Ab Initio Quantum Chemical Studies of Reactions in Astrophysical Ices: 1. Aminolysis, Hydrolysis, and Polymerization in H2CO/NH3/H2O Ices

✍ Scribed by David E. Woon

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 250 KB
• Volume: 142
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1006/icar.1999.6227

✦ Synopsis

indicate that ices containing formaldehyde (H 2 CO) and at least a trace of ammonia (NH 3 ) will undergo reactions as the samples are heated from 10 K with no additional irradiation. A residue composed of higher-boiling-point organic species persists to 190 K and beyond. Spectral changes indicate that reactions begin to occur as low as 40 K. The present study employed theoretical electronic structure methods to investigate possible microscopic mechanisms that would account for various aspects of the experimental phenomena. Reaction components were characterized in clusters composed of reactants with up to two explicit catalytically active waters present and then embedded in a continuum polarization field to incorporate the bulk solvation effects of ice. Direct dimerization and trimerization of H 2 CO were first considered, but no process was found that could account for the low-temperature reactivity observed in the laboratory. Two ice-bound aminolysis reactions were then identified that are predicted to possess barriers low enough to be viable at 40 K: H 2 CO-NH 3 → NH 2 CH 2 OH and (H 2 CO) 2 -NH 3 → NH 2 CH 2 OCH 2 OH. The latter yields an amideterminated polyoxymethylene polymer. Analogous hydrolysis reactions are enhanced in ice, but not sufficiently to occur at cold temperatures on their own, which is consistent with the experimental observation that NH 3 is a critical component and water alone is insufficient to catalyze H 2 CO reactions.