| Autor: |
Nuñez-Reyes D; Univ. Bordeaux, ISM, CNRS UMR 5255, Talence F-33400, France., Hickson KM; Univ. Bordeaux, ISM, CNRS UMR 5255, Talence F-33400, France., Loison JC; Univ. Bordeaux, ISM, CNRS UMR 5255, Talence F-33400, France., Spada RFK; Departamento de Física, Instituto Tecnológico de Aeronáutica, São José dos Campos, São Paulo 12228-900, Brazil., Vichietti RM; Departamento de Química, Instituto Tecnológico de Aeronáutica, São José dos Campos, São Paulo 12228-900, Brazil., Machado FBC; Departamento de Química, Instituto Tecnológico de Aeronáutica, São José dos Campos, São Paulo 12228-900, Brazil., Haiduke RLA; Departamento de Química e Física Molecular, Instituto de Química de São Carlos, Universidade de São Paulo, São Carlos, São Paulo 13566-590, Brazil. |
| Abstrakt: |
The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO 2 → HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO 2 reaction accelerates at low temperature as zero-point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO 2 reaction, while the equivalent effect for the CD + CO 2 reaction results instead from a submerged barrier with respect to reactants. |
