Computational Studies of the Thermochemistry of the Atmospheric Iodine Reservoirs HOI and IONO2

CCSD(T) theory with aug-cc-pVTZ-PP and aug-cc-pVQZ-PP basis sets has been applied to hypoiodous acid and iodine nitrate, two potential reservoirs for atmospheric iodine. The results are employed in bond-conserving reactions and extrapolated to the complete basis set limit, to yield ΔfH298(HOI)=−59.2±3.3kJmol−1 and ΔfH298(IONO2)=37.4±3.9kJmol−1. For iodine nitrate the bond dissociation enthalpies DH0(IO−NO2)=113.6±3.1kJmol−1 and DH0(I−NO3)=141.6±3.9kJmol−1 are derived. DH0(IO−NO2) is used in Troe's unimolecular formalism to yield the 298 K low-pressure limiting rate constant for IO + NO2 addition as (5.3–13.3)×10−31cm6molecule−1s−1 for N2 bath gas, depending on the approach taken to define the rotational term Frot. This range is in good accord with measured values. At 1 atm N2 and 298 K, the lifetime for IONO2 with respect to thermal dissociation is predicted to be of the order of 6 h, with an uncertainty of a factor of 3.5.