Atmospheric Oxidation Kinetics of Monochloramine by Hydroxyl Radical, Carbonyl Oxide, and Sulfur Trioxide Catalyzed...

Monochloramine (NH₂Cl) serves as a reservoir for atmospheric chlorine, contributing to radical chemistry and indirectly affecting ozone formation and the growth of secondary organic aerosols. We investigated the nonphotolytic atmospheric removal of NH₂Cl by the key oxidants OH, CH₂OO, and SO₃ by using dual-level kinetics calculations based on high-level quantum chemical calculations and density functional theory. The higher level of electronic structure (HL) is coupled cluster theory with single, double, triple, and quadruple excitations and noniterative pentuple excitations with a complete basis [CCSDTQ(P)/CBS] for the OH reaction and CCSDT(Q)/CBS for the CH₂OO and SO₃ reactions. The kinetics calculations are direct dynamics calculations that combine conventional transition state theory based on coupled cluster electronic structure and variational transition state theory with small-curvature tunneling based on density functional theory. The results reveal that beyond-CCSD(T) contributions can be as large as 0.8 kcal/mol for determining enthalpies of activation. Reaction-specific anharmonicity effects were found to enhance the rate constant at 190 K by a factor of 14 for NH₂Cl + OH and a factor of 8.5 for NH₂Cl···SO₃ + H₂O; these factors decrease to 6.2 and 4.4, respectively, at 280 K. Anharmonic effects are much smaller (factors ≤1.16) for NH₂Cl + CH₂OO. Tunneling was found to enhance the rate constant at 190 K by a factor of 4.5 for NH₂Cl + OH and a factor of 20 for NH₂Cl···SO₃ + H₂O; these factors decrease to 2.0 and 2.3, respectively, at 280 K. The recrossing effects reduce the NH₂Cl + OH rate coefficient by 0.46 at 190 K and 0.67 at 280 K. The contributions of the various sinks for atmospheric NH₂Cl─photolysis, reaction with OH, multiphase processes, and the reaction with SO₃ catalyzed H₂O─depend on the conditions.