Knowledge Bank

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} 259, 1143 (1993); \textit{Icarus} 104, 118 (1993).} found that H$2$CO and NH$3$ can evidentally react at temperatures under 70 K if they are embedded within an ice matrix, a conclusion that was subsequently confirmed theoretically} 142, 550 (1999).}. 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 4H$2$O and 12H$2$O, respectively. Electrostatic contributions from bulk ice were modelled with either PCM and IPCM reaction field solvation. In addition to revisiting H$2$CO + NH$3$, 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 NH$3$ and the carbonyl species in the solvated pre-reaction complexes. In small clusters, the C-N distance decreases by about 1 \AA, 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.