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4) Orban, M.; Kijrb, E. K b b , E.; Noyes, R. M. J. Am. Chem. Soc. 1972, 94, Trans. 1992,88, 917. S = 2 k~l / k~6 / &~3 &~ = 9.52 x 10" b = 2kB6kB9[R]/k~lkW[BrOS-] = 0.545 K 3 k~z / k~l = 1.6 6 = k~l / 2 k~6 = 3.33 x r = k~i k~a / k~k~s = 0.101 Q = 2 k~s k~/ k~, ' = 0.0101 0 = kB,/kB6 = 1.667 x lo3 B = k~3 / 2 k~6 = 500 T,,, = kW[Br03-]trea = 3.91t,/s Repistry No. BrO,-, 15541-45-4; gallic acid, 149-91-7; ferroin, 14708-99-7.Timaresolved fluorescmCe detection Of Cl(9j) following 266-nm laser flash photd~is Of Cl~cO/CH$CH~(DMS)/N~ mixtures has been employed to study the kinetics of the title reaction over the temperature and pressure ranges 240-421 K and 3-700 Torr. The reaction is found to be very fast, occumng on essentially every C1(2PJ) + DMS encounter. The reaction rate increases with decreasing temperature and shows a significant pressure dependence. At 297 K, for example, the rate coefficient increases from a low-pressure limit value of approximately 1.8 X cm3 molecule-' s-' to a value of (3.3 f 0.5) X cm3 molecule-' s-l at P = 700 Torr. A few experiments were carried out with CD3SCD3 or C2H5SC2Hs replacing DMS as the sulfide reactant; within experimental uncertainty, no dependence of the rate coefficient on the identity of the sulfide reactant was observed. In a separate study, time-resolved tunable diode laser spectroscopic detection of HCl has been coupled with 248-nm laser flash photolysis of Cl&O/DMS/COz/Nz mixtures to measure the HCl product yield from the title reaction as a function of pressure at T = 297 K. The HCl yield approaches unity as P -0 but decreases with increasing pressure to a value of -0.5 at P = 203 Torr. The yield experiments demonstrate that hydrogen abstraction is the dominant reaction mechanism in the low-pressure limit. With increasing pressure, stabilization of a (CH3)gCI adduct apparently becomes competitive with the hydrogen abstraction pathway. The fate of the stabilized adduct remains highly uncertain, although it clearly does not dissociate to Cl(9J) or HCl on the time scale of our experiments (several milliseconds). The potential role of the title reaction in marine atmospheric chemistry is discussed.

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A laser flash photolysis−resonance fluorescence technique has been employed to study the kinetics of the reaction of chlorine atoms with methyl iodide as a function of temperature (218−694 K) and pressure (5−500 Torr) in nitrogen buffer gas. At T ≥ 364 K, measured rate coefficients are pressure independent and a significant H/D kinetic isotope effect is observed, suggesting that hydrogen transfer is the dominant reaction pathway; the following Arrhenius expression adequately describes all kinetic data at 364 K ≤ T ≤ 694 K: k 1a = 5.44 × 10-11 exp(−1250/T) cm3 molecule-1 s-1. At T ≤ 250 K, measured rate coefficients are pressure dependent and much faster than computed from the above Arrhenius expression for the H-transfer pathway, suggesting that the dominant reaction pathway at low temperature is formation of a stable adduct; at T = 218 K and P = 500 Torr, for example, k 1 = k 1a + k 1b = 3.0 × 10-11 cm3 molecule-1 s-1, with 99.4% of the reactivity being attributable to the addition channel 1b. At temperatures in the range 263−309 K, reversible addition is observed, thus allowing equilibrium constants for CH3ICl formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction 1b: = − 53.6 ± 3.4 kJ mol-1, = − 52.2 ± 3.5 kJ mol-1, and = − 88 ± 11 J mol-1 K-1. In conjunction with the well-known heats of formation of Cl and CH3I, the above ΔH values lead to the following heats of formation for CH3ICl at 298 and 0 K: = 82.3 ± 3.5 kJ mol-1 and ΔH f,0 = 91.6 ± 3.6 kJ mol-1. Ab initio calculations using density functional theory (DFT) and G2 theory reproduce the experimental bond strength reasonably well. The DFT calculations predict a structure (used in the third-law analysis) where the C−I−Cl bond angle is 85.2° and the methyl group adopts a staggered orientation with a pronounced tilt toward chlorine. Bonding in CH3ICl is discussed as are the implications of the new kinetic data for atmospheric chemistry.

A pulsed laser photolysis-pulsed laser-induced fluorescence technique has been employed to study the detailed mechanism for the reaction of OH radicals with deuterated dimethyl sulfide [(CD&S, DMS-d6]. Equilibration of pulsed laser-generated OH with a (CD,),S-OH adduct has been directly observed, thus confirming the existence of this controversial weakly bound species. Elementary rate coefficients for adduct formation and decomposition and, therefore, the equilibrium constant for OH + (CD3)2S -(CD&SOH have been determined as a function of temperature. From the temperature dependence of the equilibrium constant over the relatively narrow temperature range 250-267 K, a 258 K adduct bond strength of 13.0 zk 3.3 kcal mol-' has been obtained (second law method). Altematively, an entropy change calculated using standard statistical mechanical methods and ab initio theory (for determining the (CD3)zS and (CD&SOH structures) has been employed in conjunction with an experimental value for the equilibrium constant at a single temperature to obtain a 258 K adduct bond strength of 10.1 f 1.1 kcal mol-' (third law method). Experiments in the presence of 0 2 confirm the previously reported dependence of the OH + DMS-d6 rate coefficient on the 0 2 partial pressure and are consistent with the previously proposed four-step mechanism involving hydrogen abstraction, addition of OH to the sulfur atom, and adduct decomposition in competition with an adduct + 0 2 reaction [Hynes et al. J. Phys. Chem. 1986, 90, 41481. The rate coefficient for the adduct + 0 2 reaction is found to be (8 f 3)x lo-', cm3 molecule-' s-l independent of pressure (100-700 Torr of N2) and temperature (250-300 K).