Magnesium monohydride

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Magnesium monohydride
Magnesium-monohydride-3D-balls.png
Magnesium-monohydride-3D-vdW.png
Names
IUPAC name
Magnesium monohydride
Other names
Magnesium(I) hydride, Hydridomagnesium(•)
Identifiers
3D model (JSmol)
  • InChI=1S/Mg.H
    Key: RZCHRULKKYOSQS-UHFFFAOYSA-N
  • [H].[Mg]
Properties
MgH
Molar mass 25.313 g/mol
Appearancegreen glowing gas [1]
reacts violently
Related compounds
Other cations
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Magnesium monohydride is a molecular gas with formula MgH that exists at high temperatures, such as the atmospheres of the Sun and stars. [2] It was originally known as magnesium hydride, although that name is now more commonly used when referring to the similar chemical magnesium dihydride.

Contents

History

George Downing Liveing and James Dewar are claimed to be the first to make and observe a spectral line from MgH in 1878. [3] [4] However they did not realise what the substance was. [5]

Formation

A laser can evaporate magnesium metal to form atoms that react with molecular hydrogen gas to form MgH and other magnesium hydrides. [6]

An electric discharge through hydrogen gas at low pressure (20 pascals) containing pieces of magnesium can produce MgH. [7]

Thermally produced hydrogen atoms and magnesium vapour can react and condense in a solid argon matrix. This process does not work with solid neon, probably due to the formation of MgH2 instead. [8]

A simple way to produce some MgH is to burn magnesium in a bunsen burner flame, where there is enough hydrogen to form MgH temporarily. Magnesium arcs in steam also produce MgH, but also produce MgO. [5]

Natural formation of MgH happens in stars, brown dwarfs, and large planets, where the temperature is high enough. The reaction that produces it is either 2 Mg + H2 → 2 MgH or Mg + H → MgH. Decomposition is by the reverse process. Formation requires the presence of magnesium gas. The amount of magnesium gas is greatly reduced in cool stars by its extraction in clouds of enstatite, a magnesium silicate. Otherwise in these stars, below any magnesium silicate clouds where the temperature is hotter, the concentration of MgH is proportional to the square root of the pressure, and concentration of magnesium, and 10−4236/T. MgH is the second most abundant magnesium containing gas (after atomic magnesium) in the deeper hotter parts of planets and brown dwarfs. [9] [10]

The reaction of Mg atoms with H2 (dihydrogen gas) is actually endothermic and proceeds when magnesium atoms are excited electronically. The magnesium atom inserts into the bond between the two hydrogen atoms to create a temporary MgH2 molecule, which spins rapidly and breaks up into a spinning MgH molecule and a hydrogen atom. [11] The MgH molecules produced have a bimodal distribution of rotation rates. When Protium is changed for Deuterium in this reaction the distribution of rotations remains unchanged. (Mg + D2 or Mg +HD). The low rotation rate products also have low vibration levels, and so are "cold". [12]

Properties

Spectrum

The far infrared contains the rotational spectrum of MgH ranging from 0.3 to 2 THz. This also contains hyperfine structure. [7] 24MgH is predicted to have spectral lines for various rotational transition for the following vibrational levels. [13]

rotationGHz for vibration level
0123
1-0343.68879332.92012321.68306309.86369
2-1687.10305665.59200643.11285619.46374
3-21030.07630997.76743964.03611928.54056

The infrared vibration rotation bands are in the range 800–2200 cm−1. [14] The fundamental vibration mode is at 6.7 μm. [15] Three isotopes of magnesium and two of hydrogen multiply the band spectra with six isotopomers: 24MgH 25MgH 26MgH 24MgD 25MgD 26MgD. Vibration and rotation frequencies are significantly altered by the different masses of the atoms. [14]

The visible band spectrum of magnesium hydride was first observed in the 19th century, and was soon confirmed to be due to a combination of magnesium and hydrogen. Whether there was actually a compound was debated due to no solid material being able to be produced. Despite this the term magnesium hydride was used for whatever made the band spectrum. This term was used before magnesium dihydride was discovered. The spectral bands had heads with fluting in the yellow green, green, and blue parts of the visible spectrum. [5]

The yellow green band of the MgH spectrum is around the wavelength 5622 Å. The blue band is 4845 Å [16]

The main band of MgH in the visible spectrum is due to electronic transition between the A2Π→X2Σ+ levels combined with transitions in rotational and vibrational state. [17]

