Kinetics of the Reactions of CH2Cl, CH3CHCl, and CH3CCl2 Radicals with Cl2 in the Temperature Range 191-363 K

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Effect of Methyl Group Substitution on the Kinetics of Vinyl Radical + O-2 Reaction

The kinetics of (CH3)(2)CCH + O-2 (1) and (CH3)(2)CCCH3 + O-2 (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k(1)(298 K) = (4.5 +/- 0.5) x 10(-12) cm(3) s(-1) and k(2)(298 = (8.9 +/- 1.0) x 10(-12) cm(3) s(-1). The observed products for reactions 1 and 2 were CH3COCH3 and CH3 + CH3COCH3, respectively. Substituting both beta-hydrogens in the vinyl radical (CH2CH) with methyl groups decreases the rate coefficient of the CH2CH + O-2 reaction by about 50%. However, the rate coefficient of the triply substituted (CH3)(2)CCCH3 radical reaction with O-2 is almost identical to the CH2CH + O-2 rate coefficient under the covered temperature range.Peer reviewe

Temperature and Pressure Dependence of the Reaction between Ethyl Radical and Molecular Oxygen : Experiments and Master Equation Simulations

Funding Information: We thank Stephen Klippenstein for providing us with the geometries, harmonic frequencies, and relative energies of the stationary points from his recent CH + O publication as well as the state sum for the loose recombination transition state. T.T.P. acknowledges support from the Doctoral Programme in Chemistry and Molecular Sciences of the University of Helsinki and the Magnus Ehrnrooth Foundation for funding. Project K129140 for G.L. was implemented with the support provided by the Ministry of Innovation and Technology of Hungary from the National Research, Development and Innovation Fund, financed under the OTKA funding scheme. T.T.P., E.A.R., and A.J.E. acknowledge support from the Academy of Finland (Grants 325250 and 346374). The authors also acknowledge CSC IT Center for Science in Finland for computational resources. 2 5 • 2 Publisher Copyright: © 2023 The Authors.We have used laser-photolysis - photoionization mass-spectrometry to measure the rate coefficient for the reaction between ethyl radical and molecular oxygen as a function of temperature (190-801 K) and pressure (0.2-6 Torr) under pseudo first-order conditions ([He] >> [O2] >> [C2H5 center dot]). Multiple ethyl precursor, photolysis wavelength, reactor material, and coating combinations were used. We reinvestigated the temperature dependence of the title reaction's rate coefficient to resolve inconsistencies in existing data. The current results indicate that some literature values for the rate coefficient may indeed be slightly too large. The experimental work was complemented with master equation simulations. We used the current and some previous rate coefficient measurements to optimize the values of key parameters in the master equation model. After optimization, the model was able to reproduce experimental falloff curves and C2H4 + HO2 center dot yields. We then used the model to perform simulations over wide temperature (200-1500 K) and pressure (10-4-102 bar) ranges and provide the results in PLOG format to facilitate their use in atmospheric and combustion models.Peer reviewe

An experimental and master-equation modeling study of the kinetics of the reaction between resonance-stabilized (CH3)(2)CCHCH2 radical and molecular oxygen

The kinetics of the reaction between resonance-stabilized (CH3)(2)CCHCH2 radical (R) and O-2 has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH3)(2)CCHCH2 + O-2 reaction (1) were carried out at low helium bath-gas pressures (0.2-5.7 Torr) and over a wide temperature range (238-660 K). Under low temperature (238-298 K) conditions, the pressure-dependent bimolecular association reaction R + O-2 -> ROO determines kinetics, until at an intermediate temperature range (325-373 K) the ROO adduct becomes thermally unstable and increasingly dissociates back to the reactants with increasing temperature. The initial association of O-2 with (CH3)(2)CCHCH2 radical occurs on two distinct sites: terminal 1(t) and non-terminal 1(nt) sites on R, leading to the barrierless formation of ROO(t) and ROO(nt) adducts, respectively. Important for autoignition modelling of olefinic compounds, bimolecular reaction channels appear to open for the R + O-2 reaction at high temperatures (T > 500 K) and pressure-independent bimolecular rate coefficients of reaction (1) with a weak positive temperature dependence, (2.8-4.6) x 10(-15) cm(3) molecule(-1) s(-1), were measured in the temperature range of 500-660 K. At a temperature of 501 K, a product signal of reaction (1) was observed at m/z = 68, probably originating from isoprene. To explore the reaction mechanism of reaction (1), quantum chemical calculations and ME simulations were performed. According to the ME simulations, without any adjustment to energies, the most important and second most important product channels at the high temperatures are isoprene + HO2 (yield > 91%) and (2R/S)-3-methyl-1,2-epoxybut-3-ene + OH (yield isoprene + HO2 (similar to 2.2 kcal mol(-1)), the ME model was able to reproduce the experimental findings. Modified Arrhenius expressions for the kinetically important reaction channels are enclosed to facilitate the use of current results in combustion models.Peer reviewe

