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The stable state of negative methane. After capturing an extra electron, the methane anion evolves over time to a final stable state: a linear exciplex (H2:CH2)-.

Negative methane is the negative ion of methane, meaning that a neutral methane molecule captured an extra electron and became an ion with a total negative electric charge: CH4-. This kind of ion is also known as anion and are relevant in nature[1] because negative ions have been observed to have important roles in several environments. For instance, they are confirmed in the interstellar space,[2] in plasma,[3] in the atmosphere of Earth[4][5] and, in the ionosphere of Titan.[6] Negative ions also hold the key for the radiocarbon dating method[7]

Negative ions can not be described with conventional atomic theory. Quantum mechanical models, including more factors than solely Coulomb attraction, have to be considered to explain their stability. Such factors are Coulomb potential screening and electron correlation.[8][9]

Relevance

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Negative methane is important for fundamental science because methane was not expected to produce a stable negative state.[10] It is also relevant because the existence of its negative ion demonstrates an extra property of this powerful greenhouse gas. It is also relevant for plasma science, specially for methane-based plasma. In addition, it may be important in some atmospheric environments, where there exists methane, like in the ionosphere of satellite Titan where negative ion species have been detected.

Negative ions are metastable because they decay over time,[11] releasing the extra electron. Therefore, they can act as time-dependent-sources of thermal electrons (low energy) in plasma environments. Negative ion's ubiquitous presence in the interstellar medium, for example, prompts the question of an efficient formation mechanism since they are expected to decay over time. In addition, their extra electron is in general weakly attached to its neutral core and as a consequence, it is also expected to lose the additional electron with a large probability, prompting again the question of the mechanism of its formation.

History

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Negative methane was not identified for at least two reasons. In mass spectrometers, its characteristic mark at m/q = -16 is similar to that of the well known anion of oxygen O-. Because, oxygen is present in most mass spectrometers as a very habitual contaminant from the atmosphere, detections of any signal at this particular mark of m/q = -16 were readily attributed to the anion of oxygen and not to methane's.

Second, in chemistry, methane is the molecular isoelectronic analogous to neon gas. Since neon does not have a known stable negative ion state, methane was not expected to support an extra electron either.

However, its molecular nature allows more degrees of freedom that allow for the formation of a negative ion. By a change of its nuclear configuration to form a Feshback negative ion resonance[12] in which the electrons or nuclei of the molecule can re-arrange to form an excited state capable of supporting the extra electron.

Detection and structure

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The existence of a stable state of negative methane was first reported in 2014.[13] In this report, some of its properties were measured, like its very large average radius (3.5 Å), its long lifetime, and the electron detachment cross-section when interacting with molecules N2 and O2.

The findings of that report (an experiment)[13] are consistent with a quantum chemistry model[14] in which it was found that its stable configuration corresponds to a linear molecular exciplex[15] (CH2:H2)- which showed stability in the timescale of hundreds of ps. However, the experiment of 2014 demonstrated stability over the larger timescale of μs, and therefore, perfectly fitted to be detected by standard mass spectrometry techniques.

The mechanism of formation of CH4- is not fully understood. However, it can be elucidated that it may form under high methane density conditions and, probably, a three body collision.

Electron Affinity of Methane

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The electron affinity (Eea) of an atom or molecule (A) is the energy difference between the ground state energy of the corresponding neutral species (EA) and the ground state energy of the negative ion (EA-):[16]

In the case of CH4-, dissociation into CH2- + H2 is more likely than releasing the extra electron, therefore, the conventional definition of Eea does not apply to methane. The energy difference between CH4- and CH2- + H2, is 0.85 kcal/mol according to the available theoretical model.[14]

