Relaxation, incubation, and dissociation in CO2

The dissociation and relaxation of CO2 has been reexamined in the incident shock wave with the laser-schlieren technique. These new experiments covered 1377-6478 K, and 42-750 Torr, and improvements partly described herein have permitted accurate determination of both rate and incubation time. In general the steady rate measurements are in agreement with other recent determinations. The one anomaly is that the new rates are not fully second order; they vary about 50% over 70-600 Torr. This unexpected feature is actually quite consistent with the recent literature, which shows a similar trend. However, attempts to produce this result with RRKM calculations were unsuccessful. Relaxation times are in agreement with available literature, and incubation time to relaxation time ratios lie between 1.5 and 3 over 4000-6600 K, consistent with findings for other molecules. These ratios are much smaller than those recently derived from reflected-shock experiments by Oehlschlaeger et al. (Z. Phys. Chem. 2005, 219, 555). A simple argument suggests such large values are indeed anomalous, although why they are too large is not clear.     

A shock tube study of the pyrolysis of NO2

NO₂ concentration profiles in shock-heated NO₂/Ar mixtures were measured in the temperature range of 1350–2100 K and pressures up to 380 atm using Ar⁺ laser absorption at 472.7 nm, IR emission at 6.25±0.25 μm, and visible emission at 300–600 nm. In the course of this study, the absorption coefficient of NO₂ at 472.7 nm was measured at temperatures from 300 K to 2100 K and pressures up to 75 atm. Rate coefficients for the reactions NO₂+M→NO+O+M (1), NO₂+NO₂→2NO+O₂ (2a), and NO₂+NO₂→NO₃+NO (2b) were derived by comparing the measured and calculated NO₂ profiles. For reaction (1), the following low- and high-pressure limiting rate coefficients were inferred which describe the measured fall-off curves in Lindemann form within 15% [FORMULA] The inferred rate coefficient at the low- pressure limit, k_1o, is in good agreement with previous work at higher temperatures, but the energy of activation is lower by 20 kJ/mol than reported previously. The pressure dependence of k₁ observed in the earlier work of Troe [1] was confirmed. The rate coefficient inferred for the high pressure limit, k_1∞, is higher by a factor of two than Troe's value, but in agreement with data obtained by measuring specific energy-dependent rate coefficients. For the reactions (2a) and (2b), least-squares fits of the present data lead to the following Arrhenius expressions: [FORMULA] For reaction (2), the new data agree with previously recommended values of k_2a and k_2b, although the present study suggests a slightly higher preexponential factor for k_2a. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 483–493, 1997.     

Formation of H‐atoms in the pyrolysis of cyclohexane and 1‐hexene: A shock tube and modeling study

Cyclohexane (cC₆H₁₂) plays an important role in the combustion of practical liquid fuels, as a major component of naphthenic compounds. Therefore, the pyrolysis of cyclohexane was investigated by measuring the formation of H‐atoms. The thermal decomposition of 1‐hexene (1‐C₆H₁₂) was also studied, because of the assumption that 1‐hexene is the sole initial product of cyclohexane decomposition. For cyclohexane, the measurements were performed over a temperature range of 1320–1550 K, at pressures ranging from 1.8 to 2.2 bar; 1‐hexene experiments were done at temperatures between 1250 and 1380 K and pressures between 1.5 and 2.5 bar. For each experiment, the time‐dependent formation of H‐atoms was measured behind reflected shock waves by using the method of atomic resonance absorption spectrometry. For the dissociation of 1‐hexene to n‐propyl (C₃H₇) and allyl (C₃H₅) radicals, the following Arrhenius expression was derived: k_R2(T) = 2.3 × 10¹⁶ exp(?36,672 K/T) s^?1. For cyclohexane, overall rate coefficients (k_ov) were deduced for the global reaction cC₆H₁₂ → products + H from the H‐atom time profiles; the following temperature dependency was obtained: k_ov(T) = 4.7 × 10¹⁶ exp(?44,481 K/T) s^?1. For both sets of rate coefficient values, an uncertainty of ±30% is estimated. Especially concerning the isomerization cC₆H₁₂ → 1‐C₆H₁₂, our experimental results are in excellent agreement with the rate coefficient values given by Tsang (Tsang, W. Int J Chem Kinet 1978, 10, 1119–1138). A reaction model was assembled that is able to reproduce the H‐atom profiles measured for both sets of experiments. According to this model, H‐atoms are mostly stemming from the thermal decomposition of allyl radicals (C₃H₅), which arise from the decomposition of 1‐hexene. Furthermore, it will be shown that the recombination of allyl radicals with H‐atoms to propene (C₃H₆) also represents a very important subsequent reaction. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 107–119, 2011     

