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Last Updated: 1/15/07

Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic Aromatic Hydrocarbons (PAHs) are a class of very stable organic molecules made up of only carbon and hydrogen. These molecules are flat with carbon atoms in rings of six carbon atoms each, think of chicken wire. In these molecules each carbon has three neighboring atoms - they are like the carbon atoms in graphite (for the chemistry students out there these carbon atoms are sp2 hybridized they have sigma and pi bonds). Any compounds that have this atomic arrangement are referred to as 'aromatic' (see note below regarding this word). There are compounds that are made mostly of carbon but have the occasional nitrogen, sulphur or oxygen atom attached or in the rings that are still 'aromatic' and sometimes when you see reference to 'aromatics' or 'aromatic compounds' they mean those too. The situation is quite different from that in diamond where the atoms form a three-dimensional lattice and there are only sigma bonds. At right is a space filling representation of the PAH coronene (C₂₄H₁₂). The structures of a variety of representative PAHs can be seen here. These molecules can be highly carcinogenic but they are also very common. They are really really stable so they are a standard survivor of combustion from automobiles and airplanes (i.e., they are found in soot) and I have been told that some (such as benzo[a]pyrene) are present in charcoal broiled hamburgers. They form in wood fires, so they are not just from mechanical or industrial processes.

Here at the astrochemistry lab we are interested in PAHs (and other related molecules because they are very common in space and as a result they can act as probes of conditions in distant regions. They light from the glow of PAHs (they emit light when excited either by heat or UV photons) has been detected coming from hot regions of space where stars are born, and around dying stars, and even from other galaxies as well. In addition, these molecules have been detected in comet and asteroidal dust and are common in meteorites. It is believed that these molecules form in the outflows of dying carbon rich stars from which they go out into the space between the stars. Those that survive the predation of supernova shocks and cosmic rays end up in dense molecular clouds, where they condense onto microscopic ice grains. Here, these molecules undergo some chemistry that we simulate in the lab. These compounds are of importance for those of us who are interested in the search for life in the Solar System, because sometimes (on earth) these molecules are used as biomarkers (chemicals that indicate life). In a now famous (or infamous) paper McKay et al., argued that there is evidence of ancient fossil life in the Martian meteorite ALH84001. One of the arguments that they presented for this were certain kinds of PAHs that were found in this meteorite. Here at the astrochem lab we are quite skeptical about this proof. The wide range of PAHs that are seen in space would seem to make PAHs of any kind a poor sign of life, even though they are sometimes biomarkers here on Earth. What would be a good indicator of life on another planet?

Here is a comparison of an infrared spectrum of a distant object with the raman spectrum of auto soot. We do not claim that they have alot of cars out there but that the high temperatures present lead to the same kinds of molecules that we see in auto engines. PAHs are now commonly thought to be the carriers of the "unidentified" infrared bands (UIRs) and We now believe that PAHs or PAH cations (a PAH with an electron missing) are also the best candidates for the diffuse interstellar bands (DIBs).

For low temperature infrared spectra of PAHs in inert gas matrix, or in H₂O see our database page. We have published man papers containing spectroscopy of PAHs some of which can be downloaded as PDF files at our publications page.

Chemists often ask us 'where do you get all of these pure PAHs'

A note about the meaning of the word 'aromatic'. In common parlance the word aromatic is used to denote something that has a fragrance (something with an aroma) and the connotation is a pleasant one as in 'aromatic herbs'. In the photograph at left you see the awning of a store in Singapore that presumably sells perfume. That is not what we mean here by aromatic. While many of the smaller aromatic compounds have an odor (like benzene and naphthalene which used to be the major component in mothballs) and that is probably how they got their name, what a chemist means by aromatic is that the molecule has a certain kind of electronic structure. PAH molecules like benzene have a ring of 4n+2 Pi electrons that are all interacting together forming a big delocalized cloud that is essentially shared by all of the carbon atoms. All of the carbon-carbon bonds are equal despite the fact that the structure is drawn as having alternating single and double bonds. These structures are very stable, and thus are the most common organic compounds that survive combustion as we mentioned above.