References

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  1. ^ Kristiansson, Moa K.; Chartkunchand, Kiattichart; Eklund, Gustav; Hole, Odd M.; Anderson, Emma K.; de Ruette, Nathalie; Kamińska, Magdalena; Punnakayathil, Najeeb; Navarro-Navarrete, José E.; Sigurdsson, Stefan; Grumer, Jon; Simonsson, Ansgar; Björkhage, Mikael; Rosén, Stefan; Reinhed, Peter (2022-10-07). "High-precision electron affinity of oxygen". Nature Communications. 13 (1): 5906. Bibcode:2022NatCo..13.5906K. doi:10.1038/s41467-022-33438-y. ISSN 2041-1723. PMC 9546871. PMID 36207329.
  2. ^ Millar, Thomas J.; Walsh, Catherine; Field, Thomas A. (2017-02-08). "Negative Ions in Space". Chemical Reviews. 117 (3): 1765–1795. doi:10.1021/acs.chemrev.6b00480. ISSN 0009-2665. PMID 28112897.
  3. ^ Stoffels, E.; Stoffels, W. W.; Kroesen, G. M. W. (May 2001). "Plasma chemistry and surface processes of negative ions". Plasma Sources Science and Technology. 10 (2): 311. Bibcode:2001PSST...10..311S. doi:10.1088/0963-0252/10/2/321. ISSN 0963-0252. S2CID 250916447.
  4. ^ Smith, David; Spanel, Patrik (July 1995). "Ions in the terrestrial atmosphere and in interstellar clouds". Mass Spectrometry Reviews. 14 (4–5): 255–278. Bibcode:1995MSRv...14..255S. doi:10.1002/mas.1280140403. ISSN 0277-7037.
  5. ^ Arnold, A. A. Viggiano, Frank (1995). "Ion Chemistry and Composition of the Atmosphere". Handbook of Atmospheric Electrodynamics, Volume I. CRC Press. doi:10.1201/9780203719503. ISBN 978-0-203-71950-3. Retrieved 2024-01-24.{{cite book}}: CS1 maint: multiple names: authors list (link)
  6. ^ Vuitton, V.; Lavvas, P.; Yelle, R. V.; Galand, M.; Wellbrock, A.; Lewis, G. R.; Coates, A. J.; Wahlund, J. -E. (2009-11-01). "Negative ion chemistry in Titan's upper atmosphere". Planetary and Space Science. Surfaces and Atmospheres of the Outer Planets, Their Satellites and Ring Systems: Part V. 57 (13): 1558–1572. Bibcode:2009P&SS...57.1558V. doi:10.1016/j.pss.2009.04.004. hdl:10044/1/18863. ISSN 0032-0633.
  7. ^ Bennett, C. L.; Beukens, R. P.; Clover, M. R.; Gove, H. E.; Liebert, R. B.; Litherland, A. E.; Purser, K. H.; Sondheim, W. E. (1977-11-04). "Radiocarbon Dating Using Electrostatic Accelerators: Negative Ions Provide the Key". Science. 198 (4316): 508–510. Bibcode:1977Sci...198..508B. doi:10.1126/science.198.4316.508. ISSN 0036-8075. PMID 17842139. S2CID 35670093.
  8. ^ Wijesundera, W. P.; Litherland, A. E. (1997-03-02). "A theoretical study of some negative ions of interest to accelerator mass spectrometry". Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 123 (1): 527–531. Bibcode:1997NIMPB.123..527W. doi:10.1016/S0168-583X(96)00427-2. ISSN 0168-583X.
  9. ^ Massey, H. S. W. (1979-01-01), "Negative Ions", in Bates, David R.; Bederson, Benjamin (eds.), Advances in Atomic and Molecular Physics Volume 15, vol. 15, Academic Press, pp. 1–36, doi:10.1016/S0065-2199(08)60293-6, ISBN 978-0-12-003815-2, retrieved 2024-01-24
  10. ^ Khamesian, Marjan; Douguet, Nicolas; Fonseca dos Santos, Samantha; Dulieu, Olivier; Raoult, Maurice; Brigg, Will J.; Kokoouline, Viatcheslav (2016-09-13). "Formation of CN-, C3N- and, C5N- Molecules by Radiative Electron Attachment and their Destruction by Photodetachment". Physical Review Letters. 117 (12): 123001. doi:10.1103/PhysRevLett.117.123001. PMID 27689267.
  11. ^ Esaulov, Vladimir A. (2017-07-24), Autodetaching States of Atomic Negative Ions, arXiv:1707.07661
  12. ^ Berrah, N.; Bilodeau, R. C.; Dumitriu, I.; Toffoli, D.; Lucchese, R. R. (2011-01-01). "Shape and Feshbach resonances in inner-shell photodetachment of negative ions". Journal of Electron Spectroscopy and Related Phenomena. Electron Spectroscopy Kai Siegbahn Memorial Volume. 183 (1): 64–69. doi:10.1016/j.elspec.2010.03.005. ISSN 0368-2048.
  13. ^ a b Hernández, E M; Hernández, L; Martínez-Flores, C; Trujillo, N; Salazar, M; Chavez, A; Hinojosa, G (2014-02-04). "Electron detachment cross sections of CH4- colliding with O2 and N2 below 10 keV energies". Plasma Sources Science and Technology. 23 (1): 015018. doi:10.1088/0963-0252/23/1/015018. ISSN 0963-0252. S2CID 100044016.
  14. ^ a b Ramírez-Solís, Alejandro; Vigué, Jacques; Hinojosa, Guillermo; Saint-Martin, Humberto (2020-02-05). "Solving the CH4- Riddle: The Fundamental Role of Spin to Explain Metastable Anionic Methane". Physical Review Letters. 124 (5): 056001. arXiv:1905.02317. Bibcode:2020PhRvL.124e6001R. doi:10.1103/PhysRevLett.124.056001. PMID 32083927. S2CID 146808326.
  15. ^ Gordon, Michael (2012-12-02). The Exciplex. Elsevier. ISBN 978-0-323-15286-0.
  16. ^ Chartkunchand, Kiattichart (May 2015). "Photodetachment Studies Of Atomic Negative Ions Through Velocity-Map Imaging Spectroscopy" (PDF).
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