Browsing by Subject "water dimer"

Sulfuric acid has a central role in atmospheric chemistry, as it is considered to have a significant contribution in cloud formation and acid rain. In the gas phase, hydrolysis of SO3 catalysed by a single water molecule is contemplated to be the primary pathway to form sulfuric acid in the atmosphere. However, in previous studies it has been calculated that when the hydrolysis reaction is catalysed by a formic acid (FA) molecule, the potential energy barrier is significantly lower than for the water molecule catalysed reaction. In this work, the role of dynamic and steric effects for both reactions were studied through ab initio molecular dynamics (AIMD) collision simulations. The simulations were done by either colliding FA or a water molecule with SO3-H2O complex or a water dimer with the SO3-molecule. Altogether 230 trajectories were calculated at PBE/6-311+G(2pd,2df) level of theory, 70 for the collision of a water dimer and SO3, and 80 for both the collision of a water molecule or FA with SO3-H2O. The collision of FA with SO3-H2O led to the formation of sulfuric acid in 5 % of the simulations, whereas for the collision of a water molecule with SO3-H2O the reaction does not occur within the simulation time. Additionally, the SO3-H2O-FA pre-reactive complex formed in the simulations is shown to be more stable, most likely due to a less constrained ring structure. The collision of a water dimer with SO3 most commonly leads to the formation of SO3-H2O, and either sticking or evading of the second water molecule of the dimer. Based on the simulation results, strictly in terms of dynamic and steric effects, the FA-catalysed mechanism seems to be favored over the H2O-catalysed one