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 Victor Talrose and his assistant Anna Konstantinovna Lyubimova. It occurs as an intermediate species in chemical reactions.
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.
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.
Infrared spectroscopy has been used to obtain information about the different conformations of the methanium ion. 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.
4 + CH
4 → CH+
5 + CH•
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.
- 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)
- Nikolaev, Eugene (1998). "Victor Talrose: an appreciation". Journal of Mass Spectrometry. 33 (6): 499–501. doi:10.1002/(SICI)1096-9888(199806)33:6<499::AID-JMS684>3.0.CO;2-C. ISSN 1076-5174.
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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