A shock tube and theoretical study on the pyrolysis of 1,4-dioxane

The dissociation of 1, 2 and 4% 1,4-dioxane dilute in krypton was studied in a shock tube using laser schlieren densitometry, LS, for 1550-2100 K with 56 ± 4 and 123 ± 3 Torr. Products were identified by time-of-flight mass spectrometry, TOF-MS. 1,4-dioxane was found to initially dissociate via C-O bond fission followed by nearly equal contributions from pathways involving 2,6 H-atom transfers to either the O or C atom at the scission site. The 'linear' species thus formed (ethylene glycol vinyl ether and 2-ethoxyacetaldehyde) then dissociate by central fission at rates too fast to resolve. The radicals produced in this fission break down further to generate H, CH(3) and OH, driving a chain decomposition and subsequent exothermic recombination. High-level ab initio calculations were used to develop a potential energy surface for the dissociation. These results were incorporated into an 83 reaction mechanism used to simulate the LS profiles with excellent agreement. Simulations of the TOF-MS experiments were also performed with good agreement for consumption of 1,4-dioxane. Rate coefficients for the overall initial dissociation yielded k(123Torr) = (1.58 ± 0.50) × 10(59) × T(-13.63) × exp(-43970/T) s(-1) and k(58Torr) = (3.16 ± 1.10) × 10(79) × T(-19.13) × exp(-51326/T) s(-1) for 1600 < T < 2100 K.     

Shock‐tube and modeling study of acetaldehyde pyrolysis and oxidation

Pyrolysis and oxidation of acetaldehyde were studied behind reflected shock waves in the temperature range 1000–1700 K at total pressures between 1.2 and 2.8 atm. The study was carried out using the following methods, (1) time‐resolved IR‐laser absorption at 3.39 μm for acetaldehyde decay and CH‐compound formation rates, (2) time‐resolved UV absorption at 200 nm for CH₂CO and C₂H₄ product formation rates, (3) time‐resolved UV absorption at 216 nm for CH₃ formation rates, (4) time‐resolved UV absorption at 306.7 nm for OH radical formation rate, (5) time‐resolved IR emission at 4.24 μm for the CO₂ formation rate, (6) time‐resolved IR emission at 4.68 μm for the CO and CH₂CO formation rate, and (7) a single‐pulse technique for product yields. From a computer‐simulation study, a 178‐reaction mechanism that could satisfactorily model all of our data was constructed using new reactions, CH₃CHO (+M) → CH₄ + CO (+M), CH₃CHO (+M) → CH₂CO + H₂(+M), H + CH₃CHO → CH₂CHO + H₂, CH₃ + CH₃CHO → CH₂CHO + CH₄, O₂ + CH₃CHO → CH₂CHO + HO₂, O + CH₃CHO → CH₂CHO + OH, OH + CH₃CHO → CH₂CHO + H₂O, HO₂ + CH₃CHO → CH₂CHO + H₂O₂, having assumed or evaluated rate constants. The submechanisms of methane, ethylene, ethane, formaldehyde, and ketene were found to play an important role in acetaldehyde oxidation. © 2007 Wiley Periodicals, Inc. 40: 73–102, 2008     

Increasing specificity of tandem mass spectrometry by laser‐induced dissociation

Mass spectrometry offers an arsenal of tools for diverse proteomic investigations. This perspective article reviews some of the recent developments in the field of coupling laser‐induced dissociation with mass spectrometry (LID‐MS). Strategies involving labelling with a chromophore to induce specific photo‐absorption properties are considered, with a focus on specific amino acid derivatization. Some of the opportunities and challenges of LID‐MS after targeted labelling for increasing specificity in complex sample analysis are discussed.     

