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Reactivity of CHI3 with OH Radicals: X‑Abstraction Reaction Pathways (X = H, I), Atmospheric Chemistry, and Nuclear Safety

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journal contribution
posted on 09.10.2014, 00:00 by Mária Sudolská, Florent Louis, Ivan Černušák
The X-abstraction (X = H, I) pathways in the reaction of CHI3 with OH radical, a possible iodoform removal process relevant to the Earth’s atmosphere and conditions prevailing in the case of a nuclear accident, have been studied applying highly correlated ab initio quantum chemistry methods and canonical transition-state theory to obtain reaction energy profiles and rate constants. Geometry optimizations of reactants, products, molecular complexes, and transition states determined at the MP2/cc-pVTZ level of theory have been followed by DK-CCSD­(T)/ANO-RCC single-point energy calculations. Further improvement of electronic energies has been achieved by applying spin–orbit coupling, corrections toward full configuration interaction, vibration contributions, and tunneling corrections. Calculated reaction enthalpies at 0 K are −108.2 and −5.1 kJ mol–1 for the H- and I-abstraction pathways, respectively; the strongly exothermic H-abstraction pathway is energetically favored over the modestly exothermic I-abstraction one. The overall rate constant at 298 K based on our ab initio calculations is 4.90 × 10–11 cm3 molecule–1 s–1, with the I-abstraction pathway being the major channel over the temperature range of 250–2000 K. The CHI3 atmospheric lifetime with respect to the removal reaction with OH radical is predicted to be about 6 h, very short compared to that of other halomethanes.