REACTIONS OF WATER CLUSTER IONS WITH NITRIC-ACID

Reactions of D+(D2O)n (n = 1-30) with DNO3 were investigated under thermal conditions in a fast-flow reactor operated over a range of temperatures extending from 298 K down to 140 K. The results reveal that the reactions take place only for clusters exceeding a specific size at low temperature; the observed products are D+(D2O).(DNO3) (n greater-than-or-equal-to 5). The incorporation of a second and third nitric acid molecule into water clusters is found to occur only for ones having a considerably higher degree of solvation. The reactions are observed to be independent of pressure, and a switching rather than an association reaction mechanism is found to be operative. These findings, as well as observations that the reactions show a strong size dependence, suggest that the degree of solvation has a direct influence on the energetics of the reactions. In addition to revealing the influence of solvation on certain switching reactions, the results also provide further insight into the general processes of formation and the stability of water nitric acid complexes related to polar stratospheric clouds implicated in the heterogeneous chemistry of the ozone cycle in the polar night.