Ethanium
2H+
7)
In chemistry, ethanium or protonated ethane is a positive ion with formula C
2H+
7. It can be described as a molecule of ethane (C
2H
6) with one extra hydrogen atom and a +1 electric charge.
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 lowere 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 inrease 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 (C2v) 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 Cs symmetry), an 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]
References
- ^ a b c d e
L. I. Yeh, J. M. Price, and Y. T. Lee (1989), "Infrared spectroscopy of the pentacoordinated carbonium ion C
2H+
7". Journal of the American Chemical Society, volume 111, pages 5591-5604. doi:10.1021/ja00197a015 - ^ a b
Margaret French and Paul Kebarle (1975), "Pyrolysis of C
2H+
7 and other ion-molecule reactions in methane containing traces of ethane". Canadian Journal of Chemistry, volume 53, pages 2268-2274. doi:10.1139/v75-318 - ^ a b G. I. Mackay, H. I. Schiff, D. K. Bohme (1981), "A room-temperature study of the kinetics and energetics for the protonation of ethane" Canadian Journal of Chemistry, volume 59, issue 12,pages 1771-1778. doi:10.1139/v81-265
- ^ Cite error: The named reference
Chongwas invoked but never defined (see the help page). - ^ Shigeki Obata and Kimihiko Hirao (1993), "Structure and Vibrational Analysis of Protonated Ethane C2H7+", Bulletin of the Chemical Society of Japan volume 66, issue 11, pages 3271-3282 doi:10.1246/bcsj.66.3271