Computational Studies of (HIO₃) Isomers and the HO₂ + IO Reaction Pathways

Abstract

The geometries, harmonic vibrational frequencies, relative energetics, and enthalpies of formation of (HIO3) isomers have been examined using quantum mechanical methods. At all levels of theory employed, MP2, B3LYP, and CCSD(T), the lowest energy structure is found to be the HOIO2 form, which shows particular stability. The two isomers HOOOI and HOOIO are closely located at the CCSD(T) level of theory. The higher energy structure is HIO3. The interisomerization transition states have been determined, along with the transition states involved in the various pathways of the reaction HO2 + IO.

Structures, Vibrational Spectra, and Relative Energetics of CH3COIO3 Isomers

“…Comparison of the enthalpies of formation for CH 3 COIO 3 isomers with HIO 3 and CH 3 IO 3 isomers calculated previously by Drougas et al 2,6 is shown in Figure 3. The enthalpies of formation for HIO 3 and CH 3 IO 3 were calculated at CCSD(T) theory.…”

“…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.…”

“…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. 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.…”

“…15 The angles of IO3O2 and O3IO4 in CH 3 COOOIO are 114.3° and 104.8°, respectively, and they are also similar to those of CH 3 OOIO (107.5° and 106.3°, respectively) 2 or HOOIO (108.9° and 106.2°, respectively). 6 The third isomeric form is CH 3 COOIO 2 as shown in Figure 1c, which is the lowest energy form of the CH 3 COIO 3 isomers. The IO 2 bond length is 2.060 Å predicted by the B3LYP/6-311＋＋G(3df,3pd) level of theory, which is apparently longer than the bond length of IO3 (1.781 Å) or IO4 (1.780 Å) at the same level of theory.…”

“…Compared to the overlap between the orbitals of two equal-sized oxygen atoms, the overlap between the 5p orbital of the large iodine atom with the 2p orbital of the oxygen atom is poor, thus, the bond length of IO4 in CH 3 COOOOI (2.080 Å) is distinctively longer than that of O3O4 (1.371 Å) or O2O3 (1.445 Å), and also longer than that of CH 3 COOOOBr (1.912 Å), 15 CH 3 OOOI (2.035 Å) 2 or HOOOI (2.012 Å). 6 Due to the greater amount of repulsion between the lone pairs of electrons on iodine and those on oxygen (comprising of O3O4I), than the repulsion occurring between the lone pairs of electrons on the two oxygen atoms (comprising of O2O3O4), the angle of O2O3O4 (108.4°) is smaller than the O3O4I (112.9°) angle.…”