Atomic carbon
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Atomic carbon, systematically named carbon and λ0-methane, also called monocarbon, is a colourless gaseous
inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
with the
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
C (also written . It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation. Atomic carbon is the simplest form of carbon, and is also the progenitor of carbon clusters. In addition, it may be considered to be the monomer of all (condensed) carbon allotropes like graphite and diamond.


Nomenclature

The
trivial name In chemistry, a trivial name is a nonsystematic name for a chemical substance. That is, the name is not recognized according to the rules of any formal system of chemical nomenclature such as IUPAC nomenclature of inorganic chemistry, IUPAC inor ...
''monocarbon'' is the most commonly used and
preferred IUPAC name In chemical nomenclature, a preferred IUPAC name (PIN) is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for choo ...
. The systematic name ''carbon'', a valid
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
name, is constructed according to the compositional nomenclature. However, as a compositional name, it does not distinguish between different forms of pure carbon. The systematic name ''λ0-methane'', also valid IUPAC name, is constructed according to the substitutive nomenclature. Along with ''monocarbon'', this name does distinguish the titular compound as they derived using structural information about the molecule. To better reflect its structure, free atomic carbon is often written as λ2-methylium () is the
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 conven ...
resulting from the gain of by atomic carbon.


Properties


Amphotericity

A
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
can join with an electron pair of atomic carbon, and an electron pair of a Lewis base can join with atomic carbon by adduction: :: + M → C: + :L → LBecause of this donation or acceptance of an adducted electron pair, atomic carbon has Lewis amphoteric character. Atomic carbon has the capacity to donate up to two electron pairs to Lewis acids, or accept up to two pairs from Lewis bases. A proton can join with the atomic carbon by protonation: :C + → Because of this capture of the proton (), atomic carbon and its adducts of Lewis bases, such as water, also have Brønsted–Lowry basic character. Atomic carbon's conjugate acid is λ2-methylium (). : + C + Aqueous solutions of adducts are however, unstable due to hydration of the carbon centre and the λ2-methylium group to produce λ2-methanol (CHOH) or λ2-methane (), or hydroxymethylium () groups, respectively. : + C → CHOH : + → The λ2-methanol group in adducts can potentially isomerise to form
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
, or be further hydrated to form
methanediol Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound with chemical formula . It is the simplest geminal diol, and the simplest sugar alcohol. In aqueous solutions it coexists with oligomers (short polyme ...
. The hydroxymethylium group in adducts can potentially be further hydrated to form dihydroxymethylium (), or be oxidised by water to form formylium ().


Electromagnetic properties

The electrons in atomic carbon are distributed among the atomic orbitals according to the aufbau principle to produce unique quantum states, with corresponding energy levels. The state with the lowest energy level, or ground state, is a triplet diradical state (3''P''0), closely followed by 3''P''1 and 3''P''2. The next two excited states that are relatively close in energy are a singlet (1''D''2) and singlet diradical (1''S''0). The non-radical state of atomic carbon is systematically named λ2-methylidene, and the diradical states that include the ground state is named carbon(2•) or λ2-methanediyl. The 1''D''2 and 1''S''0 states lie 121.9 kJ mol−1 and 259.0 kJ mol−1 above the ground state, respectively. Transitions between these three states are formally forbidden from occurring due to the requirement of spin flipping and or electron pairing. This means that atomic carbon phosphoresces in the near-infrared region of the electromagnetic spectrum at 981.1 nm. It can also fluoresce in infrared and phosphoresce in the blue region at 873.0 nm and 461.9 nm, respectively, upon excitation by ultraviolet radiation. The different states of atomic carbon exhibits varying chemical behaviours. For example, reactions of the triplet radical with non-radical species generally involves abstraction, whereas reactions of the singlet non-radical involves not only abstraction, but also addition by insertion. : sup>2•(3''P''0) + →
HOH Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "univer ...
H+ O: 1''D''2) + →
HOH Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "univer ...
→ CO + or


