Thermal decomposition of vinylacetylene in shock waves: Rate constant of initiation reaction

The thermal decomposition of vinylacetylene was studied behind incident shock waves over the temperature range 1200–1750 K and over the pressure range 0.3–0.6 atm by tracing the time variation of absorption at 230 nm. The initiation reaction and the rate constant in the thermal decomposition of vinylacetylene were determined from the initial slope of the absorption curve as C₄H₄ → h₁, C₄H₃+H, k₁ = 6.1 × 10¹³ exp (−80 kcal/RT) s⁻¹.     

Thermal decomposition of propane in shock waves

The thermal decomposition of propane was studied behind reflected shock waves over the temperature range 1100–1450 K and the pressure range 1.5–2.6 atm, by both monitoring the time variations of absorption at 3.39 μm and analyzing the concentrations of the reacted gas mixtures. The rate constants of the elementary reactions were discussed from the results. The rate constant expressions, k₁ = 1.1 × 10¹⁶ exp (?84 kcal/RT) s^?1 and k₄ = 9.3 × 10¹³ exp(?8 kcal/RT) cm³ mol^?1 s^?1, of reactions C₃H₈ → CH₃ + C₂H₅ and C₃H₈ + H → n-C₃H₇ + H₂ were evaluated, respectively.     

Thermal decomposition of ammonia in shock waves

The thermal decomposition of ammonia was studied by means of the shock-tube and vacuum ultraviolet absorption spectroscopy monitoring the concentration of atomic hydrogen. The rate constants of both the initiation reaction and the consecutive reaction were determined directly as and respectively.     

Thermal decomposition of ethane in shock waves

The thermal decomposition of ethane was studied behind reflected shock waves over the temperature range 1200–1700 K and over the pressure range 1.7?2.5 atm, by both tracing the time variation of absorption at 3.39 μm and analyzing the concentration of the reacted gas mixtures. The mechanism to interpret well not only the earlier stage of C₂H₆ decomposition, but also the later stage was determined. The rate constant of reactions, C₂H₆ → CH₃ + CH₃, C₂H₆ + C₂H₃ → C₂H₅ + C₂H₄, C₂H₅ → C₂H₄ + H were calculated. The rate constants of the other reactions were also discussed.     

Thermal decomposition of 1-butyne in shock waves

1-Butyne diluted with Ar was heated behind reflected shock waves over the temperature range of 1100–1600 K and the total density range of 1.36 × 10^?5?1.75 × 10^?5 mol/cm³. Reaction products were analyzed by gas-chromatography. The progress of the reaction was followed by IR laser kinetic absorption spectroscopy. The products were CH₄, C₂H₂, C₂H₄, C₂H₆, allene, propyne, C₄H₂, vinylacetyiene, 1,2- butadiene, 1,3-butadiene, and benzene. The present data were successfully modeled with a 80 reaction mechanism. 1-Butyne was found to isomerize to 1,2-butadiene. The initial decomposition was dominated by 1-butyne → C₃H₃ + CH₃ under these conditions. Rate constant expressions were derived for the decomposition to be k₇ = 3.0 × 10¹⁵ exp(?75800 cal/RT) s^?1 and for the isomerization to be k₄ = 2.5 × 10¹³ exp(?65000 cal/RT) s^?1. The activation energy 75.8 kcal/mol was cited from literature value and the activation energy 65 kcal/mol was assumed. These rate constant expressions are applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.     

Thermal decomposition of propyne and allene in shock waves

Propyne (p-C₃H₄) or allene (a-C₃H₄) mixtures, highly diluted with Ar, were heated to the temperature range 1200–1570 K at pressures of 1.7–2.6 atm behind reflected shock waves. The thermal decompositions of propyne and allene were studied by both measuring the profiles of the IR emission at 3.48 μm or 5.18 μm and analyzing the concentrations of reacted gas mixtures. The mechanism and the rate constant expressions were discussed from both the profiles and the concentrations of reactant and products obtained. The rate constant expressions for reactions, (1) p-C₃H₄ → a-C₃H₄, (?1) a-C₃H₄ → p-C₃H₄, and (5) p-C₃H₄ + H → CH₃ + C₂H₂ were evaluated.     

