RRKM Analysis Research Articles

Non-covalent Interactions and Charge Transfer between Propene and Neutral Yttrium-Doped and Pure Gold Clusters.

The dopant and size‐dependent propene adsorption on neutral gold (Aun) and yttrium‐doped gold (Aun−1Y) clusters in the n=5–15 size range are investigated, combining mass spectrometry and gas phase reactions in a low‐pressure collision cell and density functional theory calculations. The adsorption energies, extracted from the experimental data using an RRKM analysis, show a similar size dependence as the quantum chemical results and are in the range of ≈0.6–1.2 eV. Yttrium doping significantly alters the propene adsorption energies for n=5, 12 and 13. Chemical bonding and energy decomposition analysis showed that there is no covalent bond between the cluster and propene, and that charge transfer and other non‐covalent interactions are dominant. The natural charges, Wiberg bond indices, and the importance of charge transfer all support an electron donation/back‐donation mechanism for the adsorption. Yttrium plays a significant role not only in the propene binding energy, but also in the chemical bonding in the cluster‐propene adduct. Propene preferentially binds to yttrium in small clusters (n<10), and to a gold atom at larger sizes. Besides charge transfer, relaxation also plays an important role, illustrating the non‐local effect of the yttrium dopant. It is shown that the frontier molecular orbitals of the clusters determine the chemical bonding, in line with the molecular‐like electronic structure of metal clusters.

Unimolecular Fragmentation of Deprotonated Diproline [Pro2-H]- Studied by Chemical Dynamics Simulations and IRMPD Spectroscopy.

Dissociation chemistry of the diproline anion [Pro2-H]- is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro2- is chosen due to its reduced size and the small number of sites where deprotonation can take place. The mechanisms leading to the two dominant collision-induced dissociation (CID) product ions are elucidated. Trajectories from a variety of isomers of [Pro2-H]- were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y1- product ions are proposed, there is only one mechanism yielding the b2- ion. This mechanism leads to formation of a b2- fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b2 sequence ion is experimentally confirmed by infrared ion spectroscopy of the fragment anion. Furthermore, collisional and internal energy activation simulations are used in parallel to identify the different dynamical aspects of the observed reactivity.

RRKM and master equation kinetic analysis of parallel addition reactions of isomeric radical intermediates in hydrocarbon flames.

We have calculated the temperature-dependent rate coefficients of the addition reactions of butadien-2-yl (C4H5) and acroylyl (C3H3O) radicals with ethene (C2H4), carbon monoxide (CO), formaldehyde (H2CO), hydrogen cyanide (HCN), and ketene (H2CCO), in order to explore the balance between kinetic and thermodynamic control in these combustion-related reactions. For the C4H5 radical, the 1,3-diene form of the addition products is more stable than the 1,2-diene, but the 1,2-diene form of the radical intermediate is stabilized by an allylic delocalization, which may influence the relative activation energies. For the reactions combining C3H3O with C2H4, CO, and HCN, the opposite is true: the 1,2-enone form of the addition products is more stable than the 1,3-enone, whereas the 1,3-enone is the slightly more stable radical species. Optimized geometries and vibrational modes were computed with the QCISD/aug-cc-pVDZ level and basis, followed by single-point CCSD(T)-F12a/cc-pVDZ-F12 energy calculations. Our findings indicate that the kinetics in all cases favor reaction along the 1,3 pathway for both the C4H5 and C3H3O systems. The Rice-Ramsperger-Kassel-Marcus (RRKM) microcanonical rate coefficients and subsequent solution of the chemical master equation were used to predict the time-evolution of our system under conditions from 500 K to 2000 K and from 10-5 bar to 10 bars. Despite the 1,3 reaction pathway being more favorable for the C4H5 system, our results predict branching ratios of the 1,2 to 1,3 product as high as 0.48 at 1 bar. Similar results hold for the acroylyl system under these combustion conditions, suggesting that under kinetic control the branching of these reactions may be much more significant than the thermodynamics would suggest. This effect may be partly attributed to the low energy difference between 1,2 and 1,3 forms of the radical intermediate. No substantial pressure-dependence is found for the overall forward reaction rates until pressures decrease below 0.1 bar.

