Effects of Microhydration on the Mechanisms of Hydrolysis and Cl⁻ Substitution in Reactions of N₂O₅ and Seawater

Abstract

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 À to form ClNO 2 /NO 3 À (Cl À 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 À /nH 2 O (n = 2-5), resulting in the identification of three reaction motifs. First, we uncovered an S N 2-type Cl À 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 À 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 twostep hydrolysis (via ClNO 2 intermediate) likely only when concentrations of Cl À are high.

Mechanistic Insights into N2O5-Halide Ions Chemistry at the Air–Water Interface

“…The breaking of the N–O bond and the formation of the X-N bond occur almost simultaneously (Figure S6c), indicating that the substitution mechanism follows an S N 2 mechanism. Further analysis of the relationship between key bond lengths and collective variables (CVs) also confirms that the reaction follows the S N 2 mechanism (Figure S6e), which is consistent with previous work. ,,, (ii) As the X – ion approaches the terminal O atom of N 2 O 5 , the NO 3 – group is separated from the NO 2 + fragment, and the resulting XONO, NO 3 – species are formed (denoted channel 2) (Figure b and Movie S2). Similarly, the breaking of the N–O bond and the formation of the O-X bond were observed to occur almost simultaneously (Figure S6d).…”

“…Although PBE-D3 performs worse than SCAN in describing bulk water properties, it can reproduce the structure of bulk water and is widely used in AIMD simulations of bulk water and aqueous systems. − Prior to the MD simulations, we further evaluated the performance of PBE-D3 by calculating the energy barrier for the reaction of N 2 O 5 with Cl – /5H 2 O using the climbing image nudged-elastic-band (CI-NEB). As shown in Figure S5, the computed activation barrier is 1.8 kcal/mol, which is slightly lower than the 3.1 kcal/mol calculated at the ωB97X-D/aug-cc-pVDZ level of theory . Therefore, PBE-D3 is suitable for our system, although its description of bulk water properties is weaker compared to those of other hybrid functionals.…”

“…These calculations suggest that channel 2 plays a major role in the reaction of Br – and I – systems, with BrONO and IONO being the major products. Note that recent studies have investigated the S N 2 reaction between N 2 O 5 and Cl – in small microsolvated clusters. ,, These calculations showed that the reaction barriers increased monotonically with the number of water molecules in the clusters, indicating that the solvation is important for the N 2 O 5 -X – reaction. Obviously, the solvation of X – and N 2 O 5 at the air–water interface is different from that of X – and N 2 O 5 in water clusters.…”