Kinetics and thermochemistry of the reaction of 3-methylpropargyl radical with molecular oxygen

We have measured the kinetics and thermochemistry of the reaction of 3-methylpropargyl radical (but-2-yn-1-yl) with molecular oxygen over temperature (223-681 K) and bath gas density (1.2 - 15.0 x 10(16)cm(-3)) ranges employing photoionization mass-spectrometry. At low temperatures (223-304 K), the reaction proceeds overwhelmingly by a simple addition reaction to the -CH2 end of the radical, and the measured CH3CCCH2 center dot+O-2 reaction rate coefficient shows negative temperature dependence and depends on bath gas density. At intermediate temperatures (340-395 K), the addition reaction equilibrates and the equilibrium constant was determined at different temperatures. At high temperatures (465-681 K), the kinetics is governed by O-2 addition to the third carbon atom of the radical, and rate coefficient measurements were again possible. The high temperature CH3CCCH2 center dot +O(2 )rate coefficient is much smaller than at low T, shows positive temperature dependence, and is independent of bath gas density. In the intermediate and high temperature ranges, we observe a formation signal for ketene (ethenone). The reaction was further investigated by combining the experimental results with quantum chemical calculations and master equation modeling. By making small adjustments (2 - 3 kJ mol(-1)) to the energies of two key transition states, the model reproduces the experimental results within uncertainties. The experimentally constrained master equation model was used to simulate the CH3CCCH2 center dot+ O-2 reaction system at temperatures and pressures relevant to combustion. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

Kinetics and thermochemistry of the reaction of 1-methylpropargyl radicals with oxygen molecules : Experiments and computations

We have used laser-photolysis/photoionization mass spectrometry to measure the kinetics of the reaction of 1-methylpropargyl (but-3-yn-2-yl, CH C=CH-CH3) radicals with oxygen molecules as a function of temperature (T = 200 - 685 K) and bath gas density (1.2 - 15 x 10(16) cm(-3)). The low temperature (TPeer reviewe

Kinetics of 1-butyl and 2-butyl radical reactions with molecular oxygen : Experiment and theory

The reaction of O-2 with butyl radicals is a key early step in the oxidation of n-butane, which is a prototypical alkane fuel with combustion properties that mimic those of many larger alkanes. Current combustion mechanisms employ kinetic descriptions for such radical oxidations that are based on fairly limited information. The present work employs a combination of experiment and theory to probe the kinetics of O-2 reacting with both 1- and 2-butyl radicals. The experiments employ laser photolysis to generate butyl radicals and thereby initiate the reaction kinetics. Photoionization mass spectrometric observations of the time-dependent butyl radical concentration yield rate coefficients for the overall reaction. The experiments cover temperatures ranging from 200 to 500 K and He bath gas pressures ranging from 0.3 to 6 Torr. Ab initio transition state theory (TST) based master equation calculations are used to predict the kinetics over a broad range of conditions. The calculations consider both the barrierless R + O-2 entrance channel, treated with direct CASPT2 variable reaction coordinate TST, and the decomposition of the RO2 complex to HO2 + alkenes, treated with CCSD(T)/CBS based TST. Theory and experiment are in good agreement, with maximum discrepancies of about 30%, suggesting the appropriateness of the theory based predictions for conditions of greater relevance to combustion. The kinetic description arising from this work should be of considerable utility to combustion modeling of n-butane, as well as of other related saturated hydrocarbons. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

First direct kinetic measurement of i -C4H5 (CH2CHCCH2) + O-2 reaction : Toward quantitative understanding of aromatic ring formation chemistry

