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I am studying astronomy and came across the following term in the astrochemistry course called 'complex organic molecules' or also written as COMs. My question is: What is exactly meant with these molecules? Is it just a molecule with more than one carbon atom?

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tl;dr: two different definitions. Astronomy: multiple carbon atoms in molecule. Chemistry: polymer


Interestingly enough, after reading about COMs here, as well as reading the Wikipedia page and the corresponding arXiv paper, it seems like chemists and astronomers have different definitions of what a complex organic molecule should be!

As far as I knew, in chemistry complex organic molecules were long polymers, such as proteins, which were composed of thousands upon thousands of amino acid units. In the astronomy paper, however, they cite other types of molecules.

$\ce{CH3OH, CH3CHO, HCOOCH3 and CH3OCH3}$, all cited as "complex" (haha) organic molecules in the paper, would appear to chemists as relatively simple molecules. (I read the paper, because it piqued my interest that something like a protein could be found in space). I then read the Springer article.

The term “complex organic molecules” is used differently in astronomy and chemistry. In astronomy, complex organic molecules are molecules with multiple carbon atoms such as benzene and acetic acid. These molecules have been detected in interstellar space with radio telescopes. In chemistry, “complex organic molecules” refer to polymer-like molecules such as proteins.

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    $\begingroup$ I think the term is highly context dependent. In space molecules are pretty complex in that environment. Some chemists would argue that polymers are complex (others would disagree as they are big but repetitive so don't contain much complexity) Others (like me) would argue that molecules like taxol are complex because it has 4-fused different-sized rings in its core and 10 stereo centres and I don't have enough atoms in my modelling kit to come close to building it. $\endgroup$ – matt_black Jan 14 at 15:03
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    $\begingroup$ Don't even think about asking astronomers to define "metals" if this upsets you! $\endgroup$ – Chris H Jan 14 at 15:39
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    $\begingroup$ @Dani I can't possibly comment. I work with too many of them. Besides I'm a physicist and you should hear how we are with chemistry $\endgroup$ – Chris H Jan 14 at 19:37
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    $\begingroup$ I note that you give the example of $\ce{CH3OH}$, but your quotation specifies "multiple carbon atoms". I wonder if this means that there's disagreement about the definition used in astronomy? $\endgroup$ – ruakh Jan 14 at 21:27
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    $\begingroup$ @ruakh hmm, the thing is that was directly quoted from ArXiv $\endgroup$ – JavaScriptCoder Jan 14 at 21:38
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Here is a prime example of how much a "complex" is a simple organic molecule for Astrophysicist when the data are coming from interstellar space, which has made a Science article (Ref. 1):

Abstract: The largest noncyclic molecules detected in the interstellar medium (ISM) are organic with a straight-chain carbon backbone. We report an interstellar detection of a branched alkyl molecule, iso-propyl cyanide $(\ce{i-C3H7CN})$, with an abundance 0.4 times that of its straight-chain structural isomer. This detection suggests that branched carbon-chain molecules may be generally abundant in the ISM. Our astrochemical model indicates that both isomers are produced within or upon dust grain ice mantles through the addition of molecular radicals, albeit via differing reaction pathways. The production of iso-propyl cyanide appears to require the addition of a functional group to a nonterminal carbon in the chain. Its detection therefore bodes well for the presence in the ISM of amino acids, for which such side-chain structure is a key characteristic.

In September, 2014, a science reporter, Michael Eyre wrote an article about this discovery in BBC News under the title of "Complex organic molecule found in interstellar space," stating that iso-propyl cyanide has been detected in a star-forming cloud 27,000 light-years from Earth. This star-forming cloud or the star-forming region where the current observations made is Sagittarius B2(N) or Sgr B2(N). According to the authors of Science article, Sgr B2 is the most massive star-forming region in our Galaxy, which is located close to the Galactic Center, and contains two main sites of star formation, Sgr B2(N) and Sgr B2(M). In particular, Sgr B2(N) has been nicknamed Large Molecule Heimat because it is the source of many "complex organic molecules," most of which are found for the first time in space (Ref. 2).

