Reactions of O(³P) with Alkyl Iodides:  Rate Coefficients and Reaction Products

Abstract

The rate coefficients for the reactions of O( 3 P) with CF 3 I (1) and CH 3 I (2) were measured between 213 and 364 K to be: k 1 (T) ) (7.9 ( 0.8) x 10 -12 exp [-(175 ( 40)/T] and k 2 (T) ) (1.0 ( 0.2) x 10 -11 exp[(160 ( 50)/T] cm 3 molecule -1 s -1 . The rate coefficients for the reaction of O( 3 P) with CD 3 I, (CH 3 ) 2 CHCH 2 I, (CH 3 ) 2 -CHI, CF 3 CH 2 I, CF 3 CHFI, and CF 3 CF 2 I at 298 K were also measured. The yields of the IO and CF 3 O products in reaction 1 at 298 K were found to be 0.83 ( 0.09 and <0.01, respectively. Product yields of reaction 2 at 298 K were measured to be IO (0.44 ( 0.04), OH (0.16 ( 0.05), H (0.07 ( 0.02), CH 3 O (<0.03), and HI (<0.05). IO, OH, and CH 3 O were detected via laser-induced fluorescence, O and H atoms via resonance fluorescence, and IO, CF 3 O, and HI by chemical ionization mass spectrometry. The IO yield from reactions 1 and 2 showed a slight positive temperature dependence. Within our experimental error the OH yield from reaction 2 was temperature independent. An upper limit for the enthalpy of formation of IO, ∆ f H°(298.15 K)(IO) <28.8 kcal mol -1 , was obtained from an interpretation of our data.

Potential Energy Surfaces for the Reaction of O Atoms with CH3I: Implications for Thermochemistry and Kinetics

“…This last adjustment is within the G2 uncertainty, and we further note that the QRRK treatment imposes an energy graining or resolution of hν j, where ν j is the geometric mean frequency of the adducts, which is about 10 kJ mol -1 . Figure 4 illustrates the good accord between the QRRK results and the measurements of Gilles et al 2 The QRRK analysis is based on the assumption that all of the reaction proceeds via ISC and that the probability that a collision initially on the triplet PES leads to the singlet PES is high, J 0.1. Production of IO via the singlet surface is slightly inversely pressure dependent because as the pressure is increased adduct stabilization rather than fragmentation begins to become favored.…”

“…21,22 The Qs are the partition functions, and E 0 ‡ is the enthalpy of the TS relative to the reactants at 0 K. The k 3 results are shown in Arrhenius form in Figure 4, together with the measured rate constant for OH production (16% yield) of Gilles et al 2 The computed results differ by a factor of 10 4 from this measurement at room temperature and thereby suggest that another pathway is involved in OH production.…”

“…We note that both H and HI were observed as minor products (branching ratios approximately 7% and 2%, respectively). 2 Thus, our proposed PES is able to account qualitatively for all the observed products of reaction 1; the next section summarizes a more quantitative test.…”

“…This pathway, in combination with a possible contribution from direct abstraction on the triplet PES, presumably accounts for IO formation, which is the observed dominant product channel (44% branching ratio at 298 K). 2 CH 3 IO could isomerize via a three-center TS to more stable CH 3 OI, but this is the least favorable pathway both entropically and energetically. Consistent with this idea, CH 3 O, which would be formed easily by O-I bond fission in initially excited CH 3 OI, was not observed experimentally (branching ratio < 3%).…”

A kinetic study of the reaction of atomic hydrogen with iodobenzene

“…Reactions of hydrogen atoms with alkyl iodides have been the subject of several kinetic studies [1][2][3][4][5][6][7], motivated in part by the potential contributions of iodine to combustion chemistry [8]. Interesting kinetics have been observed in some cases, such as Cl-atom addition [9] and O-atom addition followed by elimination of HOI [10].…”