Experimental and Computational Investigations of the Reaction of OH with CF₃I and the Enthalpy of Formation of HOI

Abstract

The kinetics of the reaction of hydroxyl radicals with trifluoroiodomethane were investigated by the flash photolysis-resonance fluorescence technique. A rate constant of k ) 5.8 × 10 -12 exp((-11.3 kJ mol -1 )/RT) cm 3 molecule -1 s -1 was measured over the temperature range 280-450 K with accuracy limits of 20% (450 K) to 30% (280 K). Different product channels were investigated by ab initio methods, and the dominant products are CF 3 + HOI. The enthalpy of formation of hypoiodous acid was analyzed with Gaussian-2 theory, in conjunction with G2 energies for INO, ICN, ClCN, and other species. The transition state and reaction coordinate for OH + CF 3 I was characterized at the G2(MP2) level, and the results suggest a negligible barrier to the reverse reaction of CF 3 + HOI, so that the measured forward activation energy can be used to derive ∆ f H 298 (HOI) ) -69.6 ( 5.4 kJ mol -1 . The implications of the kinetics and thermochemistry for iodine chemistry in flames and the atmosphere are discussed, and for the range 280-2000 K a proposed rate expression is k ) 2.9 × 10 -16 (T/K) 1.5 exp(-(960 K)/T) cm 3 molecule -1 s -1 .

298 K rate coefficients for the reaction of OH with <i>i</i> - C<sub>3</sub>H<sub>7</sub>I, <i>n</i> - C<sub>3</sub>H<sub>7</sub>I and C<sub>3</sub>H<sub>8</sub>

“…However, the latest estimates for the heat of formation of HOI (Berry et al, 1998) suggest that reaction (17a) is endothermic by > 20 kJmol −1 for both n−C 3 H 7 I and i−C 3 H 7 I and therefore unlikely to be significant in the present experiments at 298 K, or in the atmosphere. As the present study gives no indications to the contrary, we assume that the reaction proceeds via H-atom abstraction to give H 2 O and an iodine containing alkyl radical as products.…”

“…The fact that both n−C 3 H 7 I and i−C 3 H 7 I react more rapidly with OH than calculated from the remaining number of primary and secondary H-atoms, suggests that the presence of the I-atom results in a weakening of the neighbouring C-H bonds as seen for the reactions of CH 3 I with OH (k = 7.2 × 10 −14 cm 3 s −1 at 298 K) and CH 4 + OH (k = 6.3 × 10 −15 cm 3 s −1 at 298 K) (DeMore et al, 1997). An alternative explanation is that the reaction partially proceeds via a complex mechanism in which OH attack at the C-I bond can lead to products, in a similar fashion to reactions of O( 3 P) with alkyl-iodides (Gilles et al, 1996;Klaasen et al, 1996) (see above) and OH with CF 3 I (Brown et al, 1990;Berry et al, 1998). There is also theoretical evidence supporting a complex mechanism for the reaction of OH with haloethanes (Sekusak and Sabljic, 1997).…”

Probing ground and low-lying excited states for HIO2 isomers

“…To assess the accuracy of the present CCSD(T)/aug-cc-pVTZ approach for the HIO 2 isomers, enthalpies of formation have been determined at this level of theory for a number of small iodine containing species (IO, OIO, HIO, and HIO 2 isomers) to compare to values computed or measured previously, 22,[44][45][46] see Table I. Since the present work is focused on the relative energies and corresponding IR and UV spectroscopy of the HIO 2 isomers, the list of previous results is not exhaustive; for a more comprehensive list, the interested reader should consult the work of Grant et al 44 IOO has been excluded as determining the enthalpy of formation directly from total atomization energies, rather than from isodesmic reaction, has been suggested to introduce errors due to the neglect of higher order correlation effects.…”

Ab Initio Calculations for Kinetic Modeling of Halocarbons

“…k 12b was roughly estimated via the relation k = A exp(-E a /RT) CF 3 I + OH (52), and an activation energy E a = 23 kJ mol" 1 . This is equalto the endothermicity at 298 K based on D 298 (HO-I) = 216 kJ mol" 1(52). 4(B) shows that, by contrast to H atom attack, the bond that is more reactive towards OH is the stronger C-H bond and that CH2 I + H 2 0 formation, the more exothermic channel, is expected to dominate at all temperatures.…”