Ab Initio Characterization of (CH₃IO₃) Isomers and the CH₃O₂ + IO Reaction Pathways

Abstract

The geometries, harmonic vibrational frequencies, relative energetics, and enthalpies of formation of (CH(3)IO(3)) isomers and the reaction CH(3)O(2) + IO have been investigated using quantum mechanical methods. Optimization has been performed at the MP2 level of theory, using all electron and effective core potential, ECP, computational techniques. The relative energetics has been studied by single-point calculations at the CCSD(T) level. Methyl iodate, CH(3)OIO(2), is found to be the lowest-energy isomer showing particular stabilization. The two nascent association minima, CH(3)OOOI and CH(3)OOIO, show similar stabilities, and they are considerably higher located than CH(3)OIO(2). Interisomerization barriers have been determined, along with the transition states involved in various pathways of the reaction CH(3)O(2) + IO.

Structures, Vibrational Spectra, and Relative Energetics of CH3COIO3 Isomers

“…Two of the experimental studies have shown that the reactions RO 2 ＋IO are fast and may indeed have an important impact on ozone chemistry. Enami et al 13 found the value k＝ (7. 2 , i.e., it is smaller than the former measurements by about 30 times. The difference is quite large, indicating that the system is very complex.…”

“…Several experimental studies reported recently the coupling of IO with the hydrogen peroxy, HO 2 , methylperoxy, CH 3 O 2 , trifluoromethylperoxy, CF 3 O 2 , and ethylperoxy, C 2 H 5 O 2 radicals and measured the corresponding rate coefficients. 2,6,8 These systems are analogous to the reactions of HO 2 and CH 3 O 2 with the ClO and BrO radicals for which several experimental and theoretical studies have been reported. [9][10][11] It was found that the reaction of HO 2 ＋IO 12 displayed a rate coefficient, k＝ (8.4±0.3)×10 -11 cm 3 •molecule -1 •s -1 at 298 K and Drougas et al 6 suggested that the HO 2 and IO reaction proceed through the formation of the bound complexes, HOOOI and HOOIO.…”

“…1 The formation of IO, and OIO 2 in the marine boundary layer and the potential role of iodine compounds in tropospheric ozone depletion cycles, have stimulated an increasing interest in the reactivity of iodine containing species. The abundant iodine-containing source including I 2 , CH 3 I, CH 2 I 2 , C 2 H 5 I, CH 2 ICl, i-C 3 H 7 I, and CH 2 IBr, 3 plays a significant role in the iodine-catalyzed ozone destruction that may contribute up to half of marine boundary layer chemical net destruction, and it is also clear that iodine is involved in the production of ultrafine particles in coastal regions. 3 The coupling of iodine oxides with other radical families is critical as such processes lead to destruction of the ozone layer.…”

“…3 The coupling of iodine oxides with other radical families is critical as such processes lead to destruction of the ozone layer. The reactions of IO, in particular with itself, ClO, BrO and HO 2 have been examined in detail and have been shown to be important processes affecting the concentrations of ozone in the lower atmosphere, 2,[4][5][6][7] which would lead to the following ozone depletion cycles: …”

Iodine chemistry in the troposphere and its effect on ozone

“…3 and 4). h Updated heats of formation for IO, OIO, and CH 3 O 2 (Dooley et al, 2008;Gómez Martín and Plane, 2009;Knyazev and Slagle, 1998) show that the only accessible exothermic product channel of CH 3 O 2 + IO (Drougas and Kosmas, 2007) is CH 3 O + I + O 2 ( H r = −5 ± 6 kJ mol −1 ), consistent with the high yield of I and low yield of OIO found experimentally (Bale et al, 2005;Enami et al, 2006). Sensitivity studies have been carried out using the preferred rate constant for this reaction of 2 × 10 −12 cm 3 molecule −1 s −1 (Dillon et al, 2006b), resulting in an enhancement of the ozone loss of 0.5% in the MBL and of less than 0.1% integrated throughout the troposphere in the J IxOy scenario, and similarly negligible enhancements in the Base scenario.…”