Production

One method of synthesis, developed by Phil Shevlin has done the principal work in the field., is by passing a large current through two adjacent carbon rods, generating an electric arc. The way this species is made is closely related to the formation of fullerenes C60, the chief difference being that a much lower vacuum is used in atomic carbon formation. Atomic carbon is generated in the thermolysis of 5-diazotetrazole upon extrusion of 3 equivalents of dinitrogen: CN6 → :C: + 3N2 A clean source of atomic carbon can be obtained based on the
thermal decomposition Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...
of
tantalum carbide Tantalum carbides (TaC) form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaC''x'', where ''x'' usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metall ...
. In the developed source, carbon is loaded into a thin-walled
tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that is ...
tube. After being sealed, it is heated by direct
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
. The solvated carbon atoms diffuse to the outer surface of the tube and, when the temperature rises, the evaporation of atomic carbon from the surface of the tantalum tube is observed. The source provides purely carbon atoms without presence of any additional species.


Carbon suboxide decarbonylation

Atomic carbon can be produced by carbon suboxide
decarbonylation Decarbonylation is a type of organic reaction that involves loss of CO. It is often an undesirable reaction since it represents a degradation. In the chemistry of metal carbonyls, decarbonylation describes a substitution process, whereby a CO lig ...
. In this process,
carbon suboxide Carbon suboxide, or tricarbon dioxide, is an organic, oxygen-containing chemical compound with formula and structure . Its four cumulative double bonds make it a cumulene. It is one of the stable members of the series of linear oxocarbons , whi ...
decomposes to produce atomic carbon and
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
according to the equation: : → 2 CO + The process involves
dicarbon monoxide Dicarbon monoxide (C2O) is a molecule that contains two carbon atoms and one oxygen atom. It is a linear molecule that, because of its simplicity, is of interest in a variety of areas. It is, however, so extremely reactive that it is not encounte ...
as an intermediate, and occurs in two steps. Photolytic far ultraviolet radiation is needed for both decarbonylations. # → CO+ CO # CO→ CO +


Uses

Normally, a sample of atomic carbon exists as a mixture of excited states in addition to the ground-state in thermodynamic equilibrium. Each state contributes differently to the reaction mechanisms that can take place. A simple test used to determine which state is involved is to make use of the diagnostic reaction of the triplet state with O2, if the reaction yield is unchanged it indicates that the singlet state is involved. The diradical ground-state normally undergoes abstraction reactions. Atomic carbon has been used to generate "true"
carbenes In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. The term "carbene" m ...
by the abstraction of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
atoms from
carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
groups: : R2C=O + :C: → R2C: + CO Carbenes formed in this way will exhibit true carbenic behaviour. Carbenes prepared by other methods such as diazo compounds, might exhibit properties better attributed to the diazo compound used to make the carbene (which mimic carbene behaviour), rather than to the carbene itself. This is important from a mechanistic understanding of true carbene behaviour perspective.


Reactions

As atomic carbon is an electron-deficient species, it spontaneously autopolymerises in its pure form, or converts to an adduct upon treatment with a Lewis acid or base. Oxidation of atomic carbon gives carbon monoxide, whereas reduction gives λ2-methane.
Non-metal In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave different ...
s, including
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, strongly attack atomic carbon, forming divalent carbon compounds: :2  + → 2 CO Atomic carbon is highly reactive, most reactions are very exothermic. They are generally carried out in the gas phase at liquid nitrogen temperatures (77 K). Typical reactions with organic compounds include:Reactive Intermediate Chemistry, Robert A. Moss, Matthew S. Platz and Maitland Jones Jr., Wiley-Blackwell, (2004), :Insertion into a C-H bond in alkanes to form a carbene :Deoxygenation of carboxyl groups in ketones and aldehydhdes to form a carbene, 2-butanone forming 2-butanylidene. :Insertion into carbon -carbon double bonds to form a cyclopropylidene which undergoes ring-opening, a simple example being insertion into an alkene to form a cumulene. With water insertion into the O-H bond forms the carbene, H-C-OH that rearranges to formaldehyde, HCHO.


References


Further reading

* * * * * * {{Allotropes of carbon Allotropes of carbon