methanium

Methanium is the common name for the protonated methane ion, CH5+. It is one of the simplest hydrocarbon cations and is often cited as a prototype of non-classical, fluxional carbocations. The ion forms when methane is protonated by a strong acid or in high-energy environments such as plasmas, mass spectrometry, or certain astrophysical processes. In the gas phase, CH5+ has been the subject of extensive theoretical work and has also been investigated in matrix isolation experiments where it can be stabilized briefly long enough to be studied spectroscopically.

Its structure is not well described by a single static geometry. Most models describe a five-coordinate carbon with rapid rearrangement of hydrogens, effectively giving a delocalized bonding situation described as a network among hydrogens with the central proton. This fluxional character means that, in experiments that average over time, a simple, highly symmetric picture emerges, even though the instantaneous structure is dynamic.

CH5+ is highly reactive and short-lived, and it readily rearranges or dissociates under typical laboratory conditions. Its existence challenges classical valence concepts for hydrocarbons and serves as an important case study in gas-phase ion chemistry and computational chemistry, illustrating how bonding can differ from textbook intuition in small hydrocarbon systems.