chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, methanium is a complex positive ion with formula []⁺, namely a molecule with one carbon atom covalent bond, bonded to three hydrogen atoms and one hydrogen molecule, bearing a +1 electric charge. It is a superacid and one of the

onium ion In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake ...

s, indeed the simplest

carbonium ion In chemistry, a carbonium ion is any cation that has a pentavalent carbon atom. The name carbonium may also be used for the simplest member of the class, properly called methanium (), where the five valences are filled with hydrogen atoms. The ...

. 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 Victor Lvovich Talrose (russian: Виктор Львович Тальрозе, April 15, 1922 – June 22, 2004) was a Russian scientist and mass spectrometrist. He was known as the “Father of Russian Mass Spectrometry” and was the author of 44 ...

and his assistant Anna Konstantinovna Lyubimova. It occurs as an intermediate species in chemical reactions. The methanium ion is named after

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

(), by analogy with the derivation of

ammonium ion The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...

() from

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

().

Structure

Fluxional methanium can be visualised as a

carbenium ion A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with a trivalent carbon that bears a +1 formal charge. In older literature the name carbonium ion was used for this class, but now it refers exclusivel ...

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 ion (a conformation change called

pseudorotation In chemistry, a pseudorotation is a set of intramolecular movements of attached groups (i.e., ligands) on a highly symmetric molecule, leading to a molecule indistinguishable from the initial one. The International Union of Pure and Applied Chem ...

, specifically the

Berry mechanism The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands (see Figure at right) for two of the equatorial ones. It is the most widely ac ...

). The methanium ion is therefore considered a

fluxional molecule In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in som ...

. The

energy barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

for the exchange is quite low and occurs even at very low temperatures.

Infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...

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 Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuri ...

(

antimony pentafluoride Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

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 Methyl (also systematically named trihydridocarbon) is an organic compound with the chemical formula (also written as •). It is a metastable colourless gas, which is mainly produced ''in situ'' as a precursor to other hydrocarbons in the petrol ...

: : + → + 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) → CH₅⁺

Stability and reactions

The cations obtained by reaction of methane with + 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.

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) {{cite journal , last1 = Schreiner , first1 = Peter R. , last2 = Kim , first2 = Seung-Joon , last3 = Schaefer , first3 = Henry F. , last4 = von Ragué Schleyer , first4 = Paul , year = 1993 , title = {{chem, CH, 5, +: The never‐ending story or the final word? , journal = Journal of Chemical Physics , volume = 99 , issue = 5, pages = 3716–3720 , doi = 10.1063/1.466147 Carbocations Superacids

Structure

Preparation

Stability and reactions

See also

References