Magnesium monohydride

Magnesium monohydride
Names
IUPAC name
Magnesium monohydride
Other names
Magnesium(I) hydride
Identifiers
14332-53-7 N
3D model (Jmol) Interactive image
PubChem 162791893
Properties
MgH
Molar mass 25.313 g/mol
Appearance green glowing gas[1]
reacts violently
Related compounds
Other cations
 Beryllium monohydride,
Calcium monohydride,
Strontium monohydride,
Barium monohydride,
Potassium hydride
Magnesium hydride Mg4H6
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

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 2Mg + H2 → 2MgH 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]

rotation GHz for vibration level
0 1 2 3
1-0 343.68879 332.92012 321.68306 309.86369
2-1 687.10305 665.59200 643.11285 619.46374
3-2 1030.07630 997.76743 964.03611 928.54056

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]

colour band wavelength band head vibration transition strength
green 4950-5330[29] 5212 (0.0) strongest
degrades to violet[30]
5182 (1,1) strong
5155 (2,2) strong
blue 4844
yellow green 5622 5621 (0,1) quite strong
5568 (1,2) weak
5516 (2,3) weak
6083 (0,2) weak
UV 2350-2330 2348.8 (0,0) and (1,1) Q branch of 2Π→X2Σ+ violet degraded
UV 2329 weak 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 JK−1 mol−1[32] and heat capacity of 29.59 JK−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] MgH3− and MgH2− 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 dissassociated 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]

