This article appeared in the Astrophysical Journal
ApJ 454 327-344 (1995)

44 Pages (including Figures)
9 Figures
2 Tables

Key Words: HMT; Infrared Spectroscopy; Organic Materials;
Interstellar Medium; Comets; UV photoprocessing

Abstract: (for more info see our file about interstellar/cometary ices).

The infrared (IR) spectra of ultraviolet (UV) and thermally-processed, methanol- containing interstellar/cometary ice analogs at temperatures from 12 to 300 K are presented. Infrared spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-mass spectrometry indicate that CO (carbon monoxide), CO2 (carbon dioxide), CH4 (methane), HCO (the formyl radical), H2CO (formaldehyde), CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), and R-C:N (nitriles) are formed. In addition, the organic materials remaining after photolyzed ice analogs have been warmed to room temperature contain (in rough order of decreasing abundance) (i) hexamethylenetetramine (HMT, C6H12N4), (ii) ethers, alcohols, and compounds related to polyoxymethylene {POM, (-CH2O-)n}, and (iii) ketones {R-C(=O)-R'} and amides {H2NC(=O)-R}. Most of the carbon in these residues is thought to come from the methanol in the original ice. Deuterium and 13C isotopic labeling demonstrates that methanol is definitely the source of carbon in HMT. For a comparison of the infrared spectra of HMT and our room temperature astrochemical residue click here.

High concentrations of HMT in interstellar and cometary ices could have important astrophysical consequences. The ultraviolet photolysis of HMT frozen in H2O ice readily produces the XCN band observed in the spectra of protostellar objects and laboratory ices, as well as other nitriles. Thus, HMT may be a precursor of XCN and a source of CN in comets and the interstellar medium. Also, HMT is known to hydrolyze under acidic conditions to yield ammonia, formaldehyde, and amino acids. Thus, HMT may be a significant source of prebiogenic compounds on asteroidal parent bodies. A potential mechanism for the radiative formation of HMT in cosmic ices is depicted below.