Ab Initio Reaction Path Analysis for the Initial Hydrogen Abstraction from Organic Acids by Hydroxyl Radicals

Hydrogen abstraction from organic acids by hydroxyl radicals is the initial rate- and selectivity-determining step in the photochemical oxidation of organic acids in the troposphere. To quantify the rate and selectivity of these reactions, the abstraction of hydrogen atoms at the acid, α, β, γ, and methyl positions was studied for valeric acid, C₄H₉COOH, using first principles calculations. At the high-pressure limit, an overall rate coefficient at 298 K of 4.3 × 10⁶ m³/(mol s) was calculated. The dominant pathways are abstraction at the β; the γ; and, to a lesser extent, the acid positions; with a selectivity of 55, 28, and 8%, respectively. This differs from the high selectivity for the acid channel for formic and acetic acids and from the thermodynamic preference for abstraction at the α position, but it is consistent with the experimentally observed preference for the β and the γ positions in larger organic acids. The rate and selectivity are controlled by the strength of hydrogen bonds between the acid group and the hydroxyl radical in the different transition states and do not correlate with the stability of the products. Natural bond orbital analysis was used to quantify the nature and strength of the hydrogen bonds. At 298 K and below 0.1 atm, the collision frequency is insufficient to stabilize the prereactive complexes, and the reaction becomes chemically activated. However, the reaction rate and the selectivity are largely unaffected by this mechanistic change.