key: HSFAAVLNFOAYQX-UHFFFAOYSA-N Yes check.svgY
  • [HeH+]
Properties
HeH+
Molar mass 5.01054 g·mol−1
Conjugate base Helium
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

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. [2]

Contents

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, [3] and it was finally detected in April 2019 using the airborne SOFIA telescope. [4] [5]

Physical properties

The helium hydrogen ion is isoelectronic with molecular hydrogen (H
2). [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] 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:

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]

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]

Chemical properties and reactions

Preparation

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]

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. [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

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]

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. [6]

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. [15]

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. [26] Improved computations were sporadically published over the next decades. [27] [28]

Tritium decay methods in chemistry

H. Schwartz observed in 1955 that the decay of the tritium molecule T2 = 3H2 should generate the helium hydride ion [3HeT]+ with high probability.

In 1963, F. Cacace at the Sapienza University of Rome conceived the decay technique for preparing and studying organic radicals and carbenium ions. [29] In a variant of that technique, exotic species like methanium are produced by reacting organic compounds with the [3HeT]+ that is produced by the decay of T2 that is mixed with the desired reagents. Much of what we know about the chemistry of [HeH]+ came through this technique. [30]

Implications for neutrino mass experiments

In 1980, V. Lubimov (Lyubimov) at the ITEP laboratory in Moscow claimed to have detected a mildly significant rest mass (30 ± 16) eV for the neutrino, by analyzing the energy spectrum of the β decay of tritium. [31] The claim was disputed, and several other groups set out to check it by studying the decay of molecular tritium T
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]+. [17] [12]

Spectral predictions and detection

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. [11] 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. [32] In 1982, P. Bernath and T. Amano detected nine infrared lines between 2,164 and 3,158 waves per cm. [16]

Interstellar space

HeH+ has long been conjectured since the 1970s to exist in the interstellar medium. [33] Its first detection, in the nebula NGC 7027, was reported in an article published in the journal Nature in April 2019. [4]

Natural occurrence

From decay of tritium

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]

Interstellar medium

It is believed to be the first compound to have formed in the universe, [2] and is of fundamental importance in understanding the chemistry of the early universe. [35] This is because hydrogen and helium were almost the only types of atoms formed in Big Bang nucleosynthesis. Stars formed from the primordial material should contain HeH+, which could influence their formation and subsequent evolution. In particular, its strong dipole moment makes it relevant to the opacity of zero-metallicity stars. [2] 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. [36]

HeH+ could be formed in the cooling gas behind dissociative shocks in dense interstellar clouds, such as the shocks caused by stellar 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. [37]

Several locations had been suggested as possible places HeH+ might be detected. These included cool helium stars, [2] H II regions, [38] and dense planetary nebulae, [38] like NGC 7027, [35] where, in April 2019, HeH+ was reported to have been detected. [4]

See also

Related Research Articles

<span class="mw-page-title-main">Hydrogen</span> Chemical element, symbol H and atomic number 1

Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest element and, at standard conditions, is a gas of diatomic molecules with the formula H2, sometimes called dihydrogen, but more commonly called hydrogen gas, molecular hydrogen or simply hydrogen. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Constituting approximately 75% of all normal matter, hydrogen is the most abundant chemical substance in the universe. Stars, including the Sun, primarily consist of hydrogen in a plasma state, while on Earth, hydrogen is found in water, organic compounds, and other molecular forms. The most common isotope of hydrogen consists of one proton, one electron, and no neutrons.

Helium-3 is a light, stable isotope of helium with two protons and one neutron. Other than protium, helium-3 is the only stable isotope of any element with more protons than neutrons. Helium-3 was discovered in 1939.

<span class="mw-page-title-main">Proton</span> Subatomic particle with positive charge

A proton is a stable subatomic particle, symbol
p
, H+, or 1H+ with a positive electric charge of +1 e (elementary charge). Its mass is slightly less than the mass of a neutron and 1,836 times the mass of an electron (the proton-to-electron mass ratio). Protons and neutrons, each with masses of approximately one atomic mass unit, are jointly referred to as "nucleons" (particles present in atomic nuclei).

In physical cosmology, Big Bang nucleosynthesis is the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. This type of nucleosynthesis is thought by most cosmologists to have occurred from 10 seconds to 20 minutes after the Big Bang. It is thought to be responsible for the formation of most of the universe's helium, along with small fractions of the hydrogen isotope deuterium, the helium isotope helium-3 (3He), and a very small fraction of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were produced: the heavy hydrogen isotope tritium and the beryllium isotope beryllium-7 (7Be). These unstable isotopes later decayed into 3He and 7Li, respectively, as above.

