Ab initio quantum chemical studies of reactions in astrophysical ices - 1.Aminolysis, hydrolysis, and polymerization in H2CO/NH3/H2O ices

Laboratory studies of astrophysical ices by SV. A. Schutte, L. J. Allamando la, and S. A. Sandford (1993, Science 259, 1143-1145; 1993, Icarus 104, 118 -137) indicate that ices containing formaldehyde (H2CO) and at least a trac e of ammonia (NH3) will undergo reactions as the samples are heated from 10 K with no additional irradiation. A residue composed of higher-boding-poin t organic species persists to 190 K and beyond. Spectral changes indicate t hat reactions begin to occur as low as 40 K. The present study employed the oretical electronic structure methods to investigate possible microscopic m echanisms that would account for various aspects of the experimental phenom ena Reaction components were characterized in clusters composed of reactant s with up to two explicit catalytically active waters present and then embe dded in a continuum polarization field to incorporate the hulk solvation ef fects of ice. Direct dimerization and trimerization of H2CO were first cons idered, but no process was found that could account for the low-temperature reactivity observed in the laboratory Two ice-bound aminolysis reactions w ere 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 h ydrolysis reactions are enhanced in ice, but not sufficiently to occur at c old temperatures on their own, which is consistent with the experimental ob servation that NH3 is a critical component and water alone is insufficient to catalyze H2CO reactions. (C) 1999 Academic Press.