Computational study on mechanisms and pathways of the atmospheric NH2 + IO reaction

Theoretical treatment of IO-X (X = N2, CO, CO2, H2O) complexes

Iodine monoxide (IO) is an important component of the biogeochemical cycle of iodine. For instance, it is present in the troposphere, where it plays a crucial role in the physical chemical processes involving iodine containing compounds. Here, we present a theoretical study on a series of atmospherically relevant complexes of IO with N₂, CO, CO₂ and H₂O, where their structural and spectroscopic properties and their interaction energies are computed. Calculations are carried out by means of ab initio post Hartree-Fock (RCCSD(T) and RMP2) methods and density functional theory DFT (PBE0 and M05-2X) based approaches with and without the inclusion of dispersion correction. After comparison to RCCSD(T), we highlight the good performance of M05-2X(+D3) DFT in describing the bonding between IO and X (X = N₂, CO, CO₂, H₂O). Moreover, we found that the IO-X (X = N₂, CO, CO₂, H₂O) complexes are formed by non-covalent interactions between the two monomers. In sum, we characterized two types of complexes: I-bonded and O-bonded, where the former is more stable. The atmospheric implications of the present findings are also discussed such as in the formation of the iodine oxide particles (IOPs).

Kinetics of IO radicals with ethyl formate and ethyl acetate: a study using cavity ring-down spectroscopy and theoretical methods

The gas-phase kinetics of the reactions of IO radicals with ethyl formate (EF) and ethyl acetate (EA) were investigated experimentally using cavity ring-down spectroscopy (CRDS). IO radicals were generated in situ in the CRD reaction zone by photolyzing a mixture of (CH₃I + O₃ + N₂) at 248 nm and thereby probed at 445.04 nm. The rate coefficients for the reactions (IO + EF) and (IO + EA) were measured at a total pressure of 65 Torr of N₂ in the temperature range of 258-358 and 260-360 K, respectively. The rate coefficients for the reactions (IO + EF) and (IO + EA) were measured experimentally at room temperature to be kExpt,298KIO+EF = (3.38 ± 0.67) × 10^-14 and kExpt,298KIO+EA = (1.56 ± 0.30) × 10^-13 cm³ molecule^-1 s^-1, respectively. The effects of pressure and photolysis laser fluence on the kinetics of test reactions were found to be negligible within the experimental uncertainties for the studied range. To complement our experimental findings, the kinetics of the title reactions were investigated theoretically using canonical variational transition state theory (CVT) with small curvature tunnelling (SCT) at the CCSD(T)//M06-2X/def2-SV(P) level of theory in temperatures between 200 and 400 K. Very good agreement was observed between the experimentally measured and theoretically calculated rate coefficients for both the reactions at 298 K. The thermochemical parameters as well as the branching ratios for the title reactions are also discussed in this study.