
Negative methane is the
negative ion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
of
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
, meaning that a neutral methane molecule captured an extra electron and became an ion with a total negative electric charge: CH
4−. This kind of
ion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
is also known as anion and are relevant in
nature
Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
because negative ions have been observed to have important roles in several environments. For instance, they are confirmed in the
interstellar space, in
plasma, in the
atmosphere of Earth
The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weather ...
and, in the ionosphere of
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
. Negative ions also hold the key for the
radiocarbon dating
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for Chronological dating, determining the age of an object containing organic material by using the properties of carbon-14, radiocarbon, a radioactive Isotop ...
method
Negative ions can not be described with conventional atomic theory.
Quantum mechanical
Quantum mechanics is the fundamental physical theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is the foundation of a ...
models, including more factors than solely Coulomb attraction, have to be considered to explain their stability. Such factors are
Coulomb potential
Electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as electric potential energy per unit of electric charge. More precisely, electric potential is the amount of work (physic ...
screening and
electron correlation
Electronic correlation is the interaction between electrons in the electronic structure of a quantum system. The correlation energy is a measure of how much the movement of one electron is influenced by the presence of all other electrons.
At ...
.
Relevance
Negative
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
is important for
fundamental science
Basic research, also called pure research, fundamental research, basic science, or pure science, is a type of scientific research with the aim of improving scientific theories for better understanding and prediction of natural or other phenome ...
because methane was not expected to produce a stable negative state. It is also relevant because the existence of its negative ion demonstrates an extra property of this powerful
greenhouse gas
Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...
. It is also relevant for plasma science, specially for methane-based
plasma. In addition, it may be important in some
atmospheric
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...
environments, where there exists methane, like in the ionosphere of satellite
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
where negative ion species have been detected.
Negative ions are
metastable
In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy.
A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...
because they decay over time, 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
Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...
. 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
Negative methane was not identified for at least two reasons. In
mass spectrometers
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...
, 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
Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...
analogous to
neon
Neon is a chemical element; it has symbol Ne and atomic number 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with approximately two-thirds the density of ...
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
In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently. For example, a point in the plane has two degrees of freedom for translation: its two coordinates; a non-infinite ...
that allow 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
In quantum mechanics
Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...
capable of supporting the extra electron.
Detection and structure
The existence of a stable state of negative methane was first reported in 2014.
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
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture and engineering 3D
* Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
...
when interacting with molecules
N2 and
O2.
The findings of that report (an experiment)
are consistent with a
quantum chemistry
Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions ...
model
in which it was found that its stable configuration corresponds to a linear molecular exciplex (CH
2:H
2)
− which showed stability in the timescale of hundreds of
ps. However, the experiment of 2014 demonstrated stability over the larger timescale of
μs
A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available.
A microsecond is to one second, ...
, and therefore, perfectly fitted to be detected by standard
mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
techniques.
The mechanism of formation of CH
4− 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
The
electron affinity
The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.
::X(g) + e− → X−(g) + energy
This differs by si ...
(''E
ea'') of an atom or molecule (''A'') is the energy difference between the
ground state
The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...
energy of the corresponding neutral species (''E
A'') and the ground state energy of the negative ion (''E
A−''):
In the case of CH
4−,
dissociation into CH
2− +
H2 is more likely than releasing the extra electron, therefore, the conventional definition of ''
Eea'' does not apply to
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
. The energy difference between CH
4− and CH
2− + H
2, is 0.85 kcal/mol according to the available theoretical model.
References
{{Reflist
Methane
Anions