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| Identifiers | |
|---|---|
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3D model (JSmol)
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| ChEBI |
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CompTox Dashboard (EPA)
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| Properties | |
| H2+ | |
| Molar mass | 2.015 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The dihydrogen cation or hydrogen molecular ion is a cation (positive ion) with formula H+
2. It consists of two hydrogen nuclei (protons), each sharing a single electron. It is the simplest molecular ion.
The ion can be formed from the ionization of a neutral hydrogen molecule (H
2) by electron impact. It is commonly formed in molecular clouds in space by the action of cosmic rays.
The dihydrogen cation is of great historical, theoretical, and experimental interest. Historically it is of interest because, having only one electron, the equations of quantum mechanics that describe its structure can be solved approximately in a relatively straightforward way, as long as the motion of the nuclei and relativistic and quantum electrodynamic effects are neglected. The first such solution was derived by Ø. Burrau in 1927,[1] just one year after the wave theory of quantum mechanics was published.
The theoretical interest arises because an accurate mathematical description, taking into account the quantum motion of all constituents and also the interaction of the electron with the radiation field, is feasible. The description's accuracy has steadily improved over more than half a century, eventually resulting in a theoretical framework allowing ultra-high-accuracy predictions for the energies of the rotational and vibrational levels in the electronic ground state, which are mostly metastable.
In parallel, the experimental approach to the study of the cation has undergone a fundamental evolution with respect to earlier experimental techniques used in the 1960s and 1980s. Employing advanced techniques, such as ion trapping and laser cooling, the rotational and vibrational transitions can be investigated in extremely fine detail. The corresponding transition frequencies can be precisely measured and the results can be compared with the precise theoretical predictions. Another approach for precision spectroscopy relies on cooling in a cryogenic magneto-electric trap (Penning trap); here the cations' motion is cooled resistively and the internal vibration and rotation decays by spontaneous emission. Then, electron spin resonance transitions can be precisely studied.
These advances have turned the dihydrogen cations into one more family of bound systems relevant for the determination of fundamental constants of atomic and nuclear physics, after the hydrogen atom family (including hydrogen-like ions) and the helium atom family.[2]
- ^ Cite error: The named reference
burr1927was invoked but never defined (see the help page). - ^ Schiller, S. (2022). "Precision spectroscopy of the molecular hydrogen ions: an introduction". Contemporary Physics. 63: 247–279.