Formic Acid Catalyzed Hydrolysis of SO_3 in the Gas Phase: A Barrierless Mechanism for Sulfuric Acid Production of Potential Atmospheric Importance

摘要

Computational studies at the B3LYP/6-311++G-(3df,3pd) and MP2/6-311++G(3df,3pd) levels are performed to explore the changes in reaction barrier height for the gas phase hydrolysis of SO_3 to form H_2SO_4 in the presence of a single formic acid (FA) molecule. For comparison, we have also performed calculations for the reference reaction involving water assisted hydrolysis of SO_3 at the same level. Our results show that the FA assisted hydrolysis of SO_3 to form H_2SO_4 is effectively a barrierless process. The barrier heights for the isomerization of the SO_3 • • • H_2O • • • FA prereactive collision complex, which is the rate limiting step in the FA assisted hydrolysis, are found to be respectively 0.59 and 0.08 kcal/mol at the B3LYP/6-311++G-(3df,3pd) and MP2/6-311++G(3df,3pd) levels. This is substantially lower than the ~7 kcal/mol barrier for the corresponding step in the hydrolysis of SO_3 by two water molecules-which is currently the accepted mechanism for ( ) production. Simple kinetic analysis of the relative rates suggests that the reduction in barrier height facilitated by FA, combined with the greater stability of the prereactive SO_3 • • • H_2O • • • FA collision complex compared to SO_3 • • • H_2O • • • H_2O and the rather plentiful atmospheric abundance of FA, makes the formic acid mediated hydrolysis reaction a potentially important pathway for atmospheric sulfuric acid production.