Deuterium Fractionation upon the Formation of Hexamethylenetetramines through Photochemical Reactions of Interstellar Ice Analogs Containing Deuterated Methanol Isotopologues

Hexamethylenetetramine (HMT) is a representative product after the photolysis of interstellar ice analogs containing methanol followed by warming-up to room temperature. Since interstellar methanol is often significantly enriched in deuterium (D), the HMT photoproduct is expected to inherit D atoms from deuterated methanol. However, D fractionation upon the formation of HMT is not well understood, especially when it is produced from partly deuterated methanol isotopologues such as CH₂DOH and CH₃OD. Here, we experimentally studied the composition of deuterated HMT (d _n-HMT, where n is the number of D atoms) at the isotopologue level formed by the photolysis of ice mixtures containing deuterated methanol, CH₂DOH or CH₃OD, at 10 or 77 K. The analyses were performed using a state-of-the-art high-resolution mass spectrometer coupled with a compound-specific separation technique. The formation of d _n-HMT (n = 0-8) was confirmed under all experimental conditions. In addition, methyl- and hydroxyl-substituted HMT and their deuterated isotopologues were also obtained in the products. The deuterium enrichment was outstanding when CH₂DOH was used rather than CH₃OD, and when photolysis was performed at 77 K rather than 10 K. We found that the deuteration level of the formed HMT far exceeded that of the reactants under the present experimental conditions. These results obtained during stable isotope probing of deuterium strongly suggest that HMT can play a role as an organic pool of interstellar D atoms. These may be distributed into other chemical species through molecular evolution in space.