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Hexamethylenetetramine (HMT) – A Major Photoproduct of the UV Irradiation of Interstellar Ices

Hexamethylenetetramine (HMT), C6H12N4 (see image) was identified as a compound over 130 years ago. It was the first organic molecule on which X-ray crystallography was performed and it was found to have tetrahedral symmetry. The infrared (IR) and Raman spectra of the solid and infrared spectra of the gas have been reported and the small number of observed fundamentals is in accord with the high degree of symmetry of this molecule (see reference at the bottom of this page). The IR spectral properties and photochemistry of HMT, especially frozen in H2O-rich ices, are of astronomical interest, as it may comprise a portion of interstellar ice grains, icy satellites, and the organic crust of comets.

Below is a representation of the HMT molecule in which the red spheres are nitrogen atoms, the black spheres are carbon atoms, and the white spheres are hydrogen atoms. Each nitrogen atom has three bonds to carbon, and each carbon has two bonds to nitrogen atoms and two bonds to hydrogen atoms.

Figure 1: the structure of a Hexamethylenetetramine C6H12N4.

Red spheres are nitrogen atoms, the black spheres are carbon atoms, and the white spheres are hydrogen atoms.

In our laboratory we simulate the thermal and photochemical processes that occur in mixed molecular ices in physical environments like interstellar dense molecular clouds, comets, and outer regions of the Solar System (for more details about these simulations, click here-main residue page). Energetic processing of these ices has been shown to convert simple, abundant molecules (like H2O, CH3OH, NH3, CO, CO2, CH4, etc.) into more complex organic materials. The resulting populations of organics are very complex and include a number of classes of compounds that are of astrobiological interest, including amino acids, quinones, and amphiphiles.

Figure 2: The infrared spectrum of (a) a sample of pure HMT frozen in H2O and 12 K and subsequently warmed to 300 K, and (b) the residue created from the photolysis of an ice containing H2O, CH3OH and NH3.

Our work suggests the HMT forms by the mechanism shown in the figure below.

Figure 3: Suggested forming mechanism for HMT.

This work suggests that HMT and similar molecules are likely to be present in some abundance in interstellar clouds, comets, and other astrophysical environments in which ices are exposed to energetic radiation.

Once HMT is made, it can be further processed by exposure to heat, liquid water, or additional radiation (see figure above). When exposed to UV radiation or heat, HMT can break down to form a variety of new compounds, including cyanide compounds and oxides of carbon and nitrogen. Thus, HMT could be one of the parent molecules of CN seen in the comae of comets. The UV photolysis of HMT also produces “XCN.”

Of particular astrobiological interest is the observation that when this molecule is hydrolyzed in acidic solutions, it decomposes into a number of new compounds, including glycine and other amino acids. Thus, HMT made in space and delivered to the early Earth may have played a role in terrestrial prebiotic chemistry and had implications for the origin of life.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., & Chang, S., (1994)., "Infrared Spectrum of Matrix-Isolated Hexamethylenetetramine in Ar and H2O at Cryogenic Temperatures", J. Phys. Chem., 98, 12206-12210

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Chang, S., & Scharberg, M. A., (1995)., "Organic Compounds Produced by Photolysis of Realistic Interstellar and Cometary Ice Analogs Containing Methanol", Astrophys. J., 454, 327-344.