RESUMEN
SO3 is an important reactive species in sulfur cycle and sulfuric acid formation processes and its reactions with some functional group substances, such as H2O, NH3, CH3OH, and organic and inorganic acids, have been extensively studied. However, its loss mechanism with multifunctional species is still lacking in detail. Herein, the reaction mechanism between SO3 and monoethanolamide (MEA) was investigated in the gas phase and on water droplets. The quantum chemical calculations indicate that the gas-phase reactions of SO3 with the OH and NH2 moieties of MEA hardly occur as their reaction energy barriers are up to 21.9-29.4 kcal mol-1. When a single water molecule is added into the SO3 + MEA reaction, it not only decreases the reaction barrier by at least 15.0 kcal mol-1 and thus enhances the rate obviously, but can also lead to the main product changing from HOCH2CH2NHSO3H to NH2CH2CH2OSO3H. The Born Oppenheimer molecular dynamics simulations on a water droplet show that the routes of the NH2CH2CH2OSO3-â¯H3O+ ion pair, HSO4- and HOCH2CH2NH3+ ions and zwitterionic formations of HOCH2CH2NH2+-SO3- and SO3--OCH2CH2NH3+ occur through a loop-structure route or chain reaction process, and can be finished within several picoseconds. Interestingly, the nucleation simulations show that the products of HOCH2CH2NHSO3H and NH2CH2CH2OSO3H have a potential ability to participate in the formation of new particles as they can form larger clusters with H2SO4, NH3 and H2O molecules within 20 ns. Thus, the present study will not only give new insight into the reaction between SO3 and multifunctional compounds, but also provide a new potential formation mechanism for particles resulting from the loss of SO3.
RESUMEN
Liu et al. (Proc. Natl. Acad. Sci. U. S. A, 2019, 116, 24966-24971) showed that at an altitude of 0 km, the reaction of SO3 with CH3OH to form CH3OSO3H reduces the amount of H2SO4 produced by the hydrolysis of SO3 in regions polluted with CH3OH. However, the influence of the water molecule has not been fully considered yet, which will limit the accuracy of calculating the loss of SO3 in regions polluted with CH3OH. Here, the influence of water molecules on the SO3 + CH3OH reaction in the gas phase and at the air-water interface was comprehensively explored by using high-level quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations. Quantum chemical calculations show that both pathways for the formation of CH3OSO3H and H2SO4 with water molecules have greatly lowered energy barriers compared to the naked SO3 + CH3OH reaction. The effective rate coefficients reveal that H2O-catalyzed CH3OSO3H formation (a favorable route for CH3OSO3H formation) can be competitive with H2O-assisted H2SO4 formation (a favorable process for H2SO4 formation) at high altitudes up to 15 km. BOMD simulations found that H2O-induced formation of the CH3OSO3-â¯H3O+ ion pair and CH3OH-assisted formation of HSO4- and H3O+ ions were observed at the droplet surface. These interfacial routes followed a loop-structure or chain reaction mechanism and proceeded on a picosecond time scale. These results will contribute to better understanding of SO3 losses in the polluted areas of CH3OH.