Theoretical Investigation into Competing Unimolecular Reactions Encountered in the Pyrolysis of Acetamide

Motivated by the necessity to understand the pyrolysis of alkylated amines, unimolecular decomposition of acetamide is investigated herein as a model compound. Standard heats of formation, entropies, and heat capacities, are calculated for all products and transition structures using several accurate theoretical levels. The potential energy surface is mapped out for all possible channels encountered in the pyrolysis of acetamide. The formation of acetamedic acid and 1-aminoethenol and their subsequent decomposition pathways are found to afford the two most energetically favored pathways. However, RRKM analysis shows that the fate of acetamedic acid and 1-aminoethenol at all temperatures and pressures is to reisomerize to the parent acetamide. 1-Aminoethenol, in particular, is predicted to be a long-lived species enabling its participation in bimolecular reactions that lead to the formation of the major experimental products. Results presented herein reflect the importance of bimolecular reactions involving acetamide and 1-aminoethenol in building a robust model for the pyrolysis of N-alkylated amides.

A combined theoretical and experimental study of mechanisms of fragmentation active for PHB oligomers in negative-ion mode multistage mass spectrometry

First-principles studies of the potential energy profiles for dissociation of poly[(R, S)-3-hydroxybutanoic acid] [PHB n -H] − anions by proton rearrangement and direct fragmentation, as well as subsequent statistical RRKM analysis, indicate that fragmentation should proceed mainly through successive loss of a single 86 Da neutral unit (propene + CO 2). This theoretically predicted fragmentation mechanism is inconsistent with that proposed in the literature based on experiments, which suggests that proton rearrangements are the only active fragmentation channels in CID of [PHB n -H] −. We have combined experimental ESI-MS/MS fragmentation studies of [PHB n -H] − with first-principles explorations of the potential energy surfaces, and molecular dynamics simulations of CID and thermal fragmentation events to understand this discordance. Both proton rearrangement and direct fragmentation channels are considered. The results reveal that the fragmentations observed at low collision energy favor the direct fragmentation channel and are more closely modeled as a thermal fragmentation process. By contrast, fragmentations observed at higher collision energy favor the indirect fragmentation channels and are more closely modeled as a fast CID process.

The Histidine Effect. Electron Transfer and Capture Cause Different Dissociations and Rearrangements of Histidine Peptide Cation-Radicals

Electron-transfer and -capture dissociations of doubly protonated peptides gave dramatically different product ions for a series of histidine-containing pentapeptides of both non-tryptic (AAHAL, AHAAL, AHADL, AHDAL) and tryptic (AAAHK, AAHAK, AHAAK, HAAAK, AAAHR, AAHAR, AHAAR, HAAAR) type. Electron transfer from gaseous Cs atoms and fluoranthene anions triggered backbone dissociations of all four N-C(alpha) bonds in the peptide ions in addition to loss of H and NH(3). Substantial fractions of charge-reduced cation-radicals did not dissociate on an experimental time scale ranging from 10(-6) to 10(-1) s. Multistage tandem mass spectrometric (MS(n)) experiments indicated that the non-dissociating cation-radicals had undergone rearrangements. These were explained as being due to proton migrations from N-terminal ammonium and COOH groups to the C-2' position of the reduced His ring, resulting in substantial radical stabilization. Ab initio calculations revealed that the charge-reduced cation-radicals can exist as low-energy zwitterionic amide pi* states which were local energy minima. These states underwent facile exothermic proton migrations to form aminoketyl radical intermediates, whereas direct N-C(alpha) bond cleavage in zwitterions was disfavored. RRKM analysis indicated that backbone N-C(alpha) bond cleavages did not occur competitively from a single charge-reduced precursor. Rather, these bond cleavages proceeded from distinct intermediates which originated from different electronic states accessed by electron transfer. In stark contrast to electron transfer, capture of a free electron by the peptide ions mainly induced radical dissociations of the charge-carrying side chains and loss of a hydrogen atom followed by standard backbone dissociations of even-electron ions. The differences in dissociation are explained by different electronic states being accessed upon electron transfer and capture.

Density Functional Study of the High-Temperature Oxidation of o-, m- and p-Xylyl Radicals

Theoretical calculations at the CBS-QB3 level of theory have been performed to investigate the potential energy surface for the reaction of o-, m- and p-xylyl with molecular oxygen. The differences of the relative potential energies for the products and the transition states of o-, m- and p-xylyl with molecular oxygen were found to be within 8.5 kJ/mol at the CBS-QB3 level of theory. Although the reaction of m- and p-xylyl radicals with molecular oxygen have the same reaction pathways and also the same reaction thermochemistry as that of benzyl radicals, the o-xylylperoxy radicals formed by the reaction of o-xylyl + O(2) had an additional intramolecular isomerization pathway to form the o-xylyl hydroperoxy radicals. The rate constants and the product branching ratios for the o-xylyl + O(2) and its subsequent reactions were evaluated by the RRKM and master equation analysis. Possible roles for these reaction pathways on the combustion of o-xylenes are discussed.

