Methanium

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True methanium, the metastable transitional state (CH5+)

In chemistry, methanium is a complex positive ion with formula [CH
3
(H
2
)
]+, namely a molecule with one carbon atom bonded to three hydrogen atoms and one hydrogen molecule, 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 Victor Talrose and his assistant Anna Konstantinovna Lyubimova.[1][2] 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[edit]

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.[3]

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.[4][5]

Infrared spectroscopy has been used to obtain information about the different conformations of the methanium ion.[6][7][8] 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[edit]

Methanium can be prepared from methane by the action of very strong acids, such as antimony pentafluoride SbF
5
in hydrogen fluoride HF.[9]

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:[10]

CH+
4
+ CH
4
CH+
5
+ CH
3

Stability and reactions[edit]

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.[10]

References[edit]

  1. ^ V. L. Talrose and A. K. Lyubimova, Dokl. Akad. Nauk SSSR 86, 909-912 (1952) (In Russian: Тальрозе, В. Л., and А. К. Любимова. "Вторичные процессы в ионном источнике масс-спектрометра." ДАН СССР 86 (1952): 909-912)
  2. ^ Nikolaev, Eugene (1998). "Victor Talrose: an appreciation". Journal of Mass Spectrometry. 33 (6): 499–501. Bibcode:1998JMSp...33..499N. ISSN 1076-5174. doi:10.1002/(SICI)1096-9888(199806)33:6<499::AID-JMS684>3.0.CO;2-C. 
  3. ^ 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
  4. ^ 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
  5. ^ 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
  6. ^ 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
  7. ^ 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
  8. ^ 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
  9. ^ 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
  10. ^ a b 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