Methanium

Methanium (CH5+)

In chemistry, methanium is a positive ion with formula CH+
5, namely a molecule with one carbon atom bonded to five hydrogen atoms and bearing a +1 electric charge. It is a superacid and one of the onium ions, indeed the simplest carbonium ion.

Methanium can be produced in the laboratory as a rarefied gas or as a dilute species in superacids. It was prepared for the first time in 1950 and published in 1952 by Talrose and Lyubimova.[1] It occurs as an intermediate species in chemical reactions.

The methanium ion is named after methane (CH
4), by analogy with the derivation of ammonium ion (NH+
4) from ammonia (NH
3).

Structure

Methanium can be visualised as a CH3+ carbenium ion with a molecule of hydrogen interacting with the empty orbital in a 3-center-2-electron bond. The bonding electron pair in the H2 molecule is shared between the two hydrogen and one carbon atoms making up the 3-center-2-electron bond.[2]

The two hydrogen atoms in the H2 molecule can continuously exchange positions with the three hydrogen atoms in the CH3+ ion (a conformation change called pseudorotation, specifically the Berry mechanism). The methanium ion is therefore considered a fluxional molecule. The energy barrier for the exchange is quite low and occurs even at very low temperatures.[3][4]

Infrared spectroscopy has been used to obtain information about the different conformations of the methanium ion.[5][6][7] The IR spectrum of plain methane has two C-H bands from symmetric and asymmetric stretching at around 3000 cm−1 and two bands around 1400 cm−1 from symmetrical and asymmetric bending vibrations. In the spectrum of CH5+ three asymmetric stretching vibrations are present around 2800 – 3000 cm−1, a rocking vibration at 1300 cm−1, and a bending vibration at 1100 1300 cm−1.

Preparation

Methanium can be prepared from methane by the action of very strong acids, such as antimony pentafluoride SbF
5 in hydrogen fluoride HF.[8]

At about 2 Torr of pressure and ambient temperature, the methane ion CH+
4 will react with neutral methane to yield methanium and a methyl radical:[9]

CH+
4 + CH
4 → CH+
5 + CH•
3

Stability and reactions

The cations obtained by reaction of methane with SbF
5 + HF are stabilized by interactions with the HF molecules.

At low pressures (around 1 mmHg) and ambient temperatures, methanium is unreactive towards neutral methane.[9]

References

  1. ↑ V. L. Talrose and A. K. Lyubimova, Dokl. Akad. Nauk SSSR 86, 909 (1952)
  2. ↑ Golam Rasul, G.K. Surya Prakash, George A. Olah (2011), "Comparative study of the hypercoordinate carbonium ions and their boron analogs: A challenge for spectroscopists". Chemical Physics Letters, volume 517, issues 1–3, pages 1–8 doi:10.1016/j.cplett.2011.10.020
  3. ↑ Peter R. Schreiner, Seung‐Joon Kim, Henry F. Schaefer, and Paul von Ragué Schleyer (1993), "CH+
    5    : The never‐ending story or the final word?" Journal of Chemical Physics, volume 99, issue 5, page 3716-3720 doi:10.1063/1.466147
  4. ↑ Hendrik Müller, Werner Kutzelnigg, Jozef Noga, and Wim Klopper (1997), "CH5+: The story goes on. An explicitly correlated coupled-cluster study". Journal of Chemical Physics, volume 106, page 1863 doi:10.1063/1.473340
  5. ↑ Edmund T. White, Jian Tang, Takeshi Oka (1999), "CH+
    5    : The infrared spectrum observed" Science, volume 284, issue=5411, page 135, doi:10.1126/science.284.5411.135 PMID 10102811
  6. ↑ Oskar Asvany, Padma Kumar P, Britta Redlich, Ilka Hegemann, Stephan Schlemmer, Dominik Marx (2005), "Understanding the infrared spectrum of bare CH+
    5    ." Science, volume 309, issue 5738, pages = 1219–1222 doi:10.1126/science.1113729 PMID 15994376
  7. ↑ Xinchuan Huang, Anne B. McCoy, Joel M. Bowman, Lindsay M. Johnson, Chandra Savage, Feng Dong, David J. Nesbitt (2006), "Quantum deconstruction of the infrared spectrum of CH+
    5    ." Science, volume 311, issue 5757, pages 60–63 doi:10.1126/science.1121166 PMID 16400143
  8. ↑ J. Sommer and R. Jost (2000), "Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes". Pure and Applied Chemistry, volume 72, pages 2309–2318. doi:10.1351/pac200072122309
  9. 1 2 F. H. Field , M. S. B. Munson (1965), "Reactions of gaseous ions. XIV. Mass spectrometric studies of methane at pressures to 2 Torr". Journal of the American Chemical Society, volume 87, issue 15, pages 3289–3294 doi:10.1021/ja01093a001
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