Experimental and kinetic modeling study of C2H4 oxidation at high pressure

A detailed chemical kinetic model for oxidation of C2H4 in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C2H3+O2 reaction was used to obtain rate coefficients over a wide range of conditions (0.003–100bar, 200–3000K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH2O and HCO, is the dominant product. The experiments, involving C2H4/O2 mixtures diluted in N2, were carried out in a high pressure flow reactor at 600–900K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C2H4 are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH2CH2OH) and 2-hydroperoxyethyl (CH2CH2OOH) radicals, formed from addition of OH and HO2 to C2H4, and vinyl peroxide, formed from C2H3+O2. Hydroxyethyl is oxidized through the peroxide HOCH2CH2OO (lean conditions) or through ethenol (low O2 concentration), while 2-hydroperoxyethyl is converted through oxirane.

Dynamics of Vibrational Overtone Excited Pyruvic Acid in the Gas Phase: Line Broadening through Hydrogen-Atom Chattering

The dynamics of overtone-excited pyruvic acid (PA) is studied using a combination of experimental and theoretical methods. It is experimentally observed that high overtone excitation of the OH-stretching mode of PA in the gas phase leads to a unimolecular decarboxylation reaction. An RRKM analysis of the rate is consistent with previous experiments for the thermal reaction but is inconsistent with the present overtone chemistry; from this it is concluded that the overtone-induced reaction is likely to be a direct reaction. Using a Fourier transform infrared spectrometer and a cavity ring-down spectrometer, the spectrum for the OH-stretch fundamental and overtone transitions is measured. We assign two conformers of PA in the spectrum, the Tc and Tt, corresponding to distinct orientations of the OH-group. The spectral peaks for the Tc-conformer broaden dramatically at the third and fourth overtones while those of the Tt-conformer remain relatively narrow. Using a three-mode quantum mechanical model for the vibrational states, the line positions and intensities are well reproduced by theory. The line widths, and the associated dynamical interpretation, are provided by a direct dynamics calculation, where the potential is computed "on-the-fly" and all degrees of freedom are included. It is found that the line broadening is due to the onset of H-atom chattering between the two O-atoms, an effect that occurs for the Tc-conformer but not the Tt-conformer. This H-atom-transfer process is the first step of the decarboxylation reaction mechanism, which subsequently involves breaking the C-C bond. The theoretical and experimental line widths agree but do not correspond to the full reaction time which is much longer than the initial chattering step.

Quantum chemical and RRKM calculations of reactions in the H/S/O system

Quantum chemical calculations at the MRCI/aug-cc-pV5Z level are used to describe the conversions between HSO, HOS, H + SO, S + OH and O + SH on the doublet H/S/O potential energy surface. An RRKM analysis of this multiple-well system was carried out in the temperature range 300–2000 K between 0.1 and 10 atm. At these pressures, the stabilization reaction H + SO → HSO or HOS is at the low pressure limit, and stabilization from S + OH and O + SH was not detected. The reactions S + OH → H + SO and O + SH → H + SO were found to be barrierless and very fast at room temperature (4 × 10 14 and 1.5 × 10 14 cm 3 mol −1 s −1, respectively). The reaction channel O + SH → S + OH is two orders of magnitude slower than the more exothermic O + SH → H + SO reaction, although a second pathway involving direct H-abstraction (O + SH → S + OH) on the quartet surface appears as a minor channel at high temperatures.

