Theoretical Study of the Gaseous Hydrolysis of NO₂ in the Presence of Amines

The effects on the hydrolysis of NO₂ in the presence of methylamine and dimethylamine molecules were investigated by theoretical calculations of a series of the molecular clusters 2NO₂-<i>m</i>H₂O–CH₃NH₂ (<i>m</i> = 1–3) and 2NO₂-<i>m</i>H₂O-(CH₃)₂NH (<i>m</i> = 1, 2). With methylamine included in the clusters, the energy barrier is reduced by 3.2 kcal/mol from that with ammonia, and the corresponding products may form without an energy barrier. The results show that amines have larger effects than ammonia in promoting the hydrolysis of NO₂ on thermodynamics. The additional water molecules can stabilize the transition states and the product complexes, and we infer that adding more water molecules in the reactions mainly act as solvent and promoting to form the methylamine nitrate (CH₃NH₃⁺NO₃^–). In addition, the interactions of CH₃NH₂ and (CH₃)₂NH on the hydration of HNO₃ are also more effective than NH₃, and the NH₄NO₃, CH₃NH₃NO₃, and (CH₃)₂NH₂NO₃ complexes tend to form the larger aerosols with the increasing of water molecules. The equilibrium geometries, harmonic vibrational frequencies, and intensities of both HONO–CH₃NH₂ and HONO–NH₃ complexes were investigated. Calculations predict that the binding energies of both HONO–CH₃NH₂ complexes are larger than HONO–NH₃ complexes, and the OH stretching vibrational frequencies and intensities are most affected. The natural bond orbital analysis was performed to describe the donor–acceptor interactions on a series of complexes in the reactions 2NO₂ + H₂O + CH₃NH₂ and 2NO₂ + H₂O + (CH₃)₂NH, as well as the complexes of HONO–NH₃ and HONO–CH₃NH₂. The results show that the interactions with amines are relatively larger, and the higher stabilization energies between CH₃NH₂ and HONO are found