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.
History George Downing Liveing and James Dewar are claimed to be the first to make and observe a spectral line from MgH in 1878. However they did not realise what the substance was.
Formation A laser can evaporate magnesium metal to form atoms that react with molecular hydrogen gas to form MgH and other magnesium hydrides. An electric discharge through hydrogen gas at low pressure (20 pascals) containing pieces of magnesium can produce MgH. 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. 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. 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. 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. 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".
Properties
Spectrum The far infrared contains the rotational spectrum of MgH ranging from 0.3 to 2 THz. This also contains hyperfine structure. 24MgH is predicted to have spectral lines for various rotational transition for the following vibrational levels.
The infrared vibration rotation bands are in the range 800–2200 cm−1. The fundamental vibration mode is at 6.7 μm. 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. 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. The yellow green band of the MgH spectrum is around the wavelength 5622 Å. The blue band is 4845 Å 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. 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. 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. 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. 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. Another 2Δ electronic level has energy 42192 cm−1 and rotation parameters B = 6.2861 cm−1 and A = -0.168 cm−1. The ultraviolet has many more bands due to higher energy electronic states. 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).
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