Abstract

Laboratory studies of astrophysical ices by W. A. Schutte, L. J. Allamandola, and S. A. Sandford (1993, Science 259, 1143-1145; 1993, Icarus 104, 118-137) indicate that ices containing formaldehyde (H2CO) and at least a trace of ammonia (NH3) 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 H2CO 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: H2CO-NH3->NH2CH2OH and (H2CO)2-NH3->NH2CH2OCH2OH. The latter yields an amide-terminated 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 NH3 is a critical component and water alone is insufficient to catalyze H2CO reactions. Copyright 1999 Academic Press.