For each electronic transition, there are different bands for changes between the different vibrational states. The transition between vibrational states is represented using parenthesis (n,m), with n and m being numbers. Within each band there are many lines organised into three sets called branches. The P, Q and R branch are distinguished by whether the rotational quantum number increases by one, stays the same or decreases by one. Lines in each branch will have different rotational quantum numbers depending on how fast the molecules are spinning. [18] For the A2Π→X2Σ+ transition the lowest vibrational level transitions are the most prominent, however the A2Π energy level can have a vibration quantum state up to 13. Any higher level and the molecule has too much energy and shakes apart. For each level of vibrational energy there are a number of different rates of rotation that the molecule can sustain. For level 0 the maximum rotational quantum number is 49. Above this rotation rate it would spin so fast it would break apart. Then for subsequently higher vibrational levels from 2 to 13 the number of maximum rotational levels decreasing going through the sequence 47, 44, 42, 39, 36, 33, 30, 27, 23, 19, 15, 11 and 6. [19]

The B'2Σ+→X2Σ+ system is a transition from a slightly higher electronic state to the ground state. It also has lines in the visible spectrum that are observable in sunspots. The bands are headless. The (0,0) band is weak compared to the (0,3), (0,4), (0,5), (0,6), (0,7), (1,3), (1,4), (1,7), and (1,8) vibrational bands. [15]

The C2Π state has rotational parameters of B = 6.104 cm−1, D = 0.0003176 cm −1, A = 3.843 cm−1, and p = -0.02653 cm−1. It has an energy level of 41242 cm−1. [20]

Another 2Δ electronic level has energy 42192 cm−1 and rotation parameters B = 6.2861 cm−1 and A = -0.168 cm−1. [20]

The ultraviolet has many more bands due to higher energy electronic states. [21] [22] [23]

The UV spectrum contains band heads at 3100 Å due to a vibrational transition (1,0) 2940 Å (2,0) 2720 Å (3,0) 2640 Å (0,1) 2567 Å (1,3). [24] [25] [26] [27] [28]

colourband wavelengthband headvibration transitionstrength
green4950-5330 [29] 5212(0.0)strongest
degrades to violet [30]
5182(1,1)strong
5155(2,2)strong
blue4844
yellow green56225621(0,1)quite strong
5568(1,2)weak
5516(2,3)weak
6083(0,2)weak
UV2350-23302348.8(0,0) and (1,1) Q branch of 2Π→X2Σ+violet degraded
UV2329weak violet degraded

[31]

Physical

The magnesium monohydride molecule is a simple diatomic molecule with a magnesium atom bonded to a hydrogen atom. The distance between hydrogen and magnesium atoms is 1.7297Å. [32] The ground state of magnesium monohydride is X2Σ+. [1] Due to the simple structure the symmetry point group of the molecule is C∞v. [32] The moment of inertia of one molecule is 4.805263×10−40 g cm2. [32]

The bond has significant covalent character. [33] The dipole moment is 1.215 Debye. [34] [35]

Bulk properties of the MgH gas include enthalpy of formation of 229.79 kJ mol−1, [32] entropy 193.20 J K−1 mol−1 [32] and heat capacity of 29.59 J K−1 mol−1. [32]

The dissociation energy of the molecule is 1.33 eV. [36] Ionization potential is around 7.9 eV with the MgH+ ion formed when the molecule loses an electron. [37]

Dimer

In noble gas matrices MgH can form two kinds of dimer: HMgMgH and a rhombic shaped (◊) (HMg)2 in which a dihydrogen molecule bridges the bond between two magnesium atoms. MgH also can form a complex with dihydrogen HMg·H2. Photolysis increases reactions which form the dimer. [6] The energy to break up the dimer HMgMgH into two MgH radicals is 197 kJ/mol. Mg(μ-H2)Mg has 63 kJ/mol more energy than HMgMgH. [38] In theory gas phase HMgMgH can decompose to Mg2 and H2 releasing 24 kJ/mol of energy exothermically. [38] The distance between the magnesium atoms in HMgMgH is calculated to be 2.861 Å. [39] HMgMgH can be considered a formal base compound for other substances LMgMgL that have a magnesium to magnesium bond. In these magnesium can be considered to be in oxidation state +1 rather than the normal +2. However these sort of compounds are not made from HMgMgH. [40] [41] [42]

MgH+ can be made by protons hitting magnesium, or dihydrogen gas H2 interacting with singly ionized magnesium atoms (H2 + Mg+ → MgH+ + H). [43]