<span class="mw-page-title-main">Isotopes of hydrogen</span> Hydrogen with different numbers of neutrons

Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted 1
H
, 2
H
, and 3
H
. 1
H
 and 2
H
 are stable, while 3
H
 has a half-life of 12.32(2) years. Heavier isotopes also exist, all of which are synthetic and have a half-life of less than one zeptosecond (10−21 s). Of these, 5
H
 is the least stable, while 7
H
 is the most.

Although there are nine known isotopes of helium (2He), only helium-3 and helium-4 are stable. All radioisotopes are short-lived, the longest-lived being 6
He
 with a half-life of 806.92(24) milliseconds. The least stable is 10
He
, with a half-life of 260(40) yoctoseconds, although it is possible that 2
He
 may have an even shorter half-life.

<span class="mw-page-title-main">Trihydrogen cation</span> Polyatomic ion (H₃, charge +1)

The trihydrogen cation or protonated molecular hydrogen is a cation with formula H+
3, consisting of three hydrogen nuclei (protons) sharing two electrons.

<span class="mw-page-title-main">Nuclear binding energy</span> Minimum energy required to separate particles within a nucleus

Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. The binding energy for stable nuclei is always a positive number, as the nucleus must gain energy for the nucleons to move apart from each other. Nucleons are attracted to each other by the strong nuclear force. In theoretical nuclear physics, the nuclear binding energy is considered a negative number. In this context it represents the energy of the nucleus relative to the energy of the constituent nucleons when they are infinitely far apart. Both the experimental and theoretical views are equivalent, with slightly different emphasis on what the binding energy means.

<span class="mw-page-title-main">Positronium hydride</span> Exotic molecule consisting of a hydrogen atom bound to a positronium atom

Positronium hydride, or hydrogen positride is an exotic molecule consisting of a hydrogen atom bound to an exotic atom of positronium. Its formula is PsH. It was predicted to exist in 1951 by A Ore, and subsequently studied theoretically, but was not observed until 1990. R. Pareja, R. Gonzalez from Madrid trapped positronium in hydrogen laden magnesia crystals. The trap was prepared by Yok Chen from the Oak Ridge National Laboratory. In this experiment the positrons were thermalized so that they were not traveling at high speed, and they then reacted with H− ions in the crystal. In 1992 it was created in an experiment done by David M. Schrader and F.M. Jacobsen and others at the Aarhus University in Denmark. The researchers made the positronium hydride molecules by firing intense bursts of positrons into methane, which has the highest density of hydrogen atoms. Upon slowing down, the positrons were captured by ordinary electrons to form positronium atoms which then reacted with hydrogen atoms from the methane.

<span class="mw-page-title-main">Dihydrogen cation</span> Molecular ion

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2. It consists of two hydrogen nuclei (protons) sharing a single electron. It is the simplest molecular ion.

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<span class="mw-page-title-main">Chromium(I) hydride</span> Chemical compound

Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (CrH)
n. It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular chromium(I) hydride with the formula CrH has been isolated in solid gas matrices. The molecular hydride is very reactive. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.

<span class="mw-page-title-main">Calcium monohydride</span> Chemical compound

Calcium monohydride is a molecule composed of calcium and hydrogen with formula CaH. It can be found in stars as a gas formed when calcium atoms are present with hydrogen atoms.

<span class="mw-page-title-main">Magnesium monohydride</span> Chemical compound

Magnesium monohydride is a molecular gas with formula MgH that exists at high temperatures, such as the atmospheres of the Sun and stars. It was originally known as magnesium hydride, although that name is now more commonly used when referring to the similar chemical magnesium dihydride.

<span class="mw-page-title-main">Helium dimer</span> Chemical compound

The helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms. This chemical is the largest diatomic molecule—a molecule consisting of two atoms bonded together. The bond that holds this dimer together is so weak that it will break if the molecule rotates, or vibrates too much. It can only exist at very low cryogenic temperatures.

Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly considered that helium compounds cannot exist at all, or at least under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point of any known substance.

<span class="mw-page-title-main">Dioxidanylium</span> Ion

Dioxidanylium, which is protonated molecular oxygen, or just protonated oxygen, is an ion with formula HO+
2. It is formed when hydrogen containing substances combust, and exists in the ionosphere, and in plasmas that contain oxygen and hydrogen. Oxidation by O2 in superacids could be by way of the production of protonated molecular oxygen.

Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.

<span class="mw-page-title-main">Argonium</span> Chemical compound

Argonium (also called the argon hydride cation, the hydridoargon(1+) ion, or protonated argon; chemical formula ArH+) is a cation combining a proton and an argon atom. It can be made in an electric discharge, and was the first noble gas molecular ion to be found in interstellar space.

In chemistry, the decay technique is a method to generate chemical species such as radicals, carbocations, and other potentially unstable covalent structures by radioactive decay of other compounds. For example, decay of a tritium-labeled molecule yields an ionized helium atom, which might then break off to leave a cationic molecular fragment.

References

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