Thermal isomerization and decomposition of 1,2-butadiene in shock waves

1,2-Butadiene diluted with Ar was heated behind reflected shock waves over the temperature and the total density range of 1100–1600 K and 1.36 × 10^?5 ? 1.75 × 10^?5 mol/cm³. The major products were 1,3-butadiene, 1-butyne, 2-butyne, vinylacetylene, diacetylene, allene, propyne, C₂H₆, C₂H₄, CH₄, and benzene, which were analyzed by gas chromatography. The UV kinetic absorption spectroscopy at 230 nm showed that 1,2-butadiene rapidly isomerizes to 1,3-butadiene from the initial stage of the reaction above 1200 K. In order to interpret the formation of 1,3-butadiene, 1-butyne, and 2-butyne, it was necessary to include the parallel isomerizations of 1,2-butadiene to these isomers. The present data were successfuly modeled with a 82 reaction mechanism. From the modeling, rate constant expressions were derived for the isomerization 1,2-butadiene = 1,3-butadiene to be k₃ = 2.5 × 10¹³ exp(?63 kcal/RT) s^?1 and for the decomposition 1,2-butadiene = C₃H₃ + CH₃ to be k₆ = 2.0 × 10¹⁵ exp(?75 kcal/RT) s^?1, where the activation energies, 63 kcal/mol and 75 kcal/mol, were assumed. These rate constants are only applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.     

Thermal decomposition of methyl nitrite in shock waves studied by laser probing

The unimolecular decomposition of methyl nitrite in the temperature range 680–955 K and pressure range 0.64 to 2.0 atm has been studied in shock-tube experiments employing real-time absorption of CW CO laser radiation by the NO product. Computer kinetic modeling using a set of 23 reactions shows that NO product is relatively unreactive. Its initial rate of production can be used to yield directly the unimolecular rate constant, which in the fall-off region, can be represented by the second-order rate coefficient in the Arrhenius form: A RRKM model calculation, assuming a loose CH₃ONO^≠ complex with two degrees of free internal rotation, gives good agreement with the experimental rate constants.     

Low-pressure thermal decomposition of ONBr and ONCl in shock waves

Rate constants for the low-pressure unimolecular decomposition of ONBr and ONCl in an argon bath have been determined at temperatures in the vicinity of 1000°K. Both molecules exhibit the usual depression of the observed activation energy below the bond dissociation energy. The Arrhenius expressions obtained are (units of cc mole^?1 sec^?1): Treatment of the data by the classical RRK theory yields s ? 2.7 ± 1 for ONCl and 3.0 ± 0.6 for ONBr. Coupling the shock tube results for ONCl with lower-temperature data from Ashmore and Burnett [3], one obtains s ? 2.5 ± 0.5 and λ ≈? 1. If it is assumed that s is also 2.5 for ONBr, then one finds the surprising (but tentative) result that λ_{ONCl? Ar/λONBr? Ar} ≈? 3 to 4.     

Measuring the rate constant of azomethane decomposition in shock waves

The rate constant of azomethane decomposition in argon was measured at temperatures of 820 to 1400 K, pressures of 0.25 to 7.5 atm, and initial azomethane concentrations of 40 to 2000 ppm. The amount of azomethane reacted was estimated as UV light absorption at the vacuum UV boundary (λ = 198 nm), and the concentration of \(\dot CH_3 \) radicals resulting from azomethane decomposition was monitored as absorption at λ = 216 nm. The observed temperature dependence of the azomethane decomposition rate constant, k ₁ ^app = 10^11.3exp(?33.5/RT)s^?1, is in good agreement with the literature. The low values of the activation energy and preexponential factor are unnatural for classical monomolecular decomposition. This confirms the assumption that azomethane decomposition at high temperatures takes place via a concerted mechanism.     

