Infrared matrix- isolation and theoretical studies of interactions between CH3I and water

International audienceGaseous methyl iodine (CH3I) is naturally emitted in the atmosphere over oceans through the algae and phytoplankton activities. The fate of naturally emitted CH3I is of great interest because of the oxidizing properties of iodine and its impact on the catalytic destruction of the ozone layer. Additionally, CH3I is one of the gaseous species that can be produced in the case of severe nuclear accident, so, its radiological impact requires knowledge about its behavior in the atmosphere. Water is one of the major species in the atmosphere, which is responsible for atmospheric aerosol nucleation and cloud condensation nuclei. Water can also act as a reactive medium leading to secondary product formation. The study of the interaction between methyl iodine and water at the molecular scale is contributing for a better understanding of the fate of such halogen alkyl into the atmosphere. Here the micro-hydration of CH3I was investigated using cryogenic matrix experiments which were supported by theoretical DFT calculations. A large excess of water regarding CH3I was used in order to mimic atmospheric conditions. Dimers and trimers of CH3I were observed despite the high water amount in the initial mixture. This may be explained by the low affinity of CH3I with water. Considering the concentration of CH3I used in the experiments, the aggregates are likely formed in the gas phase. The interaction between CH3I and H2O molecules studied for the first time experimentally and supported by DFT calculations highlights that, in the atmosphere, gaseous methyl iodine and water will likely form aggregates of water and CH3I polymers instead of (CH3I)m-(H2O)n hetero complexes. However, mixed CH3I:H2O complexes 1:1, 1:2 and 1:3 (see Figure) have been observed , whereas 2:1 and 2:2 complexes appear as minor species