Using the 30-m single-dish radio telescope of the Institut de Radioastronomie Millimétrique (IRAM) in France, the authors have previously discovered the straight-chain organic molecules, ethyl formate $(\ce{C2H5OCHO})$ and normal-propyl cyanide $(\ce{n-C3H7CN})$ in Sgr B2(N) (Ref. 3). To date, more than 165 molecules have been discovered in the interstellar medium or in circumstellar envelopes over the past five decades and approximately 2-4 "new" interstellar or circumstellar molecules are found each year. Among them, “complex” organic molecules with up to 13 atoms have been found, showing that the interstellar chemistry in some regions is efficient enough to achieve a relatively high degree of chemical complexity (For the authors credit, they have clearly stated that these molecules are “complex” for astronomers, not for biologists!). Yet, the discovery of ethyl formate (3 carbon atoms) and n-propyl cyanide (4 carbon atoms) sited in Ref. 3 is claimed to be the first clear detection of these molecules in space, and $\ce{n-C3H7CN}$ be the largest molecule yet detected in this source at the time. It is also noteworthy that the authors detected neither ethyl formate nor n-propyl cyanide toward the more evolved source, Sgr B2(M).

Even though, ammonia $(\ce{NH3})$ and water vapors $(\ce{H2O})$ have been discovered in space respectively in 1968 and 1969, the first "complex organic species" from space is believed to be formaldehyde, which was discovered and reported in 1969 (according to Ref. 2). Since then, a real Gold Rush broke out for "complex organic species," and approximately 165 molecules detected (by 2012) in the interstellar medium, 20 of which are cyanides (Ref. 4).


References:

  1. A. Belloche, R. T. Garrod, H. S. P. Müller, K. M. Menten, "Detection of a branched alkyl molecule in the interstellar medium: iso-propyl cyanide," Science 2014, 345(6204), 1584-1587 (DOI: 10.1126/science.1256678).
  2. K. M. Menten, “Perspective from a Younger Generation — The Astro-spectroscopy of Gisbert Winnewisser,” In The Dense Interstellar Medium in Galaxies: Proceedings of the 4th Cologne-Bonn-Zermatt Symposium (Zermatt, 22–26 September, 2003); S. Pfalzner, C. Kramer, C. Straubmeier, A. Heithausen (Eds.); Springer Proc. Physics: Vol. 91, Springer-Verlag: Berlin, Germany, pp. 69-82, 2004.
  3. A. Belloche, R. T. Garrod, H. S. P. Müller, K. M. Menten, C. Comito, P. Schilke, "Increased complexity in interstellar chemistry: Detection and chemical modeling of ethyl formate and n-propyl cyanide in Sagittarius B2(N)," Astronomy & Astrophysics 2009, 499, 215-232 (DOI:10.1051/0004-6361/200811550).
  4. M. H. Ordu, H. S. P. Müller, A. Walters, M. Nuñez, F. Lewen, A. Belloche, K. M. Menten, S. Schlemmer, "The quest for complex molecules in space: laboratory spectroscopy of n-butyl cyanide, $\ce{n-C4H9CN}$, in the millimeter wave region and its astronomical search in Sagittarius B2(N)," Astronomy & Astrophysics 2012, 541, A121 (8 pages) (DOI: 10.1051/0004-6361/201118738).
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"Complex" is a relative term and I would say the situation in Astronomy is more complicated than that alluded to by JavaScriptCoder, who states that methanol (amongst others) is considered complex in Astronomy.

A distinction can be made between the Interstellar Medium, where it is indeed reasonably remarkable to find a substance as complex as methanol, a Nebula (which contains a greater concentration of matter but is still by Earth standards a hard vacuum) and a Condensed Body such as a planet, moon or comet, where it is not particularly remarkable to find substances with a molecular weight several times that of methanol.

The definition of "complex" on lifeless condensed bodies seems to be at a molecular weight a little over 100. Even more complex organic molecules called tholins are responsible for the colour of some planets. The first image in the article https://en.wikipedia.org/wiki/Tholin lists "complex organics" as having a molecular weight of 100-350 (Daltons), but then lists "negative organic ions" (and presumably tholins) as having higher molecular weights still. This article makes a distinction between "simple" organic molecules with molecular weights below 50 and "complex" organic molecules with molecular weights above 200.

Astronomers are also interested in the function of "complex molecules" in the origin of life. There is a lot of interest in the presence of polycyclic aromatic hydrocarbons (which have been detected in nebulae and condensed bodies) and their potential role in mediating the synthesis of RNA / DNA type substances https://en.wikipedia.org/wiki/PAH_world_hypothesis

People who study planets are called planetary scientists. Perhaps they are not astronomers, or perhaps they are a type of astronomer. What is clear, though, is that "complex" means "at least fairly complex for that particular environment or context" so there is no fixed lower limit for what is considered complex. It is also clear that what astronomers consider "complex" is nowhere near as complex as what chemists consider "complex."

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