Relation of rate coefficients for thermal unimolecular reactions to those for chemical activation

1984 ◽  
Vol 81 (8) ◽  
pp. 3745-3746 ◽  
Author(s):  
Neil Snider
Keyword(s):  
Chemical Activation ◽  
Rate Coefficients ◽  
Unimolecular Reactions

Gas-Phase Kinetics of the Reactions of the CH2F Radicals with the Radicals CHF2, CH3, and C2H5

2004 ◽  
Vol 218 (4) ◽  
pp. 493-510 ◽  
Author(s):  
T. Beiderhase ◽  
Karlheinz Hoyermann ◽  
Jörg Nothdurft ◽  
Matthias Olzmann
Keyword(s):  
Flow Reactor ◽  
Chemical Activation ◽  
Multiphoton Ionization ◽  
Rate Coefficients ◽  
Cross Combination ◽  
Time Profiles ◽  
Gas Phase Kinetics ◽  
Stable Species ◽  
Activation Mechanisms ◽  
Kinetics Of

AbstractThe mechanisms and the rates of the reactions of the fluorinated hydrocarbon radical CH2F with the fluorinated and pure hydrocarbon radicals CHF2, CH3, and C2H5, respectively, have been studied at low pressure (around 2 mbar) and room temperature using the discharge-flow reactor technique. Mass spectrometry either with electron impact or laser induced multiphoton ionization was applied for the detection of labile and stable species. The cross combination of the radicals is mainly followed by HF elimination from the chemically activated adduct. The overall rate coefficients at 298K forCH2F + CHF2 → C2H2F2 + HF (1)CH2F + CH3 → C2H4 + HF (2)CH2F + C2H5 → C3H6 + HF (3) were derived from the analysis of the radical-time profiles by fit procedures; the following values were obtained:CH2F + CHF2 → C2H2F2 + HF (1)CH2F + CH3 → C2H4 + HF (2)CH2F + C2H5 → C3H6 + HF (3)k1 = (3.0 + 1.5/ − 0.7) × 1012cm3mol−1s−1k2 = (4.0 ± 1.5) × 1013cm3mol−1s−1k3 = (9.0 ± 3) × 1012cm3mol−1s−1.The results are discussed in terms of chemical activation mechanisms.

Double Bonds are Key to Fast Unimolecular Reactivity in First Generation Monoterpene Hydroxy Peroxy Radicals

2020 ◽  
Author(s):  
Jing Chen ◽  
Kristian H. Møller ◽  
Rasmus V. Otkjær ◽  
Henrik G. Kjaergaard
Keyword(s):  
Double Bond ◽  
Reaction Rate ◽  
First Generation ◽  
Rate Coefficients ◽  
Unimolecular Reaction ◽  
Peroxy Radicals ◽  
Double Bonds ◽  
Bimolecular Reactions ◽  
Unimolecular Reactions ◽  
Volatile Organic

<p>Monoterpenes are a group of volatile organic compounds that are emitted to the atmosphere in large amounts by natural sources. Some monoterpenes such as limonene and Δ<sup>3</sup>-carene are also widely used as additives in detergents and perfumes, and thus have a potential impact on indoor air quality and human health.</p><p>The volatile organic compounds like monoterpenes may undergo a series of autoxidation processes in the atmosphere to form highly oxygenated compounds, which have been linked to the formation of secondary organic aerosols. For this process to occur, the unimolecular reactions of the peroxy radicals formed during oxidation must have rate coefficients comparable to or greater than those of the competing bimolecular reactions with HO<sub>2</sub>, NO or other RO<sub>2</sub> radicals.</p><p>We studied the hydrogen shift (H-shift) and the cyclization reactions of all 45 hydroxy peroxy radicals formed by hydroxyl radical (OH) and O<sub>2</sub> addition to six monoterpenes (α-pinene, β-pinene, Δ<sup>3</sup>-carene, camphene, limonene and terpinolene). The reaction rate coefficients of the possible unimolecular reaction were initially studied at a lower level of theory. Those deemed likely to be atmospherically competitive were then calculated using the multi-conformer transition states theory approach developed by Møller et al. (J. Phys. Chem. A, 120, 51, 10072-10087, 2016). This approach has been shown to agree with the experimental values to within a factor of 4 for other systems.</p><p>It was found that double bonds are key to fast unimolecular reactions in the first-generation monoterpene hydroxy peroxy radicals. The H-shift reactions abstracting a hydrogen from a carbon adjacent to a double bond are found to typically be fast enough to compete with the bimolecular reactions, likely due to the resonance stability of the nascent allylic radical. The reactivity of the cyclization reaction between the carbon-carbon double bonds and the peroxy group, which forms an endoperoxide ring, is high as well. The H-shifts abstracting the hydrogen from the hydroxy group may be competitive in some cases but the reaction rate coefficients for these reactions are more uncertain. Generally, the cyclization reaction and the allylic H-shift reactions are the dominant reaction paths for the studied peroxyl radicals. Since the OH radical addition consumes one double bond, we suggest that the monoterpenes with more than one double bond in their structure are likely to have unimolecular reactions that can be important for the first-generation monoterpene peroxy radicals. On the other hand, the ones with only one double bond initially are not likely to have fast unimolecular reactions that can compete with the bimolecular reactions under the atmospheric condition, unless a double bond can be formed during their oxidation process as found for α-pinene and β-pinene. This result greatly limits the amount of potentially important unimolecular reaction paths in atmospheric monoterpene oxidation.</p>

Kinetic Analysis of Complex Chemical Activation and Unimolecular Dissociation Reactions using QRRK Theory and the Modified Strong Collision Approximation

