Thermochemistry of HIO2 Species and Reactivity of Iodous Acid with OH Radical: A Computational Study

Abstract

This paper reports computations of thermochemical properties [ΔfH298 K°, S298 K°, and Cp = f(T)] of HIO2 isomers (HOOI and HOIO) together with the kinetic parameters of the gas-phase HOIO + OH reaction. The calculations are performed using the CCSD(T) method on structures previously optimized at the B3LYP/aug-cc-pVTZ level of theory. Spin–orbit coupling is computed by employing the CASSCF/CASPT2/RASSI scheme. The standard enthalpies of formation at 298 K are equal to −24.8 ± 0.9 and 18.7 ± 0.4 kJ mol–1 for HOIO and HOOI, respectively. The H- and O(H)-abstraction pathways are considered for the reaction of iodous acid (HOIO) with OH. Our calculations reveal that the H-abstraction channel was exergonic by −135 kJ mol–1 at 298 K. The rate constants were computed as a function of the temperature (250–2500 K) using classical and variational transition state theory. At the CCSD(T)/CBS level, the rate constants are (in cm3 molecule–1 s–1): kHabs(T) = 2.22 × 10–19T1.83 exp[−4.9 (kJ mol–1)/RT] and kOHabs(T) = 7.73 × 10–21T2.66 exp[−70.2 (kJ mol–1)/RT]. The HOIO + OH overall reaction is significantly dominated by the HOIO + OH → OIO + H2O channel for tropospheric temperatures. The atmospheric lifetime of HOIO is estimated to be approximately 64 years. Thermochemical properties and kinetic parameters derived from this study can be used as input parameters in chemistry-transport models, for which iodine is important in the case of radioactive airbone species released for a severe accident occurring to nuclear power plants.