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Ion of carbon with five hydrogens From Wikipedia, the free encyclopedia
In chemistry, methanium is a complex positive ion with formula [CH5]+ (metastable transitional form, a carbon atom covalently bonded to five hydrogen atoms) or [CH3(H2)]+ (fluxional form, namely a molecule with one carbon atom covalently bonded to three hydrogen atoms and one dihydrogen molecule), bearing a +1 electric charge. It is a superacid and one of the onium ions, indeed the simplest carbonium ion.
"True" methanium, the metastable transitional state [CH5]+ | |
 Fluxional methanium, [CH3(H2)]+ | |
| Names | |
|---|---|
| IUPAC name
Methanium | |
| Other names
carbonium (discouraged due to multiple definitions)[1] | |
| Identifiers | |
3D model (JSmol) |
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PubChem CID |
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| Properties | |
| CH+5 | |
| Molar mass | 17.051 g·mol−1 |
| Conjugate base | Methane |
| Structure | |
| trigonal bipyramidal | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
It is highly unstable and highly reactive even upon having a complete octet, thus granting its superacidic properties.
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.[2][3] It occurs as an intermediate species in chemical reactions.
The methanium ion is named after methane (CH4), by analogy with the derivation of ammonium ion (NH+4) from ammonia (NH3).
Fluxional methanium can be visualised as a CH+3 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.[4]
The two hydrogen atoms in the H2 molecule can continuously exchange positions with the three hydrogen atoms in the CH+3 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.[5][6]
Infrared spectroscopy has been used to obtain information about the different conformations of the methanium ion.[7][8][9] 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 CH+5 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.
Methanium can be prepared from methane by the action of very strong acids, such as fluoroantimonic acid (antimony pentafluoride SbF5 in hydrogen fluoride HF).[10]
At about 270 Pa of pressure and ambient temperature, the methane ion CH+4 will react with neutral methane to yield methanium and a methyl radical:[11]
The methanium ion can also be made in the gas phase via the reaction of methane and an H+ ion (i.e. a proton).[citation needed]
The cations obtained by reaction of methane with SbF5 + HF, including methanium, are stabilized by interactions with the HF molecules.
At low pressures (around 1 mmHg) and ambient temperatures, methanium is unreactive towards neutral methane.[11]
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