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Concept
Ethanium
Résumé
In chemistry, ethanium or protonated ethane is a highly reactive positive ion with formula C2H7+. It can be described as a molecule of ethane (C2H6) with one extra proton (hydrogen nucleus), that gives it a +1 electric charge.
Ethanium is one of the simplest carbonium ions (after methanium CH5+). It was first detected as a rarefied gas in 1960 by S. Wexler and N. Jesse. It easily dissociates into ethenium C2H5+ and molecular hydrogen H2.
Ethanium was first detected by infrared spectroscopy among the ions produced by electrical discharges in rarefied methane or ethane gas.
Ethanium can also be produced by irradiating methane containing traces of ethane with an electron beam at low pressure (about 2 mmHg). The electron beam first creates methanium and methenium ions. The former rapidly transfer their proton to ethane:
CH5+ + C2H6 → CH4 + C2H7+
The latter reaction is also observed when CH5+, N2OH+ or HCO+ ions are injected into ethane at somewhat lower pressure.
At about 1 mmHg and 30 °C, ethanium dissociates very slowly to ethenium and H2, across an energy barrier of about 10 kcal/mol; the decomposition is considerably faster at 92 °C. The decomposition has been claimed to be nearly athermal but with 8 kcal/mol of free energy due to increase of entropy.
Like its "unsaturated" relatives ethenium and ethynium C2H3+, the ethanium ion was conjectured to have (at least momentarily) a proton bound simultaneously to the two carbon atoms, and the electrical charge evenly spread between them, as in other non-classical ions. The alternative "classical" structure would have the charge and the extra hydrogen bound to only one of the two atoms, i.e. a methylated methanium ion.
Earlier calculations had predicted that the energies of the two forms should be 4 to 12 kcal/mol lower than the dissociated state C2H5+ + H2, and they should be separated by a slightly positive energy barrier. Gas-phase infrared spectroscopy by Yeh and others (1989) has shown that both forms are stable. The bridged structure has the lowest energy, 4 to 8 kcal/mol lower than the classical one.
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