Thermal isomerization of azulene to naphthalene in shock waves

The thermal isomerization of azulene was studied in shock waves over the range 1300–1900 K. Monitoring azulene and naphthalene light absorptions in the UV, a complete conversion azulene → naphthalene was observed. After correction for some falloff effects, a limiting high pressure rate constant k_x = 10^12.93 exp(?263 kJ mol^?1/RT) s^?1 was derived. Based on this k_x, specific rate constants k(E) for photoexcitation experiments were constructed.     

Thermal decomposition mechanisms of the methoxyphenols: formation of phenol, cyclopentadienone, vinylacetylene, and acetylene

The pyrolyses of the guaiacols or methoxyphenols (o-, m-, and p-HOC(6)H(4)OCH(3)) have been studied using a heated SiC microtubular (μ-tubular) reactor. The decomposition products are detected by both photoionization time-of-flight mass spectroscopy (PIMS) and matrix isolation infrared spectroscopy (IR). Gas exiting the heated SiC μ-tubular reactor is subject to a free expansion after a residence time of approximately 50-100 μs. The PIMS reveals that, for all three guaiacols, the initial decomposition step is loss of methyl radical: HOC(6)H(4)OCH(3) → HOC(6)H(4)O + CH(3). Decarbonylation of the HOC(6)H(4)O radical produces the hydroxycyclopentadienyl radical, C(5)H(4)OH. As the temperature of the μ-tubular reactor is raised to 1275 K, the C(5)H(4)OH radical loses a H atom to produce cyclopentadienone, C(5)H(4)═O. Loss of CO from cyclopentadienone leads to the final products, acetylene and vinylacetylene: C(5)H(4)═O → [CO + 2 HC≡CH] or [CO + HC≡C-CH═CH(2)]. The formation of C(5)H(4)═O, HCCH, and CH(2)CHCCH is confirmed with IR spectroscopy. In separate studies of the (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectra, we observe the presence of C(6)H(5)OH in the molecular beam: C(6)H(5)OH + λ(275.1?nm) → [C(6)H(5)OH ?] + λ(275.1nm) → C(6)H(5)OH(+). From the REMPI and PIMS signals and previous work on methoxybenzene, we suggest that phenol results from a radical/radical reaction: CH(3) + C(5)H(4)OH → [CH(3)-C(5)H(4)OH]* → C(6)H(5)OH + 2H.     

Thermal decomposition of 1,1,2,2-tetramethylcyclopropane in a single-pulse shock tube

1,1,2,2-Tetramethylcyclopropane (TTMC) has been decomposed in a single-pulse shock tube. The main reaction process is Side reactions are unimportant. From comparative rate experiments (with cyclohexene decomposition as standard) the rate expression for these reactions are These numbers are consistent with a «best» value for cyclohexene decomposition of      

Kinetic investigations of the unimolecular decomposition of dimethylether behind shock waves

The thermal decomposition of dimethylether was studied behind reflected shock waves at total pressures of 0.3 − 1.3 bar in the temperature range 1270 − 1620 K using H-atom detection by Lyman-α resonance absorption spectroscopy at 121.6 nm.     

The NO- and NO2-catalyzed decomposition of I2 in shock waves

Measurements of the NO-catalyzed dissociation of I₂ in Ar in incident shock waves were carried out in the temperature range of 700°-1520°K and at total concentrations of 5 × 10^?6-6 × 10^?5 mol/cm³, using ultraviolet-visible absorption techniques to monitor the disappearance of I₂. It was shown that the main reaction responsible for the disappearance under these conditions is I₂ + NO → INO + I, for which a rate coefficient of (2.9 ± 0.5) × 10¹³ exp[-(18.0 ± 0.6 kcal/mol)/RT] cm²/mol·sec was determined. The INO formed dissociates rapidly in a subsequent reaction. The reaction, therefore, constitutes a “chemical model” for a “thermal collisional release mechanism.” Preliminary measurements of the rate coefficient for I₂ + NO₂ → INO₂ + I are also presented. Combined with information on the reverse reactions obtained in earlier room temperature experiments, these results lead to accurate values of ΔH°_f for INO and INO₂ equal to 29.7 ± 0.5 and 15.9 ± 1 kcal/mol, respectively.     

