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A carbonium ion is a carbocation of the penta- or tetracoordinated nonclassical type such as an ion of the type R₅C⁺.^[1]

Methanium

The parent compound methanium or CH₅⁺ is protonated methane and a superacid. This ion exists as a reactive intermediate in interstellar space and can be produced in the laboratory in low concentrations in the gas phase at low temperatures. This compound is considered a CH₃⁺ carbenium ion with a molecule of hydrogen interacting with the empty orbital in a 3-center-2-electron bond. The two hydrogen atoms in H₂ can continuously exchange positions with the three hydrogen atoms in the CH₃⁺ so the methanium ion is 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 conformations of methanium.^[2][3] 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.

See also

• More carbonium ions called non-classical ions are found in certain norbornyl systems
• Onium compounds

References

1. ^ George Andrew Olah (1972). "Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions". J. Am. Chem. Soc. 94 (3): 808–820. doi:10.1021/ja00758a020. 
2. ^ Oskar Asvany, Padma Kumar P, Britta Redlich, Ilka Hegemann, Stephan Schlemmer, Dominik Marx (2005). "Understanding the Infrared Spectrum of Bare CH5+". Science 309: 1219. doi:10.1126/science.1113729. 
3. ^ Xinchuan Huang, Anne B. McCoy, Joel M. Bowman, Lindsay M. Johnson, Chandra Savage, Feng Dong, David J. Nesbitt (2006). "Quantum Deconstruction of the Infrared Spectrum of CH5+". Science 311 (5757): 60–63. doi:10.1126/science.1121166. 

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