Gas phase reactions of hydrated halides with chlorine

A selected ion flow tube (SIFT) equipped with a supersonic expansion ion source was used to study the reactions of hydrated halides X(-)(D2O)(n), (X = F, n less than or equal to 6; X = Cl, n less than or equal to 8; X = Br, n less than or equal to 16; X = I, n less than or equal to 13) with Cl-2 at temperatures ranging from 140 to 400 K. The bare halide reactions were observed to produce Cl- with rate constants ranging from one-third to two-thirds the collision rate; At low hydration levels the reactions proceeded at nearly the collision rate via a ligand switching mechanism, displacing one or more water molecules to produce a trihalide (i.e., XCl(2)(-)(D2O)(m) + (n - m)D2O where m < n). The rate constants decreased for the fluoride and chloride clusters with each increase in hydration level at n greater than or equal to 4 and n greater than or equal to 6, respectively. The bromide and iodide clusters continued to react at nearly the collision rate over the entire range of hydration levels studied. The implications these results have on the plausibility of halide surface reactions occurring on atmospheric aerosols and the structures of halide clusters is discussed.