Interception of Excited Vibrational Quantum States by O2 in Atmospheric Association Reactions

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Science  31 Aug 2012:
Vol. 337, Issue 6098, pp. 1066-1069
DOI: 10.1126/science.1224106

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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 O2 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.

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