chemistry

Methanium

Methanium is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Methanium rather than just read about it. In short: 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.

Key takeaways

Methanium belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Methanium to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Methanium from memory before moving on to harder problems.

Reference excerpt

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. 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. 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).

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 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 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. 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 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 SbF5 in hydrogen fluoride HF). 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:

CH4•+ + CH4 → CH+5 + CH3• The methanium ion can also be made in the gas phase via the reaction of methane and an H+ ion (i.e. a proton).

CH4 + H+(g) → CH+5

Stability and reactions 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.

Further reading D. W. Boo; Z. F. Liu; A. G. Suits; J. S. Tse; Y. T. Lee (1995). "Dynamics of Carbonium Ions Solvated by Molecular Hydrogen: CH5+(H2)n (n= 1, 2, 3)". Science. 269 (5220): 57–9. Bibcode:1995Sci...269...57B. doi:10.1126/science.269.5220.57. PMID 17787703. S2CID 2100896. Kramer, G. M. (1999). "CH5+ Stability and Mass Spectrometry". Science. 286 (5442): 1051a–1051. doi:10.1126/science.286.5442.1051a. ISSN 0036-8075. Marx, D. (1999). "MOLECULAR SPECTROSCOPY:CH5+: The Cheshire Cat Smiles". Science. 284 (5411): 59–61. Bibcode:1999Sci...284...59.. doi:10.1126/science.284.5411.59. ISSN 0036-8075. S2CID 94058015. Xiao-Gang Wang; Tucker Carrington Jr (2016). "Calculated rotation-bending energy levels of CH5+ and a comparison with experiment". Journal of Chemical Physics. 144 (20): 204304. Bibcode:2016JChPh.144t4304W. doi:10.1063/1.4948549. PMID 27250303. H. Schmiedt; Per Jensen; S. Schlemmer (2017). "Rotation-vibration motion of extremely flexible molecules - The molecular superrotor". Chemical Physics Letters. 672: 34–46. Bibcode:2017CPL...672...34S. doi:10.1016/j.cplett.2017.01.045.

Illustrations

Worked examples

Example 1 — a first encounter with Methanium

Start with the simplest possible case. Write down what Methanium claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Methanium before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Methanium ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Methanium

In research: Methanium appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Methanium in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Methanium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Carbocations, Superacids, so understanding it makes those chapters shorter.
In everyday life: Look for Methanium outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Methanium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Methanium means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Methanium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Methanium in simple terms?

Why does Methanium matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Methanium?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Methanium.