References

  1. 1 2 Ziurys, L. M.; Barclay Jr., W. L.; Anderson, M. A. (1993). "The millimeter-wave spectrum of the MgH and MgD radicals". The Astrophysical Journal. 402: L21–L24. Bibcode:1993ApJ...402L..21Z. doi:10.1086/186690. ISSN 0004-637X.
  2. ↑ Bernath, Peter F. (October 2009). "Molecular astronomy of cool stars and sub-stellar objects". International Reviews in Physical Chemistry. 28 (4): 681–709. arXiv:0912.5085Freely accessible. Bibcode:2009IRPC...28..681B. doi:10.1080/01442350903292442.
  3. ↑ Liveing, G. D.; Dewar, J. (1878). "On the Reversal of the Lines of Metallic Vapours. No. IV". Proceedings of the Royal Society of London. 28 (190–195): 352–358. doi:10.1098/rspl.1878.0140. ISSN 0370-1662.
  4. ↑ Liveing, G. D.; Dewar, J. (1879). "On the Spectra of the Compounds of Carbon with Hydrogen and Nitrogen. No. II". Proceedings of the Royal Society of London. 30 (200–205): 494–509. doi:10.1098/rspl.1879.0152. ISSN 0370-1662.
  5. 1 2 3 Fowler, A. (1909). "The Spectrum of Magnesium Hydride" (PDF). Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 209 (441–458): 447–478. Bibcode:1909RSPTA.209..447F. doi:10.1098/rsta.1909.0017. ISSN 1364-503X.
  6. 1 2 Tague, Thomas J.; Andrews, Lester (1994). "Pulsed Laser Evaporated Magnesium Atom Reactions with Hydrogen: Infrared Spectra of Five Magnesium Hydride Molecules". The Journal of Physical Chemistry. 98 (35): 8611–8616. doi:10.1021/j100086a004. ISSN 0022-3654.
  7. 1 2 Zink, L. R.; Jennings, D. A.; Evenson, K. M.; Leopold, K. R. (1990). "Laboratory measurements for the astrophysical identification of MgH" (PDF). The Astrophysical Journal. 359: L65. Bibcode:1990ApJ...359L..65Z. doi:10.1086/185796. ISSN 0004-637X.
  8. ↑ Knight, Lon B.; Eltner, J. R. (1 May 1971). "Hyperfine Interaction and Chemical Bonding in MgH, CaH, SrH, and BaH Molecules". The Journal of Chemical Physics. 54 (9): 3875–3884. Bibcode:1971JChPh..54.3875K. doi:10.1063/1.1675441. ISSN 0021-9606.
  9. ↑ Visscher, Channon; Lodders, Katharina; Fegley, Bruce (2010). "ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. III. IRON, MAGNESIUM, AND SILICON". The Astrophysical Journal. 716 (2): 1060–1075. arXiv:1001.3639Freely accessible. Bibcode:2010ApJ...716.1060V. doi:10.1088/0004-637X/716/2/1060. ISSN 0004-637X. Pages 1065-1068 concentrate on magnesium.
  10. ↑ Liu, Dean-Kuo; Lin, King-Chuen; Chen, Jye-Jong (2000). "Reaction dynamics of Mg(4 [sup 1]S[sub 0], 3 [sup 1]D[sub 2]) with H[sub 2]: Harpoon-type mechanism for highly excited states". The Journal of Chemical Physics. 113 (13): 5302. Bibcode:2000JChPh.113.5302L. doi:10.1063/1.1290125.
  11. ↑ Liu, Dean-Kuo; Lin, King-Chuen (May 1999). "Reaction dynamics of Mg(3s4s) with H2: interference of the MgH product contribution from the lower Mg(3s3p) state". Chemical Physics Letters. 304 (5–6): 336–342. Bibcode:1999CPL...304..336L. doi:10.1016/S0009-2614(99)00332-2.
  12. ↑ Breckenridge, W.H.; Wang, Jiang-Hua (June 1987). "Dynamics of the reactions of Mg(3s3p1p1) with H2, HD, and D2: Rotational quantum state distributions of MgH (MgD) products". Chemical Physics Letters. 137 (3): 195–200. Bibcode:1987CPL...137..195B. doi:10.1016/0009-2614(87)80204-x.
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  16. ↑ Öhman, Yngve (3 June 1936). "On the bands of magnesium hydride in stellar spectra". Stockholms observatoriums annaler. 12 (8): 8. Bibcode:1936StoAn..12....8O.
  17. ↑ Balfour, W. J. (December 1970). "The A2Π→X2Σ+ Systems of 24Mg 25Mg 26Mg". The Astrophysical Journal. 162: 1031–1035. Bibcode:1970ApJ...162.1031B. doi:10.1086/150734.
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  20. 1 2 Caron, Nicholas; Tokaryk, D.; Adam, A.G.; (17 June 2014). "LASER SPECTROSCOPY OF THE C2Π (41242 cm−1) AND 2∆ (42192 cm−1) STATES OF MAGNESIUM HYDRIDE". Proceedings of the International Symposium on Molecular Spectroscopy.
  21. ↑ Turner, Louis; Wilbur Harris (1937). "The Ultraviolet Bands of Magnesium Hydride". Physical Review. 52 (6): 626–630. Bibcode:1937PhRv...52..626T. doi:10.1103/PhysRev.52.626. ISSN 0031-899X.
  22. ↑ Khan, M Aslam (1962). "MgH Bands at 2172, 2100 and 2088 and MgD Bands at 2172, 2358 and 2364 A". Proceedings of the Physical Society. 80 (1): 209–221. Bibcode:1962PPS....80..209A. doi:10.1088/0370-1328/80/1/324. ISSN 0370-1328.
  23. ↑ Pearse, R. W. B. (1929). "The Ultra-Violet Spectrum of Magnesium Hydride. 1. The Band at Formula 2430". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 122 (790): 442–455. Bibcode:1929RSPSA.122..442P. doi:10.1098/rspa.1929.0033. ISSN 1364-5021.
