DAVID E. WOON AND DOROTHY J. MILLER, Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana IL 61801.
While many of the reactions that can occur in icy grain mantles in cold interstellar clouds are either barrierless recombinations of open-shell radicals or are driven by energetic processing (photolysis, radiolysis, or shocks), there is a group of unusual reactions involving stable, closed-shell species that can also be efficient at very low temperatures. Previous experimental work found that H2CO and NH3 can evidentally react at temperatures under 70 K if they are embedded within an ice matrix, a conclusion that was subsequently confirmed theoretically. To assess the impact of including more water, cluster calculations were performed at the MP2/6-31+G** or B3LYP/6-31+G** level with up to 4H2O and 12H2O, respectively. Electrostatic contributions from bulk ice were modelled with either PCM and IPCM reaction field solvation. In addition to revisiting H2CO + NH3, we also characterized the reactions of ammonia with acetaldehyde and acetone. The new work confirms that the reaction barriers are very small, but it also indicates that water induces a substantial interaction between NH3 and the carbonyl species in the solvated pre-reaction complexes. In small clusters, the C-N distance decreases by about 1 Å, and partial charge-transfer complexes are formed. In large clusters, a proton transfers to yield cationic complexes. We will present an analysis of potentially observable vibrational features of these complexes.