Experimental and modeling study of carbon suboxide decomposition behind reflected shock waves

At temperatures between 1150 and 2000 K and pressures between 0.1 and 0.2 MPa, the thermal decomposition of carbon suboxide (C(3)O(2)) behind reflected shock waves was investigated with a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) connected to the end flange of a shock tube enabling rapid repetitive (100 kHz) measurements of the gas-phase composition. Concentration-time profiles for C(3)O(2) and CO were measured and compared to simulations based on an improved mechanism for C(3)O(2) decomposition and carbon cluster growth. In addition, relative concentrations of C atoms and C(2) molecules were detected and related to model predictions. For temperatures up to 1800 K, satisfactory agreement between experimental data and calculations was obtained. At higher temperatures, measurements and simulations differed noticeably. The importance of C(2) for the growth of carbon clusters was confirmed.     

Thermal decomposition of CH3I revisited: Consistent calibration of I-atom concentrations behind shock waves with dual I-/H-ARAS

Iodinated hydrocarbons are often used as precursors for hydrocarbon radicals in shock-tube experiments. The radicals are produced by C─I bond fission reaction, and their formation can be followed through time-resolved monitoring of the complementary I-atom concentrations, for example, by I-atom resonance absorption spectroscopy (I-ARAS). This very sensitive technique requires, however, an independent calibration. As a very clean source of I atoms, CH₃I is particularly well suited as calibration system for I-ARAS presumed the yield of I atoms and the rate coefficient of I-atom formation from CH₃I are known with sufficient accuracy. But if the formation of I atoms from CH₃I by I-ARAS is to be characterized, an independent calibration system is required. In this study, we propose a cross-calibration approach for I-ARAS based on the simultaneous time-resolved monitoring of I and H atoms by ARAS in C₂H₅I pyrolysis experiments. For this reaction system, it can be shown that at sufficiently short reaction times very similar amounts of I and H atoms are formed (difference <1%). As calibration of H-ARAS, with mixtures of N₂O and H₂, is a well-established technique, we calibrated I-atom absorption–time profiles with respect to simultaneously recorded H-atom concentration–time profiles. Using this approach, we investigated the thermal decomposition of CH₃I in the temperature range 950–2050 K behind reflected shock waves at two different nominal pressures (p ∼ 0.4 and 1.6 bar, bath gas: Ar). From the obtained absolute I-atom concentration–time profiles at temperatures T < 1250 K, we inferred a second-order rate coefficient k(T) = (1.7 ± 0.7) × 10¹⁵ exp(–20020 K/T) cm³ mol^–1 s^–1 for the reaction CH₃I + Ar → CH₃ + I + Ar. A small mechanism to describe the pyrolysis of CH₃I under shock-tube conditions is presented and discussed.     

NF3 decomposition behind shock waves in the intermediate pressure range

Rates of NF₃ thermal decomposition in the temperature range 1450–2050 K and pressure range 1.2–4.2 atm were measured by a shock tube technique. NF₃ decomposition was monitored by UV absorption of the produced NF₂ radicals. The bimolecular rate constant of the NF₃ decomposition reaction essentially depends upon pressure. An empirical formula was obtained from the experimental data which correctly expresses the dependence of the measured rate constant of decomposition upon the temperature and pressure of the reacting mixture behind the shock wave.     

Amidochlorination of vinylacetylene

Conclusions Amidochlorination of vinylacetylene by N,N-dichlorourethan proceeds by two pathways with the formation-of predominantly acetylenyl (but also allenyl) chlorocarbamates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1565–1567, July, 1986.