English: Schematic "roadkill" diagram of the methanium cation (positive ion) CH₅⁺, with a pentavalent carbon, which may be considered to be to methane what ammonium is to ammonia. (The cation is often called "methonium" but that seems to be incorrect by IUPAC rules.) The five hydrogen atoms seem to be equivalent in the molecule, which is explained as a dynamic equilibrium between at least 120 congruent conformations, all with the same energy. These configurations have bilateral symmetry, and are separated by very low energy barriers; so that the cation switches continuousy between these conformations by pseudorotations. It is considered an example of a fluxional molecule. Ref: "CH5+: The story goes on. An explicitly correlated coupled-cluster study" Hendrik Müller, Werner Kutzelnigg, Jozef Noga, and Wim Klopper J. Chem. Phys. 106, 1863 (1997); doi: 10.1063/1.473340