MgH−, [44] MgH−3 and MgH−2 are formed from low pressure hydrogen or ammonia over a magnesium cathode. [44] The trihydride ion is produced the most, and in a greater proportion when pure hydrogen is used rather than ammonia. The dihydride ion is produced the least of the three. [44]

HMgO and HMgS have been theoretically investigated. MgOH and MgSH are lower in energy. [45]

Applications

The spectrum of MgH in stars can be used to measure the isotope ratio of magnesium, the temperature, and gravity of the surface of the star. [46] In hot stars MgH will be mostly disassociated due to the heat breaking the molecules, but it can be detected in cooler G, K and M type stars. [47] It can also be detected in starspots or sunspots. The MgH spectrum can be used to study the magnetic field and nature of starspots. [48]

Some MgH spectral lines show up prominently in the second solar spectrum, that is the fractional linear polarization. The lines belong to the Q1 and Q2 branches. The MgH absorption lines are immune to the Hanle effect where polarization is reduced in the presence of magnetic fields, such as near sunspots. These same absorption lines do not suffer from the Zeeman effect either. The reason that the Q branch shows up in this way is because Q branch lines are four times more polarizable, and twice as intense as P and R branch lines. These lines that are more polarizable are also less subject to magnetic field effects. [49]

Related Research Articles

<span class="mw-page-title-main">Diatomic molecule</span> Molecule composed of any two atoms

Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen or oxygen, then it is said to be homonuclear. Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide or nitric oxide, the molecule is said to be heteronuclear. The bond in a homonuclear diatomic molecule is non-polar.

<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">Methanium</span> Ion of carbon with five hydrogens

In chemistry, methanium is a complex positive ion with formula [CH5]+ or [CH3(H2)]+, bearing a +1 electric charge. It is a superacid and one of the onium ions, indeed the simplest carbonium ion.

In chemistry and molecular physics, fluxionalmolecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in some respects, e.g. bond rotations in most organic compounds, the term fluxional depends on the context and the method used to assess the dynamics. Often, a molecule is considered fluxional if its spectroscopic signature exhibits line-broadening due to chemical exchange. In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by isotopic labeling and other methods.

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

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.

<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">Beryllium hydride</span> Chemical compound

Beryllium hydride is an inorganic compound with the chemical formula n. This alkaline earth hydride is a colourless solid that is insoluble in solvents that do not decompose it. Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded.

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

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3. In the early universe this ability to emit infrared light allowed the primordial hydrogen and helium gas to cool down so as to form stars.

Binary compounds of hydrogen are binary chemical compounds containing just hydrogen and one other chemical element. By convention all binary hydrogen compounds are called hydrides even when the hydrogen atom in it is not an anion. These hydrogen compounds can be grouped into several types.

<span class="mw-page-title-main">Imidogen</span> Inorganic radical with the chemical formula NH

Imidogen is an inorganic compound with the chemical formula NH. Like other simple radicals, it is highly reactive and consequently short-lived except as a dilute gas. Its behavior depends on its spin multiplicity.

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

Mercury(II) hydride is an inorganic compound with the chemical formula HgH
2. It is both thermodynamically and kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it known as a white, crystalline solid, which is kinetically stable at temperatures below −125 °C (−193 °F), which was synthesised for the first time in 1951.

<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">Iron(I) hydride</span> Chemical compound

Iron(I) hydride, systematically named iron hydride and poly(hydridoiron) is a solid inorganic compound with the chemical formula (FeH)
n (also written ([FeH])
n or FeH). It is both thermodynamically and kinetically unstable toward decomposition at ambient temperature, and as such, little is known about its bulk properties.

Stellar molecules are molecules that exist or form in stars. Such formations can take place when the temperature is low enough for molecules to form – typically around 6000 K or cooler. Otherwise the stellar matter is restricted to atoms in the forms of gas or – at very high temperatures – plasma.

<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">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.

Argon compounds, the chemical compounds that contain the element argon, are rarely encountered due to the inertness of the argon atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and molecular ions containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar1C60 is stable at room temperature. Ar1C60 was discovered by the CSIRO.

<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.

The magnesium argide ion, MgAr+ is an ion composed of one ionised magnesium atom, Mg+ and an argon atom. It is important in inductively coupled plasma mass spectrometry and in the study of the field around the magnesium ion. The ionization potential of magnesium is lower than the first excitation state of argon, so the positive charge in MgAr+ will reside on the magnesium atom. Neutral MgAr molecules can also exist in an excited state.

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