Formic Acid Catalyzed Hydrolysis of SO3 in the Gas Phase: A Barrierless Mechanism for Sulfuric Acid Production of Potential Atmospheric Importance

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 SO3 to form H2SO4 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 SO3 at the same level. Our results show that the FA assisted hydrolysis of SO3 to form H2SO4 is effectively a barrierless process. The barrier heights for the isomerization of the SO3 center dot center dot center dot H2O center dot center dot center dot 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 similar to 7 kcal/mol barrier for the corresponding step in the hydrolysis of SO3 by two water molecules which is currently the accepted mechanism for atmospheric sulfuric acid 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 SO3 center dot center dot center dot H2O center dot center dot center dot FA collision complex compared to SO3 center dot center dot center dot H2O center dot center dot center dot H2O and the rather plentiful atmospheric abundance of FA, makes the formic acid mediated hydrolysis reaction a potentially important pathway for atmospheric sulfuric acid production.