Chemistry:Negative methane

From HandWiki
Short description: Negative ion of methane
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 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]

At first approximation, negative ions can not be described with conventional atomic models. More complex effects have to be considered to model them, such as Coulomb potential screening and electron correlation.[8][9]

Relevance

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 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.

Existence

Negative methane was not believed to exist in a stable state 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, methane happens to be isoelectronic to neon. Since Ne does not have a known sufficient-stable negative ion state, methane was not expected to support an extra electron either.

However, its molecular nature allows more degrees of freedom 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 possible structure

Although it has been claimed that several experiments had detected negative methane[13] the fact[14] is that the anion of methane remained elusive for decades. Its existence was controversial because there did not existed direct reports of its identification, until it was first shown in 2014[15] in which some of its structural characteristics were measured, like its very large average radius (3.5 Å) its long stability and the electron detachment cross-section when interacting with N2 and O2.

The findings of this experiment[15] are consistent with a 2020 quantum chemistry model[13] in which it was found that its stable configuration corresponds to a linear molecular exciplex[16] (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.

References

  1. ↑ Kristiansson, Moa K.; Chartkunchand, Kiattichart; Eklund, Gustav; Hole, Odd M.; Anderson, Emma K.; de Ruette, Nathalie; Kamińska, Magdalena; Punnakayathil, Najeeb et al. (2022-10-07). "High-precision electron affinity of oxygen" (in en). Nature Communications 13 (1): 5906. doi:10.1038/s41467-022-33438-y. ISSN 2041-1723. PMID 36207329. Bibcode: 2022NatCo..13.5906K. 
  2. ↑ Millar, Thomas J.; Walsh, Catherine; Field, Thomas A. (2017-02-08). "Negative Ions in Space" (in en). Chemical Reviews 117 (3): 1765–1795. doi:10.1021/acs.chemrev.6b00480. ISSN 0009-2665. PMID 28112897. https://pubs.acs.org/doi/10.1021/acs.chemrev.6b00480. 
  3. ↑ Stoffels, E.; Stoffels, W. W.; Kroesen, G. M. W. (May 2001). "Plasma chemistry and surface processes of negative ions" (in en). Plasma Sources Science and Technology 10 (2): 311. doi:10.1088/0963-0252/10/2/321. ISSN 0963-0252. Bibcode: 2001PSST...10..311S. https://dx.doi.org/10.1088/0963-0252/10/2/321. 
  4. ↑ Smith, David; Spanel, Patrik (July 1995). "Ions in the terrestrial atmosphere and in interstellar clouds" (in en). Mass Spectrometry Reviews 14 (4–5): 255–278. doi:10.1002/mas.1280140403. ISSN 0277-7037. Bibcode: 1995MSRv...14..255S. https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/mas.1280140403. 
  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. https://www.taylorfrancis.com/chapters/edit/10.1201/9780203719503-1/ion-chemistry-composition-atmosphere-viggiano-frank-arnold. Retrieved 2024-01-24. 
  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. doi:10.1016/j.pss.2009.04.004. ISSN 0032-0633. Bibcode: 2009P&SS...57.1558V. https://www.sciencedirect.com/science/article/pii/S0032063309001068. 
  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" (in en). Science 198 (4316): 508–510. doi:10.1126/science.198.4316.508. ISSN 0036-8075. PMID 17842139. Bibcode: 1977Sci...198..508B. https://www.science.org/doi/10.1126/science.198.4316.508. 
  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. doi:10.1016/S0168-583X(96)00427-2. ISSN 0168-583X. Bibcode: 1997NIMPB.123..527W. https://www.sciencedirect.com/science/article/pii/S0168583X96004272. 
  9. ↑ Massey, H. S. W. (1979-01-01), "Negative Ions", in Bates, David R.; Bederson, Benjamin, Advances in Atomic and Molecular Physics Volume 15, 15, Academic Press, pp. 1–36, doi:10.1016/S0065-2199(08)60293-6, ISBN 978-0-12-003815-2, https://www.sciencedirect.com/science/article/pii/S0065219908602936, 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 
  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. https://www.sciencedirect.com/science/article/pii/S0368204810000502. 
  13. ↑ 13.0 13.1 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. doi:10.1103/PhysRevLett.124.056001. PMID 32083927. Bibcode: 2020PhRvL.124e6001R. https://link.aps.org/doi/10.1103/PhysRevLett.124.056001. 
  14. ↑ Ramírez-Solís, Alejandro; Hinojosa, Guillermo; Saint-Martin, Humberto (2022-09-20). "The quest for negative methane: The CH4− anion". International Journal of Modern Physics B 36 (23): 2230004. doi:10.1142/S0217979222300043. ISSN 0217-9792. Bibcode: 2022IJMPB..3630004R. https://www.worldscientific.com/doi/10.1142/S0217979222300043. 
  15. ↑ 15.0 15.1 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. https://iopscience.iop.org/article/10.1088/0963-0252/23/1/015018. 
  16. ↑ Gordon, Michael (2012-12-02) (in en). The Exciplex. Elsevier. ISBN 978-0-323-15286-0. https://books.google.com/books?id=TwiGwmJCDUgC&dq=exciplex&pg=PP1. 



Retrieved from "https://handwiki.org/wiki/index.php?title=Chemistry:Negative_methane&oldid=3321200"