List of interstellar and circumstellar molecules


This is a list of molecules that have been detected in the interstellar medium and circumstellar envelopes, grouped by the number of component atoms. The chemical formula is listed for each detected compound, along with any ionized form that has also been observed.

Background

The molecules listed below were detected through astronomical spectroscopy. Their spectral features arise because molecules either absorb or emit a photon of light when they transition between two molecular energy levels. The energy of the photon matches the energy difference between the levels involved. Molecular electronic transitions occur when one of the molecule's electrons moves between molecular orbitals, producing a spectral line in the ultraviolet, optical or near-infrared parts of the electromagnetic spectrum. Alternatively, a vibrational transition transfers quanta of energy to vibrations of molecular bonds, producing signatures in the mid- or far-infrared. Gas-phase molecules also have quantised rotational levels, leading to transitions at microwave or radio wavelengths.
Sometimes a transition can involve more than one of these types of energy level e.g. ro-vibrational spectroscopy changes both the rotational and vibrational energy level. Occasionally all three occur together, as in the Phillips band of C2, in which an electronic transition produces a line in the near-infrared, which is then split into several vibronic bands by a simultaneous change in vibrational level, which in turn are split again into rotational branches.
The spectrum of a particular molecule is governed by the selection rules of quantum chemistry and the molecular symmetry. Some molecules have simple spectra which are easy to identify, whilst others have extremely complex spectra with flux spread among many different lines, making them far harder to detect. Interactions between the atomic nuclei and the electrons sometimes causes further hyperfine structure of the spectral lines. If the molecule exists in multiple isotopologues, the spectrum is further complicated by isotope shifts.
Detection of a new interstellar or circumstellar molecule requires identifying a suitable astronomical object where it is likely to be present, then observing it with a telescope equipped with a spectrograph working at the required wavelength, spectral resolution and sensitivity. The first molecule detected in the interstellar medium was the methylidyne radical in 1937, through its strong electronic transition at 4300 angstroms. Advances in astronomical instrumentation have led to increasing numbers of new detections. From the 1950s onwards, radio astronomy began to dominate new detections, with sub-mm astronomy also becoming important from the 1990s.
The inventory of detected molecules is highly biased towards certain types which are easier to detect e.g. radio astronomy is most sensitive to small linear molecules with a high molecular dipole. The most common molecule in the Universe, H2 is completely invisible to radio telescopes because it has no dipole; its electronic transitions are too energetic for optical telescopes, so detection of H2 required ultraviolet observations with a sounding rocket. Vibrational lines are often not specific to an individual molecule, allowing only the general class to be identified. For example, polycyclic aromatic hydrocarbons are known to be common in space due to their vibrational lines, which are widely observed in the mid-infrared, but it has not been possible to identify exactly which molecules are responsible.
One of the richest sources for detecting interstellar molecules is Sagittarius B2, a giant molecular cloud near the centre of the Milky Way. About half of the molecules listed below were first found in Sgr B2, and many of the others have been subsequently detected there. A rich source of circumstellar molecules is CW Leonis, a nearby carbon star, where about 50 molecules have been identified. There is no clear boundary between interstellar and circumstellar media, so both are included in the tables below.
The discipline of astrochemistry includes understanding how these molecules form and explaining their abundances. The extremely low density of the interstellar medium is not conducive to the formation of molecules, making conventional gas-phase reactions between neutral species inefficient. Many regions also have very low temperatures, further reducing the reaction rates, or high ultraviolet radiation fields, which destroy molecules through photochemistry. Explaining the observed abundances of interstellar molecules requires calculating the balance between formation and destruction rates using gas-phase ion chemistry, surface chemistry on cosmic dust, radiative transfer including interstellar extinction, and sophisticated reaction networks.

Molecules

The following tables list molecules that have been detected in the interstellar medium or circumstellar matter, grouped by the number of component atoms. Neutral molecules and their molecular ions are listed in separate columns; if there is no entry in the molecule column, only the ionized form has been detected. Designations are those used in the scientific literature describing the detection; if none was given that field is left empty. Mass is listed in atomic mass units. Deuterated molecules, which contain at least one deuterium atom, have slightly different masses and are listed in a separate table. The total number of unique species, including distinct ionization states, is indicated in each section header.
Most of the molecules detected so far are organic. The only detected inorganic molecule with five or more atoms is SiH4. Molecules larger than that all have at least one carbon atom, with no N−N or O−O bonds.
is frequently used to trace the distribution of mass in molecular clouds.

Diatomic (43)

Triatomic (44)

Four atoms (28)

Five atoms (20)

MoleculeDesignationMassIons
—Ammonium ion 18
CH4Methane16—
CH3OMethoxy radical31—
c-C3H2Cyclopropenylidene38—
l-H2C3Propadienylidene38—
H2CCNCyanomethyl40—
H2C2OKetene42—
H2CNHMethylenimine29—
HNCNHCarbodiimide42—
—Protonated formaldehyde31H2COH+
C4HButadiynyl49C4H−
HC3NCyanoacetylene51—
HCC-NCIsocyanoacetylene51—
HCOOHFormic acid46—
NH2CNCyanamide42—
—Protonated cyanogen53NCCNH+
HCCNCyanoformaldehyde55—
C5Linear C560—
SiC4Silicon-carbide cluster92—
SiH4Silane32—

can combine with methylene to form acetamide.

Six atoms (16)

Seven atoms (12)

Eight atoms (11)

Nine atoms (10)

Ten or more atoms (17)

Deuterated molecules (20)

These molecules all contain one or more deuterium atoms, a heavier isotope of hydrogen.
AtomsMoleculeDesignation
2HDHydrogen deuteride
3H2D+, Trihydrogen cation
3HDO, D2OHeavy water
3DCNHydrogen cyanide
3DCOFormyl radical
3DNCHydrogen isocyanide
3N2D+—
4NH2D, NHD2, ND3Ammonia
4HDCO, D2COFormaldehyde
4DNCOIsocyanic acid
5NH3D+Ammonium ion
6; NHDCHOFormamide
7CH2DCCH, CH3CCDMethylacetylene

Unconfirmed (12)

Evidence for the existence of the following molecules has been reported in the scientific literature, but the detections are either described as tentative by the authors, or have been challenged by other researchers. They await independent confirmation.
AtomsMoleculeDesignation
2SiHSilylidine
4PH3Phosphine
4MgCCHMagnesium monoacetylide
4NCCPCyanophosphaethyne
5H2NCO+—
4SiH3CNSilyl cyanide
10H2NCH2COOHGlycine
12CO2Dihydroxyacetone
12C2H5OCH3Ethyl methyl ether
18Naphthalene cation
24C24Graphene
24C14H10Anthracene
26C16H10Pyrene