Molecules That Have Been Detected

In 1963, the hydroxyl radical, OH, was detected in interstellar space. Because OH is a billion times less abundant than H atoms, many astronomers felt that no molecule containing more than two atoms would be detected. However, in 1968 H₂O and ammonia, NH₃, were observed, and soon thereafter formaldehyde, H₂CO. As of 1997, 110 interstellar molecules had been detected. Many of these molecules are quite stable and are chemicals that you are probably familiar with - some of these are listed below.

Interstellar Molecules That Are Also Commonly Found on Earth
Formula	Name	Formula	Name
H₂	hydrogen	HCl	hydrogen chloride
CO	carbon monoxide	NO	nitric oxide
H₂O	water	H₂S	hydrogen sulfide
HCN	hydrogen cyanide	SO₂	sulfur dioxide
OCS	carbonyl sulfide	NH₃	ammonia
H₂CO	formaldehyde	C₂H₂	acetylene
HCOOH	formic acid	CH₃OH	methanol
CH₃CHO	acetaldehyde	CH₃ NH₂	methylamine
CH₃CH₂OH	ethanol	(CH₃)₂O	dimethyl ether
(CH₃)₂CO	acetone	NH₂CH₂COOH	glycine

Recently, an emission feature attributed to glycine has been found. Glycine is a simple amino acid, and amino acids are the building blocks of proteins, so this discovery shows that molecules important to life can be formed in the interstellar medium, under very harsh conditions.

Because gas densities in interstellar molecular clouds are much, much lower than those on Earth, many of the interstellar molecules are very reactive and are not commonly found on Earth. Some of these molecules are listed below.

Interstellar Molecules That Are Not Commonly Found on Earth (These molecules are quite reactive and many are ions or free radicals)
Formula	Name	Formula	Name
CO⁺	carbon monoxide ion	CH⁺	methylidyne ion
H₃⁺	protonated hydrogen	N₂H⁺	protonated nitrogen
HCO⁺	formyl ion	H₃O⁺	hydronium ion
OH	hydroxyl radical	CN	cyanide radical
HCO	formyl radical	HNC	hydrogen isocyanide
CH₂CO	ketene	CH₂CN	cyanomethyl radical
HC₃N	-	C₂H	-
HC₅N	-	C₃H	-
HC₇N	-	C₄H	-
HC₉N	-	C₅H	-
HC₁₁N	-	C₆H	-

These molecules fall into two broad categories: ions and unsaturated molecules. Ions are responsible for most of the chemistry that occurs in molecular clouds (this will be discussed in the next section), but most ions that have been observed are fairly simple. Of these, H₃⁺ is relatively abundant and its reactions are especially important in producing many of the larger molecules.

Unsaturated molecules lack one or more hydrogen atoms. These molecules are free radicals (they contain an odd number of electrons), such as OH, or they contain double- or triple-bonds, such as ketene, H₂C=C=O. The largest interstellar molecules are the cyano-poly-ynes. This family of molecules has the general formula HC_nN, where n is odd. The molecules are linear and contain alternating single and triple bonds. HC₃N is the largest cyano-poly-yne that is relatively stable; the larger molecules are extremely reactive.

Although the emission lines of several of these molecules were observed by radiotelescopes, for years it was not known which molecules were responsible for the observed lines. Once these molecules were synthesized and their radiofrequency spectra measured in the laboratory they were found to be responsible for some of the unidentified lines. However, many radiofrequency lines from molecular clouds have yet to be identified - presumably they are due to molecules whose radiofrequency spectra have not yet been measured in the laboratory.