Diatomic carbon
Encyclopedia
Diatomic carbon is a diatomic molecule of carbon
(C2), which occurs in electric arc
s; in comet
s, stellar atmosphere
s and the interstellar medium
; and in blue hydrocarbon
flame
s.
predicts a quadruple bond
as the only way to satisfy the octet rule for carbon. However, molecular orbital theory
shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate pi bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons in a C2 molecule. This is surprising because the MO diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds.
Bond dissociation energies
of B2
, C2, and N2
show increasing BDE, indicating single, double, and triple bonds, respectively.
C2 is an unstable molecule; carbon is far more frequently encountered as diamond
, graphite
, and fullerenes.
, forming the Swan bands.
Bond length (Angstrom): 1.24
Vibrational Mode (cm-1): 1855
The triplet state has a longer bond length than the singlet state.
and acetylaldehyde to produce acetylene
by two different pathways.
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
(C2), which occurs in electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
s; in comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s, stellar atmosphere
Stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. It is divided into several regions of distinct character:...
s and the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
; and in blue hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
flame
Flame
A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
s.
Chemistry
Valence bond theoryValence bond theory
In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...
predicts a quadruple bond
Quadruple bond
A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum...
as the only way to satisfy the octet rule for carbon. However, molecular orbital theory
Molecular orbital theory
In chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule...
shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate pi bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons in a C2 molecule. This is surprising because the MO diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds.
Bond dissociation energies
Bond dissociation energy
In chemistry, bond-dissociation energy or D0, is one measure of the bond strength in a chemical bond. It is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K...
of B2
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, C2, and N2
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
show increasing BDE, indicating single, double, and triple bonds, respectively.
C2 is an unstable molecule; carbon is far more frequently encountered as diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
, and fullerenes.
Comets
The light of fainter comets mainly originates from the emission of diatomic carbon. There are several lines of C2 light, mostly in the visible spectrumVisible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
, forming the Swan bands.
Properties
Cohesive energy (eV): 6.32Bond length (Angstrom): 1.24
Vibrational Mode (cm-1): 1855
The triplet state has a longer bond length than the singlet state.
Reactions
Diatomic carbon will react with acetoneAcetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
and acetylaldehyde to produce acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
by two different pathways.
- TripletTriplet stateA spin triplet is a set of three quantum states of a system, each with total spin S = 1 . The system could consist of a single elementary massive spin 1 particle such as a W or Z boson, or be some multiparticle state with total spin angular momentum of one.In physics, spin is the angular momentum...
C2 molecules will react through an intermolecular pathway, which is shown to exhibit radical character. The intermediate for this pathway is the ethylene radical. Its abstraction is correlated with bond energies. - Singlet C2 molecules will react through an intramolecular, nonradical pathway in which two hydrogen atoms will be taken away from one molecule. The intermediate for this pathway is singlet vinylidene. The singlet reaction can happen through a 1,1-diabstraction or a 1,2-diabstraction. This reaction is insensitive to isotope substitution. The different abstractions are possibly due to the spatial orientations of the collisions rather than the bond energies.
- Singlet C2 will also react with alkeneAlkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s. Acetylene is a major product; however, it appears C2 will insert into carbon-hydrogen bonds. - C2 is 2.5 times more likely to insert into a methyl groupMethyl groupMethyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
as into methylene groups.