The direct hydrolysis of formic acid in the gas phase is not energetically viable as an atmospheric process. However, in the presence of carboxylic acids, which are ubiquitous in the atmosphere, the reaction barrier to hydrolysis is reduced significantly, with barriers lowered by more than 40 kcal/mol to values near or below the energetic level of the entrance channel. The isolated product of this hydrolysis is methane triol, which is extremely hygroscopic and can serve as a seed for water aggregate formation. Reactions proceed through a pre-reactive complex followed by a ring-shaped transition state, which is highly stabilized by a hydrogen-bonding network. The resulting acid and triol form a post-reactive complex, which is stabilized by 10-15 kcal/mol relative to products.When this system proceeds completely to isolated products, it represents pure catalytic behaviour on the part of the mediating acid. However, the endothermicity of the reaction combined with complex stability suggests that these reactions have the potential to remain in the post-reactive well under atmospheric conditions, thereby producing robust bound species that could potentially serve as seeds for the formation of secondary organic aerosols (SOA). The study of eight carboxylic acids plus water shows a strong linear correlation between hydrolysis barrier height and pKa, providing a predictive tool for the degree of catalytic stabilization expected for a given acid.

Gas-phase hydrolysis of formic acid mediated by atmospheric carboxylic acids

248th National Meeting (2014)

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