Ethanium

Not to be confused with Ethenium.

In chemistry, ethanium or protonated ethane is a highly reactive positive ion with formula C
₂H⁺
₇. It can be described as a molecule of ethane (C
₂H
₆) with one extra proton (hydrogen nucleus), that gives it a +1 electric charge.

Ethanium (C
₂H⁺
₇)

Ethanium is one of the simplest carbonium ions (after methanium CH⁺
₅). It was first detected as a rarefied gas in 1960 by S. Wexler and N. Jesse.^[1] It easily dissociates into ethenium C
₂H⁺
₅ and molecular hydrogen H
₂.

Production

Ethanium was first detected by infrared spectroscopy among the ions produced by electrical discharges in rarefied methane or ethane gas.^[1]

Ethanium can also be produced by irradiating methane containing traces of ethane with an electron beam at low pressure (about 2 mmHg).^[2] The electron beam first creates methanium and methenium ions. The former rapidly transfer their proton to ethane:

CH⁺
₅ + C
₂H
₆ → CH
₄ + C
₂H⁺
₇

The latter reaction is also observed when CH⁺
₅, N
₂OH⁺
or HCO⁺
ions are injected into ethane at somewhat lower pressure.^[3]

Stability and reactions

At about 1 mmHg and 30 °C, ethanium dissociates very slowly to ethenium and H
₂, across an energy barrier of about 10 kcal/mol; the decomposition is considerably faster at 92 °C.^[2][3] The decomposition has been claimed to be nearly athermal but with 8 kcal/mol of free energy due to increase of entropy.^[4]

Structure

Like its "unsaturated" relatives ethenium and ethynium C
₂H⁺
₃, 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 C
₂H⁺
₅ + H
₂, and they should be separated by a slightly positive energy barrier.^[1] Gas-phase infrared spectroscopy by Yeh and others (1989) has shown that both forms are stable.^[1] The bridged structure has the lowest energy, 4 to 8 kcal/mol lower than the classical one.^[1]

Refined calculations by Obata and Hirao (1993) predict that the most stable form has three orthogonal planes of symmetry (C_2v) with the two CH
₃ subgroups in the eclipsed configuration (unlike ethane, whose ground state has the staggered configuration). Four "bottom" H atoms lie on a plane opposite to the bridging H atom and the other two "top" H atoms. The approximate computed distances are C–C 0.211 nm, C–H 0.124 nm (bridging), 0.107 nm (bottom) and 0.108 nm (top); the C–H–C angle at the bridge is about 116 degrees, the H–C–H angles are 116 degrees (bottom-bottom) and 114 degrees (bottom-top). However, there are other configurations with near-minimum energy, including one where the two CH
₃ subgroups are slightly staggered (with C_s symmetry), another where one of the carbons of a C
₂H⁺
₅ ion is loosely bound to an H
₂ molecule 0.250 nm away.^[5]

See also

• Doubly protonated ethane C
  ₂H²⁺
  ₈ ^[6]