The decomposition of normal hexyl radicals

Normal hexyl radicals generated from the decomposition of n-hexyl iodide have been decomposed in single pulse shock tube experiments. All the products arising from the decomposition of 1-hexyl (the initial reactant) and 2-hexyl and 3-hexyl (isomerization products) have been detected in the temperature range 890–1020 K and 1.5–5 bar pressures. We find that k ( C 6 H 13 - 3 = > C 2 H 5 + 1 - C 4 H 8 ) / k ( C 6 H 13 - 3 = > CH 3 + 1 - C 5 H 10 ) = 3.4 ± 0.1 when this is combined with literature values of the beta bond scission reactions of hexyl radicals we find the following high pressure rate expressions for the isomerization processes; k ( C 6 H 13 - 1 < = > C 6 H 13 - 2 ) = 1.83 × 10 2 T 2.55 exp ( - 5516 / T ) s - 1 k ( C 6 H 13 - 1 < = > C 6 H 13 - 3 ) = 6.98 × T 3.2 exp ( - 8333 / T ) s - 1 with an uncertainty of a factor of 1.5 in the rate constants and 5 kJ/mol in the activation energy. They are compatible with low as well as the high temperature results. The effects of energy transfer have been deduced from a master equation RRKM analysis covering the pressure range of 0.1–100 bar, 500–1900 K. The low reaction thresholds and multichannel nature of the process leads to special problems. These include a non-monotonic decrease in rate constants from high pressure values with increasing temperature and rate constants in the “fall-off” region that are dependent on the initial isomer. Overall effects are minimal in the 100 bar region. Maximum deviation from high pressure behavior is approximately an order of magnitude near 1000 K for reactions in the 1 bar region.

Dissociation of the ground state vinoxy radical and its photolytic precursor chloroacetaldehyde: electronic nonadiabaticity and the suppression of the H+ketene channel.

This work is a study of the competition between the two unimolecular reaction channels available to the vinoxy radical (CH(2)CHO), C-H fission to form H+ketene, and isomerization to the acetyl radical (CH(3)CO) followed by C-C fission to form CH(3) + CO. Chloroacetaldehyde (CH(2)ClCHO) was used as a photolytic precursor to the vinoxy radical in its ground state; photodissociation of chloroacetaldehyde at 193 nm produces vinoxy radicals with internal energies spanning the G3//B3LYP calculated barriers to the two available unimolecular reaction channels. The onset of the CH(3) + CO channel, via isomerization to the acetyl radical, was found to occur at an internal energy of 41 +/- 2 kcal/mol, agreeing well with our calculated isomerization barrier of 40.8 kcal/mol. Branching to the H+ketene channel was too small to be detected; we conclude that the branching to the H+ketene channel must be at least a factor of 200 lower than what is predicted by a RRKM analysis based on our electronic structure calculations. This dramatic result may be explained in part by the presence of a conical intersection at planar geometries along the reaction coordinate leading to H+ketene, which results in electronically nonadiabatic recrossing of the transition state.

A shock tube, laser‐schlieren study of the pyrolysis of isobutene: Relaxation, incubation, and dissociation rates

AbstractDissociation, vibrational relaxation, and unimolecular incubation have all been observed in shock waves in isobutene with the laser‐schlieren technique. Experiments covered a wide range of high‐temperature conditions: 900–2300 K, and post‐incident shock pressures from 7 to 400 torr in 2, 5, and 10% mixtures with krypton. The surprising observation is that of vibrational relaxation, well resolved over the full temperature range. The resolved process is completely exponential, with relaxation times in the range 20–120 ns atm. Relaxation and dissociation are clearly separated for T &gt; 1850 K, with estimated incubation times near 200 ns atm. Incubation is essential for modeling of the very low‐pressure decomposition. Modeling of gradients with a chain mechanism initiated by CH fission produces an excellent fit and accurate dissociation rates that show severe falloff. A restricted‐rotor, Gorin‐model RRKM analysis fits these rates quite well with the known bond‐energy as barrier and 〈ΔE〉down = 680 cm−1. The extrapolated k∞ is log k∞(s−1) = 19.187–0.865 log T −87.337 (kcal/mol)/RT, in good agreement with previous work. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 381–390, 2003

Theoretical study on the comparative fate of the 1-butoxy and β-hydroxy-1-butoxy radicals

Theoretical high level ab initio BAC-MP4 and DFT calculations followed by a kinetic RRKM analysis have been performed in this work for the study of unimolecular reactions of the 1-butoxy and β-hydroxy-1-butoxy radicals. We have shown that the substitution of H by OH on the carbon in the β position of the 1-butoxy radical (leading to the β-hydroxy-1-butoxy radical) results in an important lowering of the decomposition barrier and a slight increase of the isomerisation barrier. Coupled to the rate constant calculations, this study suggests that, contrary to the fate of the 1-butoxy radical, the thermal decomposition is the major pathway for the β-hydroxy-1-butoxy radical. We have also shown that, under atmospheric conditions (760 Torr and 298 K), both isomerisation and decomposition processes are still in the fall-off range for the hydroxy radical. These behaviors have been interpreted in terms of electronic structures and intramolecular hydrogen bonding. This is the first theoretical study of the β-hydroxy-1-butoxy radical unimolecular reactions. As there are no experimental measurements on the β-hydroxy-1-butoxy radical rate constants, this theoretical study is the first to predict kinetic parameters for the decomposition and isomerisation reactions of this compound.