A shock tube study of the reaction S + H2 ⇌ SH + H in pyrolysis and photolysis systems

Atomic resonance absorption spectroscopy (ARAS) was applied to measure S atoms, behind shock waves in COS/H₂ pyrolysis or CS₂/H₂ photolysis systems. Both the pyrolysis of COS and the photolysis CS₂ was used to generate the S atoms, which subsequently reacts with H₂ via the reaction: The photolysis experiments were designed to provide clear first-order conditions for reaction (R3); i.e., the H₂ concentration exceeds that of S by at least a factor of 100. The S atom profiles obtained during pyrolysis of highly diluted COS/H₂/Ar mixtures were analyzed by computer simulations based on a simplified reaction mechanism using the rate coefficient k₃ as a fitting parameter. Both groups of experiments covered the temperature range of 1257 K ? T ? 3137 K and lead to a rate coefficient of: . © 1995 John Wiley & Sons, Inc.     

Reaction kinetics of NH in the shock tube pyrolysis of HNCO

The high temperature kinetics of NH in the pyrolysis of isocyanic acid (HNCO) have been studied in reflected shock wave experiments. Time histories of the NH(X³Σ^?) radical were measured using a cw, narrow-linewidth laser absorption diagnostic at 336 nm. The second-order rate coefficients of the reactions: (1) were determined to be: cm³?mol^?1?s^?1, where f and F define the lower and upper uncertainty limits, respectively. The data for k_1a are somewhat better fit by:      

Shock tube and modeling study of propene pyrolysis

The thermal decomposition of propene behind reflected shock waves with 1200 < T₅ < 1800 K and 1.6 × 10^?5 < ρ₅ < 2.7 × 10^?5 mol/cm³ was studied by IR laser kinetic absorption spectroscopy and gas-chromatographic analysis of reaction products. The present data together with earlier shock tube data were satisfactorily modeled with a 51-reaction mechanism. As the initial step of the reaction, three channels, C₃H₆ = CH₃ + C₂H₃ (1), C₃H₆ = H + AC₃H₅ (2), and C₃H₆ = CH₄ + C₂H₂ (3), were necessary to interpret all the experimental data. © John Wiley & Sons, Inc.     

A shock tube study of the reactions of NH with NO,O2, and O

The reactions of NH(X³Σ⁻) with NO, O₂, and O have been studied in reflected and incident shock wave experiments. The source of NH in all the experiments was the thermal dissociation of isocyanic acid, HNCO. Time-histories of the NH(X³Σ) and OH(X²Π) radicals were measured behind the shock waves using cw, narrow-linewidth laser absorption at 336 nm and 307 nm, respectively. The second-order rate coefficients of the reactions: were determined to be: and cm³ mol⁻¹ s⁻¹, where ƒ and F define the lower and upper uncertainty limits, respectively. The branching fraction of channel defined as k_3b/k_3total, was determined to be 0.19 ± 0.10 over the temperature range of 2940 K to 3040 K.     

Shock tube and modeling study of 1,3-butadiene pyrolysis

1,3-Butadiene (1,3-C₄H₆) was heated behind reflected shock waves over the temperature range of 1200–1700 K and the total density range of 1.3 × 10⁻⁵ −2.9 × 10⁻⁵ mol/cm³. Reaction products were analyzed by gas-chromatography. The concentration change of 1,3-butadiene was followed by UV kinetic absorption spectroscopy at 230 nm and by quadrupole mass spectrometry. The major products were C₂H₂, C₂H_4, C₄H₄, and CH₄. The yield of CH₄ for a 0.5% 1,3-C₄H₆ in Ar mixture was more than 10% of the initial 1.3-C₄H₆ concentration above 1500 K. In order to interpret the formation of CH₄ successfully, it was necessary to include the isomerization of 1,3-C₄H₆ to 1,2-butadiene (1,2-C₄H₆) and to include subsequent decomposition of the 1,2-C₄H₆ to C₃H₃ and CH₃. The present data and other shock tube data reported over a wide pressure range were qualitatively modeled with a 89 reaction mechanism, which included the isomerizations of 1,3-C₄H₆ to 1,2-C₄H₆ and 2-butyne (2-C₄H₆). © 1996 John Wiley & Sons, Inc.     

