Negative methane
Negative methane is the negative ion of methane, meaning that a neutral methane molecule has captured an extra electron and become an ion with a total negative electric charge: CH4-. This kind of ion are also known as an anion and are relevant in nature because negative ions have been observed in several environments. For instance, they are confirmed in the interstellar space, in plasma, in the atmosphere of Earth and, in the ionosphere of Titan. Negative ions also hold the key for the radiocarbon dating method.
At first approximation, negative ions can not be described. More complex effects have to be considered to model them, such as Coulomb potential screening and electron correlation.
Relevance
The formation of negative methane is important from the perspective of fundamental science because it's an important property of some atomic and molecular species to form stable negative species. 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. Also, 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 also referred to as negative ion resonances. They are of metastable nature because they decay over time, releasing the extra electron. Therefore, they can act as punctual 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.
On the existence of negative methane
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 for the formation of a negative ion by a change of its nuclear configuration to form a Feshback negative ion resonance, 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[1] the fact[2] is that the anion of methane remained elusive for decades. Its existence was controversial because there did not exist direct reports of its identification, until it was first shown in 2014 in a scientific article[3] 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[3] are consistent with a 2020 quantum chemistry model,[1] where it was found that its stable configuration corresponds to a linear molecular exciplex (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
- ↑ 1.0 1.1 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. arXiv:1905.02317. Bibcode:2020PhRvL.124e6001R. doi:10.1103/PhysRevLett.124.056001. PMID 32083927 Check
|pmid=value (help). Unknown parameter|s2cid=ignored (help) - ↑ 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. Bibcode:2022IJMPB..3630004R. doi:10.1142/S0217979222300043. ISSN 0217-9792. Unknown parameter
|s2cid=ignored (help) - ↑ 3.0 3.1 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. Unknown parameter
|s2cid=ignored (help)
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