Poly (ethylene terephthalate) oligomers cationized by alkali ions: structures, energetics, and their effect on mass spectra and the matrix-assisted laser desorption/ionization process

In this article the folding dynamics and energetics for a set of poly(ethylene terephthalate) (PET) oligomers cationized by various alkali ions are studied: M +PET n for n = 2 to 7 and M = Li, Na, and K. Experimental cross sections were determined for matrix-assisted laser desorption/ionization (MALDI) generated ions using the ion mobility based ion chromatography method. Very good agreement was obtained with cross sections generated by the amber 4.0 suite of molecular dynamics programs. For n = 2 and 4 the benzene rings of the oligomers π stack with the metal ion coordinated to both terminal hydroxyl oxygen atoms and several of the nearby carbonyl oxygen atoms. For n = 3, two isomers are both observed and predicted by theory: an open form where the third PET monomer attaches to the dimer and extends into space and a closed form where the third PET moiety bends back and coordinates its hydroxyl oxygen with the metal ion. For Na +PET 3, equilibrium is observed between 100 and 180 K with an Arrhenius analysis yielding an open to closed form isomerization barrier of 1.6 kcal/mol. For this same system the two isomeric forms are frozen out at 80 K and coupling the observed isomeric distribution with an RRKM analysis indicates the closed form is more stable by 0.5 kcal/mol. For K +PET 3 the barrier to isomerization is too low to observe (<1.0 kcal/mol), whereas for Li +PET 3 a temperature independent isomer distribution is observed (80 to 55°K). Using methods developed for determining isomerization barriers in carbon clusters it was possible to obtain an open to closed form isomerization barrier of 7 ± 2 kcal/mol for Li +PET 3. In this system, the open and closed form isomer populations were observed to be strong functions of the laser power in the MALDI source. This allowed a detailed description of the formation mechanism to be formulated and indicated alkali ion attachment to the polymer during expansion of the plume emanating from the surface. Finally, the mass spectrum of a PET oligomer sample has been shown to strongly depend on the cationizing alkali metal ion. It is qualitatively shown that M +–PET n binding energies and structures are responsible.

Thermal Decomposition and Isomerization Processes of Alkyl Radicals

The thermal decomposition and isomerization processes of C3−C4 alkyl radicals, 1-C5H11, and 1-C6H13 have been investigated by using a shock-tube apparatus coupled with atomic resonance absorption spectrometry (ARAS). Isomeric alkyl radicals were generated by the thermal decomposition of respective alkyl iodides. Branching fractions for the competitive pathways (C−C bond cleavage, C−H bond cleavage, and isomerization) have been determined by following the hydrogen-atom concentration by ARAS. In the investigated temperature range (900−1400 K), for all alkyl radicals, the energetically favored C−C bond cleavage was found to dominate over the C−H bond cleavage. The 1,2 or 1,3 isomerization reaction was found to be minor in C3 and C4 alkyl radicals. On the other hand, the results for 1-C5H11 and 1-C6H13 radicals clearly show the occurrence of 1,4 and 1,5 isomerization reactions. From an RRKM analysis of the present result and the previous lower temperature data, with consideration of the tunneling effect, the ...

Isomerization and Dissociation in Competition: The Two-Component Dissociation Rates of Energy Selected Methyl Formate Ions

Threshold photoelectron−photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase, energy-selected methyl formate ions, CH3OCHO•+. The 0 K dissociation onsets for CO loss from CH3OCHO•+ and CH3OCDO•+ were 11.550 ± 0.020 and 11.582 ± 0.020 eV, respectively. The two-component dissociation rates are interpreted in terms of competitive fast dissociation and isomerization steps, followed by a slow dissociation from the lower energy isomer, a distonic ion with the structure CH2OCHOH•+. Ab initio molecular orbital calculations (G2 theory) are used to obtain energies and vibrational frequencies of various ion structures and transition states. The experimental rate constants are then modeled with the RRKM statistical theory for dissociation of both CH3OCHO•+ and CH2OCHOH•+. From the RRKM analysis, we found that the distonic ion CH2OCHOH•+ lies 16.2 ± 1.5 kcal/mol lower than the methyl formate ion. Rearrangement of CH3OCHO•+ to its distonic isomer requires 15.2...

