Methanium

Methanium

"True" methanium, the metastable transitional state [CH 5]+
Fluxional methanium, [CH 3(H 2)]+
Names
IUPAC name Methanium
Other names carbonium (discouraged due to multiple definitions)[1]
Identifiers
3D model (JSmol)	true methanium: Interactive image fluxional methanium: Interactive image
PubChem CID	21881157
InChI true methanium: InChI=1S/CH5/h1H5/q+1 Key: PXOFOHGGCICFQD-UHFFFAOYSA-N fluxional methanium: InChI=1S/CH5/c1-2/h2H,1H3/q+1 Key: AJLDAZFHECSILY-UHFFFAOYSA-N
SMILES true methanium: [C+H5] fluxional methanium: [CH3+].[HH]
Properties
Chemical formula	CH+ 5
Molar mass	17.051 g·mol−1
Conjugate base	Methane
Structure
Molecular shape	trigonal bipyramidal
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

In chemistry, methanium is a complex positive ion with formula [[[Carbon|C]]H
5]+
(metastable transitional form, a carbon atom covalently bonded to five hydrogen atoms) or [CH
3(H
2)]+
(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.

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 (CH
4), by analogy with the derivation of ammonium ion (NH+
4) from ammonia (NH
3).

Structure

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 H
2 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 H
2 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.

Preparation

Methanium can be prepared from methane by the action of very strong acids, such as fluoroantimonic acid (antimony pentafluoride SbF
5 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]

CH+
4 + CH
4 → CH+
5 + CH
3•

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]}

CH
4 + H+
(g) → CH+
5

Stability and reactions

The cations obtained by reaction of methane with SbF
5 + 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]

See also

• Methenium
• Ammonium
• Ethanium

References

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