Methanium

Structure

Fluxional methanium can be visualised as a 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 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 molecule can continuously exchange positions with the three hydrogen atoms in the 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 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 in hydrogen fluoride HF).

At about 270 Pa of pressure and ambient temperature, the methane ion will react with neutral methane to yield methanium and a methyl radical: : The methanium ion can also be made in the gas phase via the reaction of methane and an ion (i.e. a proton).

:

Stability and reactions

The cations obtained by reaction of methane with , 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.

References

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)

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