Theoretical Study of the Gas-Phase Reactions of Iodine Atoms (²P_3/2) with H₂, H₂O, HI, and OH

Abstract

The rate constants of the reactions of iodine atoms with H(2), H(2)O, HI, and OH have been estimated using 39, 21, 13, and 39 different levels of theory, respectively, and have been compared to the available literature values over the temperature range of 250-2500 K. The aim of this methodological work is to demonstrate that standard theoretical methods are adequate to obtain quantitative rate constants for the reactions involving iodine-containing species. Geometry optimizations and vibrational frequency calculations are performed using three methods (MP2, MPW1K, and BHandHLYP) combined with three basis sets (cc-pVTZ, cc-pVQZ, and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVnZ (n = T, Q, and 5), aug-cc-pVnZ (n = T, Q, and 5), 6-311G(d,p), 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Canonical transition state theory with a simple Wigner tunneling correction is used to predict the rate constants as a function of temperature. CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory provide accurate kinetic rate constants when compared to available literature data. The use of the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ and CCSD(T)/6-311++G(3df,3pd) levels of theory allows one to obtain a better agreement with the literature data for all reactions with the exception of the I + H(2) reaction R(1) . This computational procedure has been also used to predict rate constants for some reactions where no available experimental data exist. The use of quantum chemistry tools could be therefore extended to other elements and next applied to develop kinetic networks involving various fission products, steam, and hydrogen in the absence of literature data. The final objective is to implement the kinetics of gaseous reactions in the ASTEC (Accident Source Term Evaluation Code) code to improve speciation of fission transport, which can be transported along the Reactor Coolant System (RCS) of a Pressurized Water Reactor (PWR) in case of a severe accident.

Gas-Phase Reactivity of Cesium-Containing Species by Quantum Chemistry

“…3 (1) with the cardinal number n = 3 (aVTZ), 4 (aVQZ), 5 (aV5Z) and corresponding to the CCSD(T)/aug-cc-pVnZ (n = T, Q, 5) level of theory (two-parameter extrapolation), and second of Peterson et al 30…”

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

“…In the course of this process, gas-phase reactive iodine is involved in two important chemical processes of the background troposphere: ozone depletion and new particle formation. Iodine is thought to be responsible for 9 to 16 % of the contemporary ozone removal in the tropical troposphere (Saiz-Lopez et al, 2014;Sherwen et al, 2016), and there is evidence that anthropogenic ozone pollution enhances iodine release from the sea surface (Carpenter et al, 2013;Chance et al, 2014;MacDonald et al, 2014;Prados-Roman et al, 2015;Cuevas et al, 2018), which in turn has accelerated ozone loss in the last decades (Cuevas et al, 2018). The atmospheric chemistry of iodine is in principle fairly simple, since the main atmospheric fate of the T. R. Lewis et al: Determination of the absorption cross sections iodine atoms is reaction with ozone to form iodine monoxide (IO).…”

“…Recent nitrate ion chemical ionization-atmospheric pressure interface-time-of-flight mass spectrometry (NO − 3 CI-API-ToF-MS) observations of IO − 3 -and IO 3 -containing ion clusters in coastal and polar environments, as well as complementary laboratory experiments in the absence of HO x , have been interpreted as direct measurements of gas-phase iodic acid (HOIO 2 , hereafter denoted as HIO 3 ) and HIO 3 clusters (Sipilä et al, 2016) by ionization of the ambient species by NO − 3 in the instrument inlet. Since all possible reaction paths for I, IO, and OIO with H 2 O are very endothermic (Canneaux et al, 2010;Hammaecher et al, 2011;Khanniche et al, 2017a) and IO x −H 2 O 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):…”

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):…”