Carbonium Ion - Methanium Cation

Methanium Cation

The simplest carbonium ion, methanium (protonated methane, with the formula CH₅+), is a superacid. This ion exists as a reactive intermediate in the interstellar medium and can be produced in the laboratory as a dilute, low-temperature gas. This molecular ion can be visualised as a CH₃+ 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 CH₃+ ion; 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 CH₅+ 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.