Effects of Microhydration on the Mechanisms of Hydrolysis and Cl - Substitution in Reactions of N 2 O 5 and Seawater.

The reaction of N 2 O 5 at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N 2 O 5 reacts preferentially with Cl - to form ClNO 2 /NO 3 - (Cl - substitution), but can also react with H 2 O to form 2HNO 3 (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N 2 O 5 /Cl - /nH 2 O (n=2-5), resulting in the identification of three reaction motifs. First, we uncovered an S N 2-type Cl - substitution reaction of N 2 O 5 that occurs very quickly due to low barriers to reaction. Second, we found a low-lying pathway to hydrolysis via a ClNO 2 intermediate (two-step hydrolysis). Finally, we found a direct hydrolysis pathway where H 2 O attacks N 2 O 5 (one-step hydrolysis). We find that Cl - substitution is the fastest reaction in every cluster. Between one-step and two-step hydrolysis, we find that one-step hydrolysis barriers are lower, making two-step hydrolysis (via ClNO 2 intermediate) likely only when concentrations of Cl - are high.