Carbonium ion
Encyclopedia
A carbonium ion is a carbocation
of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+.
methane
and a superacid
. This ion exists as a reactive intermediate
in the interstellar medium
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. 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.
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+.
Methanium
The parent compound methanium or CH5+ is protonatedProtonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and a superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...
. This ion exists as a reactive intermediate
Reactive intermediate
In chemistry a reactive intermediate is a short-lived, high energy, highly reactive molecule. When generated in a chemical reaction it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation...
in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic 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
Carbenium ion
A carbenium ion is a carbocation of the trivalent and classical type R3C+. It is one of two types of carbocation, the other being a carbonium ion. In older literature a carbocation of the type R3C+ may still be referred to as a carbonium ion, a term that is used now for five-coordinate carbon...
with a molecule of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
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
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
has been used to obtain information about the conformations of methanium. 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 ionNon-classical ionNon-classical ions in organic chemistry are a special type of carbonium ions displaying delocalization of sigma bonds in 3-center-2-electron bonds of bridged systems. The term non-classical ion was first used by John D...
s are found in certain norbornyl systems - Onium compoundsOnium compoundsOnium compounds are cations derived by the protonation of mononuclear parent hydrides of elements of the nitrogen group , chalcogens , or halogens , and similar cations derived by the substitution of hydrogen atoms in the former by other groups, such as organic radicals, or halogens, for example...