Measurements and Automated Mechanism Generation Modeling of OH Production in Photolytically Initiated Oxidation of the Neopentyl Radical | Sarah V. Petway, † Huzeifa Ismail, † William H. Green,* ,† Edgar G. Estupin ˜ a ´ n, ‡,§ Leonard E. Jusinski, ‡ and Craig A. Taatjes ‡ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, LiVermore, California 94551-0969 ReceiVed: October 18, 2006; In Final Form: March 21, 2007 Production of OH in the reaction of the neopentyl radical with O 2 has been measured by a laser photolysis/ cw absorption method for various pressures and oxygen concentrations at 673, 700, and 725 K. The MIT Reaction Mechanism Generator (RMG) was used to automatically generate a model for this system, and the predicted OH concentration profiles are compared to present and literature experimental results. Several reactions significantly affect the OH profile. The experimental data provide useful constraints on the rate coefficient for the formally direct chemical activation reaction of neopentyl radical with O 2 to form OH (CH 3 ) 3 CCH 2 + O 2 f OH + 3,3-dimethyloxetane (Rxn 1) At 673 K and 60 Torr, log k 1 (cm 3 molecule -1 s -1 ) )-13.7 ( 0.5. Absolute absorbance measurements on OH and I indicate that the branching ratio for R + O 2 to OH is about 0.03 under these conditions. The data suggest that the ab initio neopentyl + O 2 potential energy surface of Sun and Bozzelli is accurate to within 2 kcal mol -1 . Introduction Reactions of alkyl radicals (R) with O 2 are important for understanding low and intermediate temperature hydrocarbon oxidation and autoignition and are especially important in predicting negative temperature coefficient behavior. R + O 2 reactions involve the formation of an alkyl peroxy radical, RO 2 , Figure 1. At temperatures above 600 K or so, most RO 2 radicals form HO 2 and the conjugate alkene as the major reaction product. The RO 2 radical can also undergo intramolecular hydrogen abstraction to form a hydroperoxy alkyl radical (QOOH). The principal decomposition pathway of QOOH produces OH and a cyclic ether. The QOOH radical can also undergo a second O 2 addition; the species formed in this reaction leads to the chain branching that drives moderate temperature oxidation chemistry. Because of the many pathways, and the convolution of chemically activated and thermal reactions, it is very difficult to isolate and measure the rates of individual steps. Because the competing formation of a conjugate alkene + HO 2 is impossible in the reaction of the neopentyl radical with O 2 , this reaction is used to highlight the pathway shown in bold in Figure 1. Several experimental and modeling studies have investigated the oxidation of the neopentyl radical. Walker and co-workers 1-3 performed slow-flow reactor experiments to analyze the products of neopentane oxidation and suggested a mechanism to explain their results. Hughes et al. 4-5 measured OH production following pulsed photolysis of neopentyl iodide in the presence of O 2 and derived a rate constant for the isomerization of the neopentyl peroxy radical, assuming that the isomerization was effectively irreversible under their experimental conditions. Curran et al. 6 developed a detailed mechanism for the oxidation of neopentane and compared it to experimental results. They later modified the mechanism on the basis of data from high-pressure flow reactor experiments. 7 DeSain et al. 8 measured production of OH and HO 2 in pulsed-photolytic Cl-initiated neopentane oxidation and rationalized their results using a simple model on the basis of analogous time-dependent master equation calculations for the reaction of n-propyl with O 2 . Sun and Bozzelli 9 calculated thermochemical and kinetic properties for important species in the oxidation of the neopentyl radical using ab initio and density functional calculations. They reported Δ f H° 298 values for relevant species and calculated high-pressure limit rate constants using canonical transition-state theory and pressure-dependent rate constants using QRRK and master equation analyses. From comparison of an ad hoc model of neopentyl + O 2 with their measurements of HO 2 and OH formation in Cl- | Part of the special issue “James A. Miller Festschrift”. * To whom correspondence should be addressed. E-mail: whgreen@ MIT.edu. † Massachusetts Institute of Technology. ‡ Sandia National Laboratories. § Present address: Osram Sylvania, Inc., 71 Cherry Hill Drive, Beverly, MA 01915 Figure 1. Main reaction pathways for alkyl radicals R • in autoignition. For R ) neopentyl, unlike most alkyl radicals, there is no direct route to HO2. The present work focuses on the reaction path shown in bold. 3891 J. Phys. Chem. A 2007, 111, 3891-3900 10.1021/jp0668549 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007