Quantum chemical calculations on a selection of iodine-containing species (IO, OIO, INO3, (IO)2, I2O3, I2O4 and I2O5) of importance in the atmosphere

Abstract

The electronic and geometric structures of the title complexes are studied quantum chemically using ab initio and density functional approaches. Coupled cluster calculations at the scalar relativistic (basis set) level are performed, and the results are corrected for spin–orbit coupling using data from relativistic density functional theory studies. The heats of formation (kJ mol⁻¹) at 298 K are found to be: IO₃ 147.8, INO₃ 33.1, OIO 110.1, I₂O₃ 64.0, I₂O₄ 111.3, I₂O₅ 33.0, IOIO 141.3, IOOI 179.9 and OI(I)O 157.9. These data are used to draw a number of conclusions regarding three important aspects of iodine chemistry in the marine boundary layer. (i) Although the IO self reaction produces the asymmetric dimer, IOIO, it is unlikely that this species plays a further role in the atmosphere as it is short-lived. (ii) INO₃ is sufficiently stable to explain the kinetics of the recombination reaction between IO and NO₂, and the reaction between I₂ and NO₃ to produce I + INO₃ is almost certainly the major source of iodine oxides at night. (iii) The higher iodine oxides I₂O₃ and I₂O₅ are very stable molecules, by contrast to the OIO dimer, I₂O₄, which is much less stable but which should still survive long enough in the marine boundary layer to provide a building block for iodine oxide particle formation.