Pulsed‐laser–deposited and plasma‐polymerized polytetrafluoroethylene (PTFE)‐like thin films: A comparative study on PTFE‐specific properties

Glass‐like and structural first‐order phase transitions are investigated in polytetrafluoroethylene (PTFE) foils and PTFE‐like films prepared by pulsed‐laser deposition (PLD) and plasma polymerization (PP). A structural comparison of the investigated polymers is performed by infrared spectroscopy and dielectric dilatometry. It is shown that dielectric dilatometry (the measurement of the susceptance vs. temperature) provides a simple and elegant means for detecting volumetric transitions in thin nonpolar polymer films. In conventional PTFE foils, the known glass‐like and structural first‐order phase transitions are identified. The structure of pulsed‐laser deposited PTFE strongly depends on the target material, ranging from highly crystalline films showing only structural phase transitions to films strongly deviating from PTFE foils, with structural characteristics comparable to plasma‐polymerized fluorocarbons. The dielectric loss of the highly crystalline PLD films compares favorably with conventional PTFE foils, making the films attractive for new applications in miniature electret devices. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2115–2125, 1999     

Chitosan‐based hydrogels: A new polymer‐based system with excellent laser‐damage threshold properties

Two types of chitosan hydrogel systems have been prepared that have a laser damage threshold (LDT) up to 35 times higher than commercial PMMA bulk materials. For these samples, the LDT increases with increasing water content. The mechanism of laser damage and the contribution of water to their high laser damage resistance have been examined. DSC measurements indicate water within the hydrogels exist in various states, each with different laser damage resistance properties. These various states play a key role in determining the LDT by controlling the dissipation of laser energy and providing a mechanism for self‐healing. This preliminary research shows that polymer hydrogels have potential for high power laser applications because they combine good mechanical integrity due to the polymer frame and good energy dissipation and healing characteristics due to the molecular mobility of water. These two traits allow for bulk shape‐retaining films with high laser damage thresholds and potential reversibility in damage processes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 769–778, 1999     

Gas‐phase pyrolysis of 2‐heteroaromatic‐1‐dimethylaminoethylenes: Kinetic and mechanistic study

Gas‐phase pyrolysis reactions of 4(2′‐dimethylaminoethenyl)‐2‐oxo‐2H‐benzo[b]pyran‐3‐carbonitrile ( 1 ), 4(2′‐dimethylaminoethenyl)‐2‐oxo‐2H‐naphtho[1,2‐b]pyran‐3‐carbonitrile ( 2 ), 1,6‐dihydro‐4‐(2′‐dimethylaminoethenyl)‐6‐oxo‐1‐phenylpyridazine‐3,5‐dicarbonitrile ( 3 ), 2‐cyano‐5‐dimethylamino‐3‐phenyl‐2,4‐pentadienonitrile ( 4 ), 2‐cyano‐5‐dimethylamino‐3‐(2‐thienyl)‐2,4‐pentadienonitrile( 5 ), 1,2‐dihydro‐4‐(2′‐dimethylaminoethenyl)‐oxo‐quinoline‐4‐carbonitrile ( 6 ), 6‐(ethylthio)‐4‐(2′‐dimethylaminoethenyl)‐2‐phenylpyrimidine‐5‐carbonitrile ( 7 ) (Scheme 1) have been carried out. The rates of gas‐phase pyrolytic reactions of compounds 3, 4, 5, and 7 have been measured and found to correspond to unimolecular first‐order reactions. Product analyses together with kinetic data were used to outline a feasible pathway for the pyrolytic reactions of the compounds under study. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:47–51, 2001     

A shock tube study of the reaction of the hydroxyl radical with propane

The rate coefficient for the reaction of the hydroxyl radical, OH, with propane has been measured at 1220 K in shock tube experiments, and a value of (1.58 ± 0.24) × 10¹³ cm³/mol s was obtained. This measured value is compared with previous experimental results and a transition-state theory calculation.