Some recent laboratory work of ours on the infrared spectra of PAHs:

Mattioda, A. L.; Hudgins, D. M.; Bauschlicher C. W.; Allamandola, L. J.; (2005) Infrared spectroscopy of matrix-isolated polycyclic aromatic compounds and their ions. 7. Phenazine, a dual substituted polycyclic aromatic nitrogen heterocycle. Advances in Space Research 36, 156-165.

Bernstein, M. P.; Sandford, S. A.; Allamandola, L. J. (2005) The Mid-Infrared Absorption Spectra of Neutral Polycyclic Aromatic Hydrocarbons in Conditions Relevant to Dense Interstellar Clouds. The Astrophysical Journal Supplement Series, Volume 161, Issue 1, pp. 53-64.

Mattioda, A. L.; Allamandola, L. J.; Hudgins, D. M. (2005) Experimental Near-Infrared Spectroscopy of Polycyclic Aromatic Hydrocarbons Between 0.7 and 2.5 microns. The Astrophysical Journal, 629, 1188-1210.

Bernstein, M. P.; Mattioda, A. L.; Sandford, S. A.; Hudgins, D. M. (2005) Laboratory Infrared Spectra of Polycyclic Aromatic Nitrogen Heterocycles: Quinoline and Phenanthridine in Solid Argon and H2O. The Astrophysical Journal, 626, 909-918.

Bernstein, M. P.; Sandford, S. A.; Walker, R. L., (2005) Laboratory IR spectra of 4-azachrysene in solid H2O Advances in Space Research Volume 36, 166-172

Sandford, S.A., Bernstein, M.P., and Allamandola, L. J. The mid-infrared laboratory spectra of naphthalene (C10H8) in solid H2O ApJ, 607, 346-360, (2004)

Mattioda, A. L., Hudgins, D. M., Bauschlicher, Jr., C. W., Rosi, M., Allamandola, L. J. (2003). Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Hydrocarbons, 6. Polycyclic Aromatic Nitrogen Heterocycles and their Cations. J. Phys Chem A, 107, 1486-1498.

Some recent work of ours on the modeling of PAHs and its connection to astronomical observations:

Mattioda, A. L.; Allamandola, L. J.; Hudgins, D. M. (2005) The Ultraviolet to Near-Infrared Optical Properties of Polycyclic Aromatic Hydrocarbons: A Semiempirical Model. The Astrophysical Journal, 629, 1183-1187.

T. M. Halasinski, L.J. Allamandola and F. Salama. Investigation of the UV, Visible and Near-IR Absorption Spectra of Hydrogenated Polycyclic Aromatic Hydrocarbons and Their Cations. The Astrophys. J., 628, 555. (2005)

E. Peeters, A. L. Mattioda, D. M. Hudgins, and L. J. Allamandola (2004) PAH Emission in the 15-21 micron Region. The Astrophysical Journal, 617: L66-L68.

Peeters, E., Allamandola, L. J., Bauschlicher Jr., C. W., Hudgins, D. M., Sandford, S. A., and Tielens, A. G. G. M., Deuterated interstellar PAHs. ApJ.604, 252-257, (2004)

Hudgins, D.M. (2002) "Interstellar Polycyclic Aromatic Compounds and Astrophysics." Polycyclic Aromatic Compounds, 22, 469-488.

Some older work of ours on IR spectra of PAHs and astronomy:

Hudgins, D. M., Bauschlicher, Jr., C. W., Allamandola, L. J., & Fetzer, J. C. (2000). Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Hydrocarbon Ions 5. PAHs Incorporating a Cyclopentadienyl Ring. J. Phys. Chem. A 104, 3655.

Allamandola, L. J., Hudgins, D. M., & Sandford, S. A. (1999). Modeling the Unidentified Infrared Emission with Combinations of Polycyclic Aromatic Hydrocarbons. Astrophys. J. (Letters) 511, L115-L119.

Langhoff, S. R., Bauschlicher, Jr., C. W., Hudgins, D. M., Sandford, S. A., & Allamandola, L. J. (1998). Infrared spectra of substituted polycyclic aromatic hydrocarbons. J. Phys. Chem. A 102, 1632-1646.