2000 ◽  
Vol 214 (11) ◽  
Author(s):  
A.Y. Chang ◽  
J.W. Bozzelli ◽  
A.M. Dean
Keyword(s):  
Chemical Activation ◽  
Peroxy Radical ◽  
Dissociation Rate ◽  
Rate Coefficients ◽  
Unimolecular Dissociation ◽  
Frequency Model ◽  
Dependent Rate ◽  
Vinyl Radical ◽  
Temperature And Pressure ◽  
Collision Approximation

A method to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions is described. A three-frequency version of QRRK theory is developed and collisional stabilization is estimated using the modified strong-collision approximation. The methodology permits analysis of reaction systems with an arbitrary degree of complexity in terms of the number of isomer or product channels. Specification of both high and low pressure limits is also provided. The chemically activated reaction of vinyl radical with molecular oxygen is used to demonstrate the approach. Subsequent dissociation of the stabilized vinyl peroxy radical is used to illustrate prediction of dissociation rate coefficients. These calculations confirm earlier results that the vinoxy + O channel is dominant under combustion conditions. The results are also consistent with RRKM results using the same input conditions. This approach provides a means to provide reasonably accurate predictions of the rate coefficients that are required in many detailed mechanisms. The major advantage is the ability to provide reasonable estimates of rate coefficients for many complex systems where detailed information about the transition states is not available. It is also shown that a simpler 1-frequency model appears adequate for high temperature conditions.

Chemical Activation Study of the Unimolecular Reactions of CD3CD2CHCl2 and CHCl2CHCl2 with Analysis of the 1,1-HCl Elimination Pathway

2016 ◽  
Vol 120 (42) ◽  
pp. 8244-8253 ◽  
Author(s):  
Allie C. Larkin ◽  
Matthew J. Nestler ◽  
Caleb A. Smith ◽  
George L. Heard ◽  
D. W. Setser ◽  
...  
Keyword(s):  
Chemical Activation ◽  
Unimolecular Reactions ◽  
Elimination Pathway

Chemical Activation and Nonequilibrium Unimolecular Reactions of C2H5Cl and 1,2‐C2H4Cl2 Molecules

1966 ◽  
Vol 45 (9) ◽  
pp. 3231-3236 ◽  
Author(s):  
J. C. Hassler ◽  
D. W. Setser ◽  
R. L. Johnson
Keyword(s):  
Chemical Activation ◽  
Unimolecular Reactions

Photochemical studies of unimolecular processes IV. Quenching of vibrationally excited cycloheptatriene by added gases

1972 ◽  
Vol 329 (1577) ◽  
pp. 233-240 ◽  
Keyword(s):  
Chemical Activation ◽  
Vibrationally Excited ◽  
Unimolecular Reactions

The relaxation of highly vibrationally excited cycloheptatriene molecules by a variety -of added gases has been studied photochemically. The following mean energies removed per downward step were deduced; He, 1.8; No, 2.4; Ar, 3.5; Kr, 3.6; Xe, 3.3; H 2 , 2.6; D 2 , 2.4; n-C 5 H 12 , 11.7; neo-C 5 H 12 , 10.2 kJ mol -1 . These agree better with data from the ‘fall off’ of unimolecular reactions than with chemical activation studies.

Introduction

1996 ◽  
Author(s):  
Tomas Baer ◽  
William L. Hase
Keyword(s):  
Internal Energy ◽  
Vibrational Energy ◽  
Chemical Activation ◽  
Experimental Studies ◽  
Spectroscopic Studies ◽  
Dissociation Limit ◽  
Major Advance ◽  
Thermal Systems ◽  
Unimolecular Reactions ◽  
Intramolecular Vibrational Energy Redistribution

The field of unimolecular reactions has witnessed impressive advances in both experimental and theoretical techniques during the past 20 years. These developments have resulted in experimental measurements that finally permit critical tests of the major assumptions made more than 60 years ago when Rice and Ramsperger (1927, 1928) and Kassel (1928) first proposed their statistical RRK theory of unimolecular decay. At the heart of these advances is our ability to prepare molecules in narrow ranges of internal energy, even in single quantum states, at energies below and above the dissociation limit. This has led to detailed spectroscopic studies of intramolecular vibrational energy redistribution (IVR), a process that is intimately related to the assumption of random energy flow in the statistical theory of unimolecular decay. This book is devoted exclusively to the study of state- or energy-selected systems. However, in order to place these studies in the context of the much larger field of unimolecular reactions in general, we provide a brief background of the field up to about 1970. The experimental studies of unimolecular reactions developed in three stages. The early studies involved strictly thermal systems in which molecules were energized by heating the sample either in a bulb (Chambers and Kistiakowsky, 1934; Schlag and Rabinovitch, 1960; Flowers and Frey, 1962; Schneider and Rabinovitch, 1962), or by more sophisticated methods such as shock tubes which were applied to unimolecular reactions by Tsang (1965, 1972, 1978, 1981) and others (Astholz et al., 1979; Brouwer et al.,1983). The drawback of these studies is that molecules were prepared in a very broad (albeit well characterized) distribution of internal energy states. A major advance was the use of chemical activation in the early 1960s in which a species such as CH2 reacted with a molecule, thereby forming an energized species which could either isomerize or be stabilized by collisions (Rabinovitch and Flowers, 1964; Rabinovitch and Setser, 1964; Kirk et al., 1968; Hassler and Setser, 1966; Simons and Taylor, 1969). This approach permitted the reacting species to be prepared in a narrow range of internal energies.

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