Investigation of the Reaction Mechanism and Kinetics of Iodic Acid with OH Radical Using Quantum Chemistry

Abstract

In this paper, the mechanism of the HOIO2 + OH → products reaction is investigated using quantum chemistry tools. Two pathways are considered: HOIO2 + OH → OIO2 + H2O and HOIO2 + OH → OIO + H2O2. The potential energy surfaces are calculated at the CCSD(T)/CBS(D,T)//B3LYP/aug-cc-pVTZ level of theory. The OH radical was found to attack iodic acid from either the frontside (TSHabs) or the topside (TSOHabs), subsequently leading to H- or OH-transfer channels. Reaction enthalpies and standard Gibbs free reaction energies at 298 K are provided. The values indicate that only the HOIO2 + OH → OIO2 + H2O channel is exothermic and exergonic. Theoretical prediction of the kinetic parameters is performed within the framework of the transition state theory (TST) and variational transition state theory (VTST). The rate constants (in cm3 molecule–1 s–1) for temperatures from 250 to 2500 K are kHabs(T) = 1.76 × 10–22T2.39 exp(−3.5 (kJ mol–1)/RT) and kOHabs(T) = 3.16 × 10–21T2.57 exp(−96.2 (kJ mol–1)/RT). The HOIO2 + OH overall reaction is significantly dominated by the HOIO2 + OH → OIO2 + H2O channel for tropospheric temperatures. The main outcomes of this work are as follows: (i) The lifetime of iodic acid toward its removal by OH radicals is extremely long (1336 years), enabling its transportation to different locations around the Earth (marine, polar, and continental), as confirmed by recent field measurements; other possible loss pathways under clear sky (gas phase) and cloudy (aqueous phase) conditions could reduce its atmospheric lifetime. (ii) The thermochemical properties of the OIO2 radical are provided: ΔfH298 K° = 168.3 kJ mol–1, S298 K° = 301.45 J mol–1 K–1, and Cp(300 K) = 65.82 J mol–1 K–1.