Experimental and Theoretical Kinetic Studies of the Ozonolysis of Selected Allyl Sulfides (H₂C═CHCH₂SR, R = CH₃, CH₃CH₂): The Effect of Nascent OH Radicals

Abstract

Rate coefficients of the O 3 -initiated oxidation of allyl methyl sulfide (H 2 C�CHCH 2 SCH 3 , AMS) and allyl ethyl sulfide (H 2 C�CHCH 2 SCH 2 CH 3 , AES) were determined at atmospheric conditions by "in situ" FTIR. The relative kinetic experiments were performed using methylcyclohexane (McH) and carbon monoxide (CO) as nascent OH radical scavengers and in the absence of any scavenger, to determine the impact that the formation of OH radicals has on the rate coefficients. In the absence of scavengers, values of k AMS+Od 3 = (5.23 ± 3.57) × 10 −18 and k AES+Od 3 = (5.76 ± 1.80) × 10 −18 cm 3 molecule −1 s −1 were obtained. In the presence of the scavengers, however, the rates decreased to k AMS+Od 3 +McH = (3.92 ± 1.92) × 10 −18 and k AMS+Od 3 +CO = (2.63 ± 0.47) × 10 −18 cm 3 molecule −1 s −1 for AMS, and k AES+Od 3 +McH = (4.78 ± 1.38) × 10 −18 and k AES+Od 3 +CO = (3.50 ± 0.27) × 10 −18 cm 3 molecule −1 s −1 for AES. On the basis of these results, we have decided to recommend the values obtained using CO as scavenger as those best representing the rate coefficient for the reactions of O 3 with AMS and AES. The reaction mechanism was explored using DFT and post-Hartree−Fock computational methods. It is shown that the barrier for the common cyclization to primary ozonide (−3.7 ± 0.1 kcal mol −1 ) followed by other reactions, as well as that for the reaction with the sulfur atom (−5.1 ± 0.1 kcal mol −1 ), is small and quite close, meaning that both reaction paths should contribute significantly to the global reaction rate.

Computational Studies of Rubber Ozonation Explain the Effectiveness of 6PPD as an Antidegradant and the Mechanism of Its Quinone Formation

“…Nevertheless, many single-reference (SR) approaches to O 3 modeling have been reported. ,,− Errors in these predictions can be substantial. As Wheeler et al have noted, generally respectable SR methodologies predict barrier heights with discrepancies in excess of 10 kcal mol –1 for small systems like C 2 H 2 and C 2 H 4 , and even the CCSD­(T) (coupled-cluster with single, double, and perturbative triple excitations) method gives unsatisfactory results .…”

Computational studies of rubber ozonation explain the effectiveness of 6PPD as an antidegradant and the mechanism of its quinone formation

“…46,91 Nevertheless, many single-reference (SR) approaches to O 3 modeling have been reported. 47,59,[92][93][94][95][96][97][98][99][100][101] Errors in these predictions can be substantial. As Wheeler et al have noted, generally respectable SR methodologies predict barrier heights with discrepancies in excess of 10 kcal mol −1 for small systems like C 2 H 2 and C 2 H 4 , and even the generally reliable CCSD(T) (coupled-cluster with single, double, and perturbative triple excitations) method gives unsatisfactory results.…”

Advances in Atmospheric Chemical and Physical Processes

“…Initiation of the VOC oxidation process is primarily driven by the sunlight-derived OH radical but also via reactions involving other oxidants. Papers in this virtual special issue include studies of the initiation reactions of VOC species by OH, Cl, − O 3 , , NO 3 , and HCO, as well as by direct photolysis . Chamber studies, in which the products of a VOC oxidation process are identified and quantified, are also prevalent; two studies quantify the formation of secondary organic aerosol (SOA) in the reaction of OH or NO 3 with monoterpenes, , while a combined experimental/theoretical study shows that methylmethanimine oxidation via the OH radical is unlikely to compete with atmospheric hydrolysis of this species .…”