Hudgins, D. M., & Sandford, S. A. (1998a). Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Hydrocarbons 1. PAHs Containing 2 to 4 Rings. J. Phys. Chem. 102, 329-343.

Hudgins, D. M., & Sandford, S. A. (1998b). Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Hydrocarbons 2. PAHs Containing 5 or More Rings. J. Phys. Chem. 102, 344-352.

Hudgins, D. M., & Sandford, S. A. (1998c). Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Hydrocarbons 3. Fluoranthene and the Benzofluoranthenes. J. Phys. Chem. 102, 353-360.

Bauschlicher, Jr., C. W., Langhoff, S. R., Sandford, S. A., & Hudgins, D. M. (1997). Infrared Spectra of Perdeuterated Naphthalene, Phenanthrene, Chrysene, and Pyrene. J. Phys. Chem. A 101, #13, 2414-2422.

Hudgins, D. M., Sandford, S. A., & Allamandola, L. J. (1994). Infrared Spectroscopy of Polycyclic Aromatic Hydrocarbon Cations I: Matrix-Isolated Naphthalene and Perdeuterated Naphthalene. J. Phys. Chem. 98, 4243-4253.

Witteborn, F. C., Sandford, S. A., Bregman, J. D., Allamandola, L. J., Cohen, M., & Wooden, D. (1989). New Emission Features in the 11-13 µm Region and Their Relationship to Polycyclic Aromatic Hydrocarbons. Astrophys. J. 341, 270-277.

The Ultraviolet/Visible Spectra of PAHs

Salama, F. (1999). Polycyclic Aromatic Hydrocarbons in the Interstellar Medium. A Review. In Solid Interstellar Matter: The ISO Revolution, L. d'Hendecourt, C. Joblin & A. Jones, eds., EDP Sciences, (Springer-Verlag: Les Ulis), pp. 65-87.

Romanini, D., Biennier, L., Salama, F., Kachanov, A., Allamandola, L. J., & Stoeckel, F. (1999). Jet-discharge cavity ring down spectroscopy of ionized polycyclic aromatic hydrocarbons: Progress in testing the PAH hypothesis for the diffuse interstellar band problem. Chem. Phys. Letters 303, 165-170.

Joblin, C., Salama, F., & Allamandola, L. J. (1999). Absorption and emission spectroscopy of perylene (C₂₀H₁₂) isolated in Ne, Ar, and N₂ matrices. J. Chem. Phys. 110, 7287-7297.

Joblin, C., Salama, F., & Allamandola, L. J. (1995). Photoinduced Fluorescence from the Perylene Cation Isolated in Ne and Ar Matrices. J. Chem. Phys. 102, 9743-9745.

Salama, F., Joblin, C., & Allamandola, L. J. (1994). Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. II. The phenanthrene cation (C₁₄H₁₀+) and its 1-Methyl derivative. J. Chem. Phys. 101, 10252-10262.

Du, P., Salama, F., & Loew, G. H. (1993). Theoretical study on the electronic spectra of a polycyclic aromatic hydrocarbon, naphthalene, and its derivatives. Chem. Phys. 173, 421-437.

Salama, F., & Allamandola, L. J. (1991). The role of matrix material and CCl₄ (electron acceptor) on the ionization mechanisms of matrix-isolated naphthalene. J. Chem. Phys. 95, 6190-6191.

Salama, F., & Allamandola, L. J. (1991). Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. I. The naphthalene cation (C₁₀H₈+). J. Chem. Phys. 94, 6964-6977.

The Chemistry of PAHs and PAHs in Extraterrestrial Materials

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Gillette, J. S., Clemett, S. J., & Zare, R. N. (1999). UV Irradiation of Polycyclic Aromatic Hydrocarbons in Ices: Production of Alcohols, Quinones, and Ethers. Science 283, 1135-1138.

Allamandola, L. J., Sandford, S. A., & Wopenka, B. (1987). Interstellar polycyclic aromatic hydrocarbons and carbon in interplanetary dust particles and meteorites. Science 237, 56-59.