The kinetics of the i -C 4 H 5 (buta-1,3-dien-2-yl) radical reaction with molecular oxygen has been measured over a wide temperature range (275-852 K) at low pressures (0.8-3 Torr) in direct, time-resolved experiments. The measurements were performed using a laminar flow reactor coupled to photoionization mass spectrometer (PIMS), and laser photolysis of either chloroprene (2-chlorobuta-1,3-diene) or isoprene was used to produce the resonantly stabilized i -C 4 H 5 radical. Under the experimental conditions, the measured bimolecular rate coefficient of i -C 4 H 5 + O 2 reaction is independent of bath gas density and exhibits weak, negative temperature dependency, and can be described by the expression k 3 = (1.45 +/- 0.05) & times; 10 & minus;12 & times; ( T /298 K) & minus;(0.13 +/- 0.05) cm 3 s & minus;1 . The measured bimolecular rate coefficient is surprisingly fast for a resonantly stabilized radical. Under combustion conditions, the reactions of i -C 4 H 5 radical with ethylene and acetylene are believed to play an important role in forming the first aromatic ring. However, the current measurements show that i C 4 H 5 + O 2 reaction is significantly faster under combustion conditions than previous estimations suggest and, consequently, inhibits the soot forming propensity of i -C 4 H 5 radicals. The bimolecular rate coefficient estimates used for the i -C 4 H 5 + O 2 reaction in recent combustion simulations show significant variation and are up to two orders of magnitude slower than the current, measured value. All estimates, in contrast to our measurements, predict a positive temperature dependency. The observed products for the i -C 4 H 5 + O 2 reaction were formaldehyde and ketene. This is in agreement with the one theoretical study available for i C 4 H 5 + O 2 reaction, which predicts the main bimolecular product channels to be H 2 CO + C 2 H 3 + CO and H 2 CCO + CH 2 CHO. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

An experimental and computational study of the reaction between pent-3-en-2-yl radicals and oxygen molecules : switching from pure stabilisation to pure decomposition with increasing temperature

We have used laser-photolysis-photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between (E/Z)-pent-3-en-2-yl (CH3-CH - CH - CH-CH3), a resonance-stabilised hydrocarbon radical, and molecular oxygen. The time-resolved experiments were performed over a wide temperature range (240-750 K) at relatively low pressures (0.4-7 Torr) under pseudo-first-order conditions (excess [O-2]). Helium bath gas was used in most experiments, but nitrogen was employed in a few measurements to investigate the effect of a heavier collider on the kinetics of the studied reaction. The experimental traces were directly used to optimise parameters in the master equation model using the recently implemented trace fitting feature in the MESMER program. At low temperatures (T < 300 K), the reaction proceeds by barrierless recombination reactions to form peroxyl adducts, and the radical traces are single-exponential. Between 326 K and 376 K, equilibration between the reactants and the peroxyl adducts is observed, and the radical traces are multi-exponential. Interestingly, at temperatures above 500 K, single-exponential decays were again observed, although the reaction is much slower than at low temperatures. The master equation simulations revealed that at both low and high temperatures, the radical decay rate is governed by a single eigenvalue. At low temperatures, this eigenvalue corresponds to recombination reactions, and at high temperatures to the phenomenological formation of bimolecular products. Between low and high temperatures (the exact temperature thresholds depend on [O-2]), there is a region of avoided crossing in which the rate coefficient "jumps" from one eigencurve to the other. Although chemically significant eigenvalues are not well separated from internal energy relaxation eigenvalues at elevated temperatures (600 K at 0.01 bar, 850 K at 100 bar), we observed that many of the Bartis-Widom rate coefficients produced by the master equation model were valid up to 1500 K. Our simulations predict that the most important reaction channel at high temperatures is the formation of (E/Z)-penta-1,3-diene and hydroperoxyl. The experimentally constrained master equation model was used to simulate the title reaction over a wide range of conditions. To facilitate the use of our results in autoignition and combustion models, modified Arrhenius representations are given for the most important reaction channels.Peer reviewe

Oxidation Kinetics and Thermodynamics of Resonance-Stabilized Radicals : The Pent-1-en-3-yl + O-2 Reaction

The kinetics and thermochemistry of the pent-1-en-3-yl radical reaction with molecular oxygen (CH2CHCHCH2CH3 + O-2) has been studied by both experimental and computational methods. The bimolecular rate coefficient of the reaction was measured as a function of temperature (198-370 K) and pressure (0.2-4.5 Torr) using laser photolysis-photoionization mass-spectrometry. Quantum chemical calculations were used to explore the potential energy surface of the reaction, after which Rice-Ramsperger-Kassel-Marcus theory/master equation simulations were performed to investigate the reaction. The experimental data were used to adjust key parameters, such as well depths, in the master equation model within methodological uncertainties. The master equation simulations suggest that the formation rates of the two potential RO2 adducts are equal and that the reaction to QOOH is slower than for saturated hydrocarbons. The initial addition reaction, CH2CHCHCH2CH3 + O-2, is found to be barrierless when accounting for multireference effects. This is in agreement with the current experimental data, as well as with past experimental data for the allyl + O-2 reaction. Finally, we conducted numerical simulations of the pent-1-en-3-yl + O-2 reaction system and observed significant amounts of penta-1,3-diene being formed under engine-relevant conditions.Peer reviewe