Kinetic and mechanistic study of the pressure and temperature dependence of the reaction CH3O+NO

New kinetic measurements for the CH3O + NO reaction have been performed using two different techniques. The discharge flow (DF) technique has been used to investigate the 0.5–5 Torr and 248–473 K pressure and temperature ranges and pulsed laser photolysis (PLP) has been used for the 30–500 Torr and 284–364 K ranges. These new results represent an extension of the pressure and temperature ranges investigated previously. This reaction is known to present two reaction pathways, the association pathway yielding CH3ONO and the disproportionation pathway yielding CH2O+HNO. Based on literature and present experimental data, using the results of ab initio calculations, a multichannel RRKM analysis was developed to interpret the experimental results. This analysis has shown that the disproportionation reaction occurs simultaneously by both a direct hydrogen abstraction reaction, and via the formation of energized CH3ONO* complex in competition with the association reaction. The RRKM analysis, fitted to present and previous data, has yielded a second-order limiting low-pressure value of 2.5 × 10−12 cm3 molecule−1 s−1 at 298 K, with a complex temperature dependence. The limiting high-pressure rate constant derived in the same way is k∞ = (3.4 ± 0.4) × 10−11(T/298)−0.75. The model allows the prediction of CH3O loss rate constants and of the branching ratios in the 1–760 Torr and 220–600 K ranges. For a convenient presentation of the overall rate constant, an analytical expression using the conventional Troe expression with a temperature-dependent addition constant, has been fitted to the results of the RRKM analysis.

Vibrational relaxation and dissociation in the perfluoromethyl halides, CF3Cl, CF3Br, andCF3I

The processes of vibrational relaxation and unimolecular dissociation of the perfluoromethyl halides CF3Cl, CF3Br, and CF3I have been studied in the shock tube with the laser-schlieren technique. Vibrational relaxation was resolved in pure CF3Cl and CF3Br (400–484 K and 400–500 K, respectively), and in the mixtures; 2% CF3Cl/Kr (500–1000 K), 10% CF3Cl/Kr (440–670 K), 4% CF3Br/Kr (450–850 K), and 2% CF3I/Kr (620–860 K). Relaxation in the pure gases is extremely rapid, but shows a well-resolved, accurately exponential decay which provides very precise relaxation times in close agreement with ultrasonic results. Relaxation times as short as 0.1 μs-atm can be resolved, showing the method has a resolution within a factor 2–3 of the best ultrasonic methods. Relaxation dilute in rare gas shows a complex double exponential behavior consistent with a two-stage series process. Rates of CF3(SINGLEBOND)X fission in these mixtures were measured over 1800–3000 K, P<0.55 atm, for CF3Cl; 1600–2500 K, P<0.55 atm, in CF3Br; and 1260–2100 K, P<0.34 atm, in CF3I. Rates for dissociation were derived from a full profile modeling using a secondary mechanism of six CF3 reactions. RRKM analysis showed all dissociations to lie near the low pressure limit. Using literature barriers, these rates are best fit with (ΔE)all=−270 cm−1 for CF3Cl, 〈ΔE〉down=0.3 T for CF3Br, and 〈ΔE〉down=800 cm−1 for CF3F. All these transfers are on the large side, similar to those found in other halogenated methanes. © 1997 John Wiley & Sons, Inc.

Dissociation and chain reaction in the pyrolysis of pyraine

Experiments on pyrazine decomposition in shock waves that use both the laser-schlieren technique (1680–2300 K, 2 and 4% pyrazine in Kr) providing time-resolved density gradient, and time-of-flight mass spectrometry (1460–2267 K, 2% pyrazine in 2% Ar/Ne and in 2% Ar/5%H2/Ne) giving time-dependent major-product concentrations are reported. The gradient measurements resolve the induction period of this extremely fst chain, allowing an unambiguous determination of the C-H fission initiation rate. Derived rate constants are in the fall-off regime; RRKM analysis of these suggests a barrier of 105±2 kcal/mol, below the 112 of benzene but above the 99±2 also derived here for pyrimidine. A revised chain mechanisms is presented incorporating new ab initio thermochemistry, which is then used in, RRKM calculations of key radical dissociations rates. Another departure is a path to cyanoacetylene (CHCCN) beginning with is consistent with theory and observed strong maxima in CHCCN concentration that the greatly reduced by added H2.