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

Abstract

This paper reports computations of thermochemical properties [Δf H 298 K °, S 298 K °, and C p = 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): k Habs(T) = 2.22 × 10–19 T 1.83 exp­[−4.9 (kJ mol–1)/RT] and k OHabs(T) = 7.73 × 10–21 T 2.66 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.

Unraveling the Tropospheric Microhydration Processes of Iodous Acid HOIO

“…Comparison with the previous data obtained by Khanniche et al. 18 at the B3LYP/aug-cc-pVTZ level of theory shows slight differences between the bond lengths (<0.034 Å) and angles (<1°). 37 ).…”

“…Optimized geometry parameters and vibrational frequencies for H 2 O at the ωB97X-D/aug-cc-pVTZ level of theory have been already presented in our last studies. , The corresponding data for iodous acid are presented in Tables S1 and S2 of the Supporting Information. Comparison with the previous data obtained by Khanniche et al at the B3LYP/aug-cc-pVTZ level of theory shows slight differences between the bond lengths (<0.034 Å) and angles (<1°).…”

“…[HOIO] was about 2 × 10 6 molecules cm –3 . Previous computational studies ,− have been mainly performed to determine the structural and thermochemical properties [Δ f H 298K ° , S 298K ° , and C p = f ( T )] of gas-phase HIO 2 isomers (HOIO, HOOI, and HIOO). HOIO was the most stable isomer in terms of standard enthalpy of formation at 298 K .…”

“…Previous computational studies ,− have been mainly performed to determine the structural and thermochemical properties [Δ f H 298K ° , S 298K ° , and C p = f ( T )] of gas-phase HIO 2 isomers (HOIO, HOOI, and HIOO). HOIO was the most stable isomer in terms of standard enthalpy of formation at 298 K . The same study determined also its reactivity toward the most important atmospheric photo-oxidant, that is, hydroxyl radicals.…”

Determination of the absorption cross-sections of higher order iodine oxides at 355 nm and 532 nm

“…Recent Chemical Ionization -Atmospheric Pressure Interface-Time of Flight Mass Spectrometry (CI-API-ToF-MS) observations of IO3and IO3-containing ion clusters in coastal and polar environments, as well as complementary laboratory experiments in the absence of HOx, have been interpreted as direct measurements of gas-phase iodic acid (HOIO2, hereafter denoted as HIO3) and HIO3 clusters (Sipilä et al, 2016). Since all possible reaction paths between H2O and I, IO and OIO are very endothermic (Canneaux et al, 2010;Hammaecher et al, 2011;Khanniche et al, 2017a), and IOx-H2O complexes are very weakly bound (Galvez et al, 2013), the formation of oxyacids may rather proceed via hydrolysis of higher iodine oxides (Kumar et al, 2018):…”