  24. ↑ Pearse, R. W. B. (1929). "The Ultra-Violet Spectrum of Magnesium Hydride. II. The Many-Lined Formula-System". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 125 (796): 157–179. Bibcode:1929RSPSA.125..157P. doi:10.1098/rspa.1929.0159. ISSN 1364-5021. JSTOR 95255.
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  30. ↑ Branch, David (1970). "Isotopes of Magnesium in the Sun". The Astrophysical Journal. 159: 39. Bibcode:1970ApJ...159...39B. doi:10.1086/150288. ISSN 0004-637X.
  31. ↑ Sotirovski, P. (2 July 1971). "The Molecular Spectrum of Sunspot Umbrae". Astronomy and Astrophysics. 14: 319. Bibcode:1971A&A....14..319S.
  32. 1 2 3 4 5 6 "CCCBDB Listing of experimental data for MgH (magnesium monohydride)". Retrieved 3 January 2015.
  33. ↑ Bucchino, Matthew P.; Lucy M. Ziurys (2013). "Terahertz Spectroscopy of 25MgH (X2Σ+) and 67ZnH (X2Σ+): Bonding in Simple Metal Hydrides". The Journal of Physical Chemistry A. 117 (39): 9732–9737. Bibcode:2013JPCA..117.9732B. doi:10.1021/jp3123743. ISSN 1089-5639. PMID 23517252.
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  36. ↑ Balfour, W. J.; H. M. Cartwright (December 1976). "A2Π-X2Σ+ system and dissociation energy of magnesium hydride". Astronomy and Astrophysics Supplement Series. 26: 389–397. Bibcode:1976A&AS...26..389B.
  37. ↑ Singh, P. D.; W. J. Maciel (1976). "Possibility of 24MgH+ in the solar atmosphere-high resolution rotation-vibration spectra". Solar Physics. 49 (2): 217–230. Bibcode:1976SoPh...49..217S. doi:10.1007/BF00162446. ISSN 0038-0938.
  38. 1 2 Schnepf, Andreas; Hans-Jörg Himmel (2005). "Subvalent Compounds Featuring Direct Metal-Metal Bonds: The Zn–Zn Bond in [Cp*2Zn2]". Angewandte Chemie International Edition. 44 (20): 3006–3008. doi:10.1002/anie.200500597. ISSN 1433-7851.
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  40. ↑ Green, S. P.; C. Jones, A. Stasch; Stasch, Andreas (2007). "Stable Magnesium(I) Compounds with Mg-Mg Bonds". Science. 318 (5857): 1754–1757. Bibcode:2007Sci...318.1754G. doi:10.1126/science.1150856. ISSN 0036-8075. PMID 17991827.
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  42. ↑ Liu, Yanyan; Shaoguang Li; Xiao-Juan Yang; Peiju Yang; Biao Wu (2009). "Magnesium−Magnesium Bond Stabilized by a Doubly Reduced α-Diimine: Synthesis and Structure of [K(THF)3]2[LMg−MgL] (L = [(2,6-iPr2C6H3)NC(Me)]22−)". Journal of the American Chemical Society. 131 (12): 4210–4211. doi:10.1021/ja900568c. ISSN 0002-7863. PMID 19271703.
  43. ↑ Højbjerre, K; Hansen, A K; Skyt, P S; Staanum, P F; Drewsen, M (14 May 2009). "Rotational state resolved photodissociation spectroscopy of translationally and vibrationally cold MgH ions: toward rotational cooling of molecular ions" (PDF). New Journal of Physics. 11 (5): 055026. Bibcode:2009NJPh...11e5026H. doi:10.1088/1367-2630/11/5/055026.
  44. 1 2 3 Middleton, Roy (February 1990). "A Negative Ion Cookbook" (PDF). pp. 10, 40–42. Retrieved 7 January 2015.
  45. ↑ Zaidi, A; Lahmar, S; Ben Lakhdar, Z; Diehr, M; Rosmus, P; Chambaud, G (November 2003). "Electronic structure and spectroscopy of the ground and excited states of the HMgO and HMgS radicals". Chemical Physics. 295 (1): 89–95. Bibcode:2003CP....295...89Z. doi:10.1016/j.chemphys.2003.08.010.
  46. ↑ Yadin, Benjamin; Thomas Veness; Pierandrea Conti; Christian Hill; Sergei N. Yurchenko; Jonathan Tennyson (2012). "ExoMol line lists - I. The rovibrational spectrum of BeH, MgH and CaH in theX 2Σ+state". Monthly Notices of the Royal Astronomical Society. 425 (1): 34–43. arXiv:1204.0137Freely accessible. Bibcode:2012MNRAS.425...34Y. doi:10.1111/j.1365-2966.2012.21367.x. ISSN 0035-8711.
  47. ↑ Pavlenko, Ya. V.; G. J. Harris, J. Tennyson, H. R. A. Jones, J. M. Brown, C. Hill, L. A. Yakovina; Tennyson, J.; Jones, H. R. A.; Brown, J. M.; Hill, C.; Yakovina, L. A. (2008). "The electronic bands of CrD, CrH, MgD and MgH: application to the 'deuterium test'" (PDF). Monthly Notices of the Royal Astronomical Society. 386 (3): 1338–1346. arXiv:0710.0368Freely accessible. Bibcode:2008MNRAS.386.1338P. doi:10.1111/j.1365-2966.2008.12522.x. ISSN 0035-8711. Retrieved 5 January 2015.
  48. ↑ Afram, Nadine (2008). Molecular Diagnostics of Solar and Stellar Magnetic Fields. Cuvillier Verlag. p. 95. ISBN 9783867277631. Retrieved 5 January 2015.
  49. ↑ Berdyugina, S. V.; Stenflo, J. O.; Gandorfer, A. (June 2002). "Molecular line scattering and magnetic field effects: Resolution of an enigma". Astronomy and Astrophysics. 388 (3): 1062–1078. Bibcode:2002A&A...388.1062B. doi:10.1051/0004-6361:20020587.

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