Ab initio quantum chemical studies of reactions in astrophysical ices 3. Reactions of HOCH₂NH₂ formed in H₂CO/NH₃/H₂O ices

Prior experimental and theoretical studies have demonstrated that reactions involving H₂CO and NH₃ can occur in water ice mixtures even at temperatures below 100 K. The key reaction between H₂CO and NH₃ generates monohydroxymethylamine, HOCH₂NH₂. The present study examines further reactions between this product and H₂CO in ices. Quantum chemical calculations at the MP2/6-31+G** level indicate that while H₂CO may add across the OH bond in HOCH₂NH₂ to yield the amine-terminated polyoxymethylene species HOCH₂OCH₂NH₂, it is much more favorable to add across one of the two remaining NH bonds of the original ammonia molecule to yield (HOCH₂)₂NH. In fact, another H₂CO may add across the final NH to form the tertiary amine (HOCH₂)₃N. On the basis of the favorable energetics of these reactions, it is suggested that one or both of these products may be an intermediate in the pathway to the formation of hexamethylenetetramine (C₆H₁₂N₄), which is produced when ices containing methanol, ammonia, and water are subjected to UV irradiation and heated to room temperature.