A Mechanistic Study of the Reaction of OH with Dimethyl-d6 Sulfide. Direct Observation of Adduct Formation and the Kinetics of the Adduct Reaction with O2

Abstract

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).

Kinetic Study on the Reaction of OH Radical with Dimethyl Sulfide in the Absence of Oxygen

“…[16,8] At 298 K the experimental reported KIE was 2.20, while at 360 K it was 2.08, comparable to the theoretical values reported in Table 5, which range from 2.35 to 2.12 at 298 K and from 2.24 to 2.02 at 350 K, for the low-and high-pressure limits, respectively. The factorization of the overall KIEs indicates that the main contribution comes from the ratio of the vibrational partition functions.…”

“…No complex confined in the well AD would be formed (that is, the vibrationally excited AD* adducts flying over the well AD would not become thermalized species inside it). However, it seems quite clear from experimental works [7,8,10] that the addition reaction takes place in its high-pressure regime, because it is always assumed that the adduct (AD) exhibits a Boltzmann thermal distribution of its corresponding energetic states. For this reason we only apply the high-pressure limit expression to obtain the rate coefficient for this channel.…”

“…The kinetic isotope effect (KIE) at 298 K was about 2.2. Although the existence of the DMS-OH adduct was proposed in all the mechanisms, it was not directly observed until 1995 by Wine et al, [8] using the deuterated analog [D 6 ]DMS. One year later, Ravishankara et al [10,11] published a complete analysis of the equilibrium rate constant for adduct formation, the kinetics of the reaction between the adduct and oxygen and an exhaustive study on the products and the mechanistic implications, also using [D 6 ]DMS.…”

“…Therefore, further measurements with higher precision are suggested to determine more accurately the Arrhenius parameters. Most theoretical works concerning the DMS + OH reaction before Wine et al [8] confirmed the existence of the adduct, focused on the nature of that interaction. [19][20][21][22] To the best of our knowlege, two theoretical works have been published concerning the kinetics of reaction (R1), both of which focused on the H-abstraction channel.…”

FT-IR kinetic and product study of the Br-initiated oxidation of dimethyl sulfide

“…which has been used to describe the O 2 dependence of the reactions of OH radicals with DMS [26] and CS 2 [27], is apparently not valid in this case, or at least under the present experimental conditions. The results from the kinetic measurements suggest that the relative competitive technique under the present experimental conditions is not providing reliable kinetic data.…”