Formation of Hydrogen Peroxide from O-(H2O)n Clusters

Intracluster Reaction Dynamics of Ionized Micro-Hydrated Hydrogen Peroxide (H2O2): A Direct Ab Initio Molecular Dynamics Study

Hydrogen peroxide (H₂O₂) is a unique molecule that is applied in various fields, including energy chemistry, astrophysics, and medicine. H₂O₂ readily forms clusters with water molecules. In the present study, the reactions of ionized H₂O₂-water clusters, H₂O₂ ⁺(H₂O) _n , after vertical ionization of the parent neutral cluster were investigated using the direct ab initio molecular dynamics (AIMD) method to elucidate the reaction mechanism. Clusters with one to five water molecules, H₂O₂-(H₂O) _n (n = 1-5), were examined, and the reaction of [H₂O₂ ⁺(H₂O) _n ]_ver was tracked from the vertical ionization point to the product state, where [H₂O₂ ⁺(H₂O) _n ]_ver is the vertical ionization state (hole is localized on H₂O₂). After ionization, fast proton transfer (PT) from H₂O₂ ⁺ to the water cluster (H₂O) _n was observed in all clusters. The HOO radical and H₃O⁺(H₂O) _n-1 were formed as products. The PT reaction proceeds directly without an activation barrier. The PT times for n = 1-5 were calculated to be 36.0, 9.8, 8.3, 7.7, and 7.1 fs, respectively, at the MP2/6-311++G(d,p) level, indicating that PT in these clusters is a very fast process, and the PT time is not dependent on the cluster size (n), except in the case of n = 1, where the PT time was slightly longer because the bond distance and angle of the hydrogen bond in n = 1 were deformed from the standard structure. The reaction mechanism was discussed based on these results.

Reaction mechanism of an intracluster SN2 reaction induced by electron capture

Bimolecular nucleophilic substitution (S_N2) reactions have been widely investigated from both experimental and theoretical points of view because they represent one of the simplest organic reactions. Most studies on S_N2 reactions have been focused on bimolecular collision. In contrast, information on intracluster S_N2 reactions is limited. In this study, an intracluster S_N2 reaction of NF₃-CH₃Cl triggered by electron attachment was investigated using a direct ab initio molecular dynamics (AIMD) method. In the structure of NF₃-CH₃Cl, the N-F bond in NF₃ is oriented collinearly toward the carbon atom of CH₃Cl. After electron capture by NF₃-CH₃Cl, the F^- ion that is generated from the (NF₃)^- moiety collides with the carbon atom of CH₃Cl. The intracluster S_N2 reaction occurs as follows: (NF₃-CH₃Cl)^- (electron capture state) → NF₂-(F^-)-CH₃Cl (pre-reaction complex) → transition state (TS) → NF₂-CH₃F-Cl^- (post-reaction complex) → NF₂ + CH₃F + Cl^- (product state). The reaction energy is efficiently transferred to the translational mode of Cl^-, and the Cl^- ion with a high translational energy is then removed from the system. This energy is significantly larger than that of Cl^- formed in the bimolecular S_N2 reaction (F^- + CH₃Cl). The reaction mechanism is discussed based on the theoretical results.

Reaction Dynamics of NO+ with Water Clusters

The reaction of NO⁺ with water molecules plays a crucial role in the D-region of the atmosphere because the reaction provides nitrous acid (HONO) and protonated water species (H₃O⁺). In this study, the reaction of NO⁺ with water clusters, NO⁺ + (H₂O)_n (n = 1-7), was investigated by means of the direct ab initio molecular dynamics method to elucidate the reaction mechanism of NO⁺ in the atmosphere from a theoretical viewpoint. At n = 1 and 2, the reaction of NO⁺ with (H₂O)_n led to the formation of a complex: NO⁺ + (H₂O)_n → NO⁺(H₂O)_n (n = 1 and 2). At n = 3, the formation channel of HONO was open, and HONO was formed according to NO⁺ + (H₂O)_n → HONO---H⁺(H₂O)_n-1 (n = 3), through which H₃O⁺ was also formed as H⁺(H₂O)₂. However, the HONO formation efficiency was significantly low for n = 3. In large clusters with n = 4-7, the HONO formation channel became the main channel, and the dissociation of HONO from the HONO--H⁺(H₂O)_n-1 complex occurred in part: NO⁺ + (H₂O)_n → HONO---H⁺(H₂O)_n-1 → HONO + H⁺(H₂O)_n-1. The energetics and reaction mechanism were discussed on the basis of theoretical results.