An Ab Initio Molecular Dynamics Study of the Hydrolysis Reaction of Sulfur Trioxide Catalyzed by a Formic Acid or Water Molecule.

Ab initio molecular dynamics (AIMD) calculations have been performed to investigate the role of dynamical and steric effects in formic acid (FA) or HO-catalyzed gas phase hydrolysis of SO to form sulfuric acid. This was done by colliding FA or HO with the SO-HO complex and the water dimer with the SO molecule and analyzing the outcomes of 230 AIMD trajectories. Our calculations show that, within simulation times used, sulfuric acid is formed in 5% of FA collisions but is not produced when HO collides with the SO-HO complex or when the water dimer collides with the SO molecule. We also find that FA collisions have about 2 times higher probability to form the prereactive complex than HO collisions. Moreover, our simulations show that the SO-HO-FA prereactive complex is more stable in time than the SO-HO-HO prereactive complex. These findings indicate that the FA-catalyzed mechanism is favored over the HO one when looking from the steric and dynamic effect point of view. Additionally, AIMD simulations starting from the optimized structure of the SO-HO-FA prereactive complex have been computed to qualitatively estimate the rate of the sulfuric acid formation. Collisional energy has been observed to promote sulfuric acid formation more effectively than thermal excitation.