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Mechanism and Kinetics of the Water-Assisted Formic Acid + OH Reaction under Tropospheric Conditions

2011; American Chemical Society; Volume: 115; Issue: 20 Linguagem: Inglês

10.1021/jp201517p

1520-5215

Cristina Iuga, J. Raúl Alvarez‐Idaboy, Annik Vivier‐Bunge,

Advanced Chemical Physics Studies

In this work, we have revisited the mechanism of the formic acid + OH radical reaction assisted by a single water molecule. Density functional methods are employed in conjunction with large basis sets to explore the potential energy surface of this radical–molecule reaction. Computational kinetics calculations in a pseudo-second-order mechanism have been performed, taking into account average atmospheric water concentrations and temperatures. We have used this method recently to study the single water molecule assisted H-abstraction by OH radicals (Iuga, C.; Alvarez-Idaboy, J. R.; Reyes, L.; Vivier-Bunge, A. J. Phys. Chem. Lett.2010, 1, 3112; Iuga, C.; Alvarez-Idaboy, J. R.; Vivier-Bunge, A. Chem. Phys. Lett.2010, 501, 11; Iuga, C.; Alvarez-Idaboy, J. R.; Vivier-Bunge, A. Theor. Chem. Acc.2011, 129, 209), and we showed that the initial water complexation step is essential in the rate constant calculation. In the formic acid reaction with OH radicals, we find that the water–acid complex concentration is small but relevant under atmospheric conditions, and it could in principle be large enough to produce a measurable increase in the overall rate constant. However, the water-assisted process occurs according to a formyl hydrogen abstraction, rather than abstraction of carboxylic hydrogen as in the water-free case. As a result, the overall reaction rate constant is considerably smaller. Products are different in the water-free and water-assisted processes.