Tachikawa Hiroto
Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628, Japan.
J Chem Phys. 2024 Sep 7;161(9). doi: 10.1063/5.0221836.
Nitric oxide (NO) and NO-water clusters play crucial roles in the D-region of the atmosphere because it is postulated that NO+ reacts with H2O to produce nitrous acid (HONO) and H3O+. HONO is the major precursor of the hydroxyl radicals leading to the formation of secondary pollutants. The sources of atmospheric HONO, however, are not fully understood. Previously, the sequential H2O addition reaction, H2O + NO+(H2O)n, and the bi-molecular collision reaction, NO+ + (H2O)n, have been investigated by both experiments and theoretical calculations to determine the formation mechanism of HONO. However, the photo-reactions from NO(H2O)n neutral clusters were not considered for the formation mechanism of HONO. In this study, the intra-cluster reactions of NO+(H2O)n clusters, following ionization of the parent neutral cluster of NO(H2O)n, were investigated using the direct ab initio molecular dynamics method. When n = 4, [NO+(H2O)4]ver [vertical ionization state of NO(H2O)n] yielded HONO and hydrated H3O+ after the intra-cluster reaction, and the reaction time was calculated to be 150 fs. The reaction is expressed as [NO+(H2O)n]ver → HONO + H3O+(H2O)n-2 (reactive) (n > 3). Larger clusters of [NO+(H2O)n]ver (n = 5-8) also yield HONO. In contrast, in smaller clusters (n = 1-3), only solvent re-orientation around NO+ occurred after the ionization: [NO+(H2O)n]ver → NO+(H2O)n (solvent re-orientation) (n = 1-3). The hydration energy of H3O+, which depends on the cluster size (n), plays an important role in promoting the formation of HONO. The reaction mechanism is discussed based on theoretical results.
一氧化氮（NO）和NO-水团簇在大气的D区域中起着至关重要的作用，因为据推测NO⁺与H₂O反应生成亚硝酸（HONO）和H₃O⁺。HONO是导致二次污染物形成的羟基自由基的主要前体。然而，大气中HONO的来源尚未完全明确。此前，通过实验和理论计算研究了水的连续加成反应H₂O + NO⁺(H₂O)ₙ以及双分子碰撞反应NO⁺ + (H₂O)ₙ，以确定HONO的形成机制。然而，HONO的形成机制未考虑来自NO(H₂O)ₙ中性团簇的光反应。在本研究中，使用直接从头算分子动力学方法研究了NO(H₂O)ₙ母体中性团簇电离后NO⁺(H₂O)ₙ团簇的团簇内反应。当n = 4时，[NO⁺(H₂O)₄]ver[NO(H₂O)ₙ的垂直电离态]在团簇内反应后产生HONO和水合H₃O⁺，计算得出反应时间为150飞秒。该反应表示为[NO⁺(H₂O)ₙ]ver → HONO + H₃O⁺(H₂O)ₙ₋₂（反应性）（n > 3）。更大的[NO⁺(H₂O)ₙ]ver团簇（n = 5 - 8）也会产生HONO。相比之下，在较小的团簇（n = 1 - 3）中，电离后仅发生围绕NO⁺的溶剂重新取向：[NO⁺(H₂O)ₙ]ver → NO⁺(H₂O)ₙ（溶剂重新取向）（n = 1 - 3）。取决于团簇大小（n）的H₃O⁺水合能在促进HONO的形成中起重要作用。基于理论结果讨论了反应机制。
