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 H2O-catalyzed gas phase hydrolysis of SO3 to form sulfuric acid. This was done by colliding FA or H2O with the SO3-H2O complex and the water dimer with the SO3 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 H2O collides with the SO3-H2O complex or when the water dimer collides with the SO3 molecule. We also find that FA collisions have about 2 times higher probability to form the prereactive complex than H2O collisions. Moreover, our simulations show that the SO3-H2O-FA prereactive complex is more stable in time than the SO3-H2O-H2O prereactive complex. These findings indicate that the FA-catalyzed mechanism is favored over the H2O one when looking from the steric and dynamic effect point of view. Additionally, AIMD simulations starting from the optimized structure of the SO3-H2O-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.