Recombination and dissociation of 2-methyl allyl radicals: Experiment and theory

doi:10.1016/j.proci.2016.06.040

Proceedings of the Combustion Institute

Volume 36, Issue 1, 2017, Pages 211-218

https://doi.org/10.1016/j.proci.2016.06.040 Get rights and content

Abstract

The recombination and dissociation of the resonantly stabilized 2-methylallyl radical has been studied in a diaphragmless shock tube by laser schlieren densitometry (LS) over temperatures of 700–1350 K and pressures of 60–260 Torr. Both 2,5-dimethyl-1,5-hexadiene and the new low temperature precursor 3-methylbut-3-enyl nitrite were used to generate 2-methylallyl radicals under these conditions. Rate coefficients were obtained for dissociation of the precursors, recombination of 2-methylallyl, and dissociation of 2-methylallyl by simulation of the LS profiles. The experiments are complemented by a priori theoretical calculations for both the recombination and dissociation of 2-methylallyl. The experimental results and theoretical predictions are in excellent agreement with one another. The calculated high pressure limit rate coefficient for recombination of 2-methylallyl is log(k₁) = 14.737−0.641logT+251.39/(2.303×T) and that for dissociation of 2-methylallyl is log(k₃) = 11.100−1.2295logT−28545/(2.303×T). The uncertainties in k₁ and k₃ are estimated as factors of 1.5. Rate coefficients are provided over a broad range of pressures for chemical kinetic modeling.

Introduction

Resonantly stabilized radicals (RSRs) are typically much less reactive than other common combustion radicals such as OH, H, and O. Consequently, RSRs can accumulate in relatively large concentrations and play important roles in combustion. For instance, allyl radicals have been studied extensively ([1–3] and references therein) and recently Curran et al. showed the importance of allyl recombination in simulating propene ignition delay times [4], [5]. The related RSR, 2-methylallyl, 2MA, plays a similarly important role in the low temperature ignition of isobutene [6] and is responsible for the effectiveness of tert-butyl ethers in suppressing engine knock [7], [8].

The literature on recombination and dissociation of 2MA is limited. Roth et al. [9] reported rate coefficients for dissociation of 2,5-dimethyl-1,5-hexadiene (25DM15HD), the reverse of reaction 1 (Note: all reaction numbers refer to Tables 1 and S2), from single pulse shock tube (SPST) experiments over 873–1073 K and obtained k₁ from calculated thermochemical quantities and the equilibrium constant. Bayraceken et al. [10] obtained k₁ at 295 K from flash photolysis of 2-methylbut-1-ene (1–20 Torr). Tsang studied the dissociation of 2,4-dimethylhex-1-ene [11] by SPST (970–1180 K, 1–5 atm) and reported k₃ for dissociation of 2MA.

Previously we have studied the recombination of allyl radicals using a combination of shock tube methods over a broad range of conditions (10–7600 Torr, 650–1700 K) [1], [2].The current work presents complementary experimental and ab initio theoretical treatments of the recombination and dissociation of the related RSR 2-methyl allyl. Together these span conditions relevant to low-temperature ignition and master equation modeling provides rate expressions that encompass the low pressure experimental work (c.f. [1]) and engine relevant pressures. The synthesis and dissociation of a new low temperature precursor for 2MA is also presented.

Section snippets

Experimental section

Experiments were performed behind incident shock waves in a diaphragmless shock tube (DFST) that created very reproducible and predictable reaction conditions. The DFST has been fully described previously [12]. The temperature, T₂, and pressure, P₂, behind the incident shock wave were calculated from the ideal shock relations, initial loading conditions and incident shock velocity, assuming frozen conditions. The shock velocity, with an estimated error of 0.5% (< 10 K in T₂) was calculated from

Recombination of 2-methylallyl

Variable reaction coordinate transition state theory [17], [18] (VRC-TST) was used to calculate the high-pressure-limit, HPL, (capture) rate coefficient for reaction 1. Our procedure was similar to that of Georgievskii et al. [19], who previously considered several RSR + RSR reactions, including the allyl + allyl recombination reaction. Here, a more approximate treatment of fragment relaxation was used in the VRC-TST calculation. We therefore first applied our approximate treatment to the allyl +

Results and discussion

Sample raw LS signals and corresponding semi-log density gradient plots are shown in Fig 1. The chemical signal starts on the falling edge of the large peak which obscures t₀, Fig. 1a. Consequently, t₀ is located by a well-established procedure to within 0.1–0.2 µs and the location of t₀ for one experiment is shown in Fig. 1a. The density gradients due to chemical processes are first observable from the discontinuity around 0.8 µs in each of the main plots in Fig. 1 and the preceding signal is

Conclusions

The recombination of 2-methylallyl radicals has been determined by two sets of experiments that together span a temperature range, 700–1300 K, of interest to ignition. The radical is very stable and its chemistry is well-described by only recombination at low temperatures. At higher temperatures, T > ∼1100 K, recombination dominates, although dissociation of 2MA is also significant and addition of CH₃ radicals to 2MA also becomes important high level theoretical calculations for the dissociation

Acknowledgments

This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, U.S. Department of Energy. The work at ANL was performed under Contract number DE-AC02-06CH11357. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract no. DE-AC04-94-AL85000. JPP is grateful to the National Science Foundation for support on contract CBET

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Recombination and dissociation of 2-methyl allyl radicals: Experiment and theory

Abstract

Introduction

Section snippets

Experimental section

Recombination of 2-methylallyl

Results and discussion

Conclusions

Acknowledgments

Combust. Flame

Combust. Flame

Combust. Flame

Combust. Flame

Proc. Combust. Inst.

Combust. Flame

J. Phys. Chem. A

J. Phys. Chem. A

Phys. Chem. Chem. Phys.

Phys. Chem. Chem. Phys.

Int. J. Chem. Kinet.

Chem. Ber.

J. Chem. Soc. Faraday Trans.

Int. J. Chem. Kinet.

Rev. Sci. Instrum.

Appl. Opt.