Interception of excited vibrational quantum states by O2 in atmospheric association reactions

Abstract

Bimolecular reactions in Earth's atmosphere are generally assumed to proceed between reactants whose internal quantum states are fully thermally relaxed. Here, we highlight a dramatic role for vibrationally excited bimolecular reactants in the oxidation of acetylene. The reaction proceeds by preliminary adduct formation between the alkyne and OH radical, with subsequent O2 addition. Using a detailed theoretical model, we show that the product-branching ratio is determined by the excited vibrational quantum-state distribution of the adduct at the moment it reacts with O2. Experimentally, we found that under the simulated atmospheric conditions O 2 intercepts ∼25% of the excited adducts before their vibrational quantum states have fully relaxed. Analogous interception of excited-state radicals by O2 is likely common to a range of atmospheric reactions that proceed through peroxy complexes.