Cyclopropenylidene
Encyclopedia
Cylopropenylidene, or c-C3H2 belongs to a highly reactive class of organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 molecules known as carbene
Carbene
In 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 RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...

s. Due to its reactivity, cyclopropenylidene is only seen terrestrially in the laboratory. However, it is found in significant concentrations in 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...

 (ISM) due to the extreme environment. The linear isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

 of c-C3H2 is also found in the ISM, but its column density is generally about an order of magnitude lower.

History

The astronomical detection of c-C3H2 was first confirmed in 1985. Four years earlier, several ambiguous lines had been observed in the radio
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...

 region of spectra taken of the ISM, but the observed lines were not identified at the time. These lines were later matched with a spectrum of c-C3H2 using an acetylene-helium discharge.
Surprisingly, c-C3H2 has been found to be ubiquitous in the ISM. Detections of c-C3H2 in the diffuse medium
Interstellar cloud
Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size and temperature of a given cloud, the hydrogen in it...

 were particularly surprising because of the low densities. It had been believed that the chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 of the diffuse medium did not allow for the formation of larger molecules, but this discovery, as well as the discovery of other large molecules, continue to illuminate the complexity of the diffuse medium.
More recently, observations of c-C3H2 in dense clouds
Interstellar cloud
Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size and temperature of a given cloud, the hydrogen in it...

 have also found concentrations that are significantly higher than expected. This has led to the hypothesis that the photodissociation of polycyclic aromatic hydrocarbons (PAHs) enhances the formation of c-C3H2.

Formation

The major formation reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 of c-C3H2 is the dissociative recombination
Dissociative recombination
Dissociative recombination is a process where a positive molecular ion recombines with an electron, and as a result, the neutral molecule dissociates. This reaction is important for extraterrestrial and atmospheric chemistry. On Earth, dissociative recombination is practically non-existent, as...

 of c-C3H3+.
C3H3+ + e- → C3H2 + H


c-C3H3+ is a product of a long chain of carbon chemistry that occurs in the ISM. Carbon insertion reactions are crucial in this chain for forming C3H3+. The protonation of NH3 by c-C3H3+ is the second most important formation reaction. However, under typical dense cloud conditions, this reaction contributes less than 1% of the formation of C3H2.

Destruction

Cyclopropenylidene is generally destroyed by reactions between ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s and neutral molecules. Of these, protonation
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...

 reactions are the most common. Any species of the type HX+ can react to convert the c-C3H2 back to c-C3H3+. Due to rate constant and concentration considerations, the most important reactants for the destruction of c-C3H2 are HCO+, H3+, and H3O+.
C3H2 + HCO+ → C3H3+ + CO


Notice that c-C3H2 is mostly destroyed by converting it back to C3H3+. Since the major destruction pathways only regenerate the major parent molecule, C3H2 is essentially a dead end in terms of interstellar carbon chemistry. However, in diffuse clouds or in the photodissociation region
Photodissociation region
Photodissociation regions are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source of heat...

 (PDR) of dense clouds, the reaction with C+ becomes much more significant and C3H2 can begin to contribute to the formation of larger organic molecules
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

.

Spectroscopy

Detections of c-C3H2 in the ISM rely on observations of molecular transitions using rotational spectroscopy
Rotational spectroscopy
Rotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation by molecules associated with a corresponding change in the rotational quantum number of the molecule...

. Since c-C3H2 is an asymmetric top, the rotational energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

s are split and the spectrum becomes complicated. Also, it should be noticed that C3H2 has spin isomers much like the spin isomers of hydrogen
Spin isomers of hydrogen
Molecular hydrogen occurs in two isomeric forms, one with its two proton spins aligned parallel , the other with its two proton spins aligned antiparallel...

. These ortho and para forms exist in a 3:1 ratio and should be thought of as distinct molecules. Although the ortho and para forms look identical chemically, the energy levels are different, meaning that the molecules have different spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 transitions.

When observing c-C3H2 in the interstellar medium, there are only certain transitions that can be seen. In general, only a few lines are available for use in astronomical detection. Many lines are unobservable because they are absorbed by the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. The only lines that can be observed are those that fall in the radio window
Radio window
The radio window is the range of frequencies of electromagnetic radiation that the earth's atmosphere lets through. The wavelengths in the radio window run from about one centimetre to about eleven-metre waves.-See also:*Astronomical window...

. The more commonly observed lines are the 110 to 101 transition at 18343 MHz and the 212 to 101 transition at 85338 MHz of ortho c-C3H2.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK