Rates of elementary reactions involving the BrO (X²Π) and IO (X²Π) radicals. Part 2.—Reactions of the BrO and IO radicals

Abstract

The mechanisms and rate constants at 293 K of a number of elementary reactions of the ground state BrO (X211, v" = 0) radical are reported. BrO is found to react rapidly only with other free radicals (NO,O),It is unreactive towards singlet ground-state molecules (H2, CH4, C2H4, C21-16), BrO+H2 +products ; k5< ; BrO + CH4, C2H4, C2H6 +products ; k< 101o-o.Free X atoms (X = Br, C1) (formed by bimolecular disproportionation of 2 XO radicals) are normally present in low-pressure systems containing XO radicals. The possible effects on reaction rates of the coexistence of XO and X are considered.The ground state 10(X211, v" = 0,l) radical, produced by the reaction, * rate constants are in units of cm3 niol-I s-l. 22272228 REACTIONS INVOLVING BrO (x") A N D 1 0 ( x " ) the I 0 radical, which might be expected to be closely analogous to those of the BrO radical. The results show a similarity between the bimolecular disproportionation reaction of two BrO radicals, and the decay reaction of I 0 radicals. Detailed quantitative studies on I 0 using electronic absorption spectrophotometry have proved difficult, but some kinetic data on reactions of this radical have been obtained.The I 0 radical has been identified by means of its absorption band system (425-470 nm) following the flash photolysis of mixtures of I2 and 02.4 Subsequently, this identification was confirmed by rotational analysis of a number of emission bands of the same ~y s t e m . ~ A linear extrapolation of the upper state (A211) vibrational levels led to a value of (176f21) kJ mol-l for the dissociation energy, in good agreement with an earlier estimate of 184 kJ mol-l. No quantitative data on the kinetics of chemical reactions of 10(X211) appear to have been reported, although several workers have reported rapid decay of this radicaL4* l 2

Kinetic studies of the reactions of IO, BrO, and ClO with dimethylsulfide

“…This value is in good agreement with the currently recommended pressure independent value for the rate coefficient of k5(298 K) = 5.2 x lo-'' cm3 molecule-' s-l [17]. It has been discussed in the literature that the rate coefficient for the reaction may be pressure dependent and could be a factor of 2 slower under the conditions employed in this study [14,16,17,19,20]. However, we prefer to use the value of k5 obtained in this study for the evaluation of the experimental data; use of a value of 2.5 x lo-" cm3 molecule-' s-' for ks in the data evaluation would lower kl by ca.…”

Decomposition of BrO studied by kinetic spectroscopy

“…Since reactions 7, 8, and 9 appear to be irreversible, one may expect that BrO decomposition should be faster than C10 decomposition. This conclusion is supported in the work of Clyne and Coxon (3,13). Although we were unable to measure our rate constants, qualitatively, by examining our plates, we concur at this point with Clyne and Coxon (3, 13).…”

Photochemical aspects of tropospheric iodine behaviour

“…Kinetics studies of this reaction [3,4] suggest that for a typical 03 concentration of 40 p,p.b., iodine atoms have a lifetime of about 1 s before conversion to IO radicals. Calculations based on the measured absorption spectrum of IO [5] indicate that photolysis occurs rapidly in the atmosphere, so that an equilibrium between I atoms and IO radicals is quickly established.…”