The kinetics and the mechanism of the thermal decomposition of H2S and subsequent reactions have been studied. The rate constant for the initiation reaction H2S + M → products (1) was determined by a shock tube−infrared emission spectroscopy at temperatures 2740−3570 K to be k 1 = 10-10.44±0.31 exp[−(268.6±18.4)kJ mol-1/RT] cm3 molecule-1 s-1, which is about one-fifth to one-tenth of the recent results reported by Woiki and Roth (J. Phys. Chem. 1994, 98, 12958) and Olschewski et al. (J. Phys. Chem. 1994, 98, 12964). An ab initio (MRCI+Q) calculation suggested that a spin-forbidden product channel (→S(3P) + H2) is energetically favorable compared to a H−S bond fission channel; that is, the singlet−triplet intersystem crossing occurs at an energy lower than the dissociation threshold for HS + H by about 17 kJ mol-1. The present rate constant for reaction 1 could be well reproduced by an unimolecular decomposition theory with the calculated energy for the crossing and with a reasonable collision parameter, βc. The rate constants for important subsequent reactions, S(3P) + H2 → products (3) and S(3P) + H2S → products (4), were also determined by a laser photolysis−shock tube−atomic resonance absorption spectrometry method:  k 3 = 10-9.58±0.16 exp[−(82.5±4.0) kJ mol-1/RT] (1050−1660 K) cm3 molecule-1 s-1, and k 4 = 10-9.86±0.17 exp[−(30.9±4.1) kJ mol-1/RT] (1050−1540 K) cm3 molecule-1 s-1. The ARAS measurement of H atoms revealed that the main products for reaction 3 are HS + H at pressures below 2 atm.

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To reexamine the possibility of the spin-forbidden, molecular-elimination mechanism of the thermal decomposition of H 2 S, H 2 S + M f S( 3 P) + H 2 + M (1), recently indicated by experimental and theoretical studies, as well as to examine the cause of discrepancies of its rate constants among recent works, the reverse insertion channel of the S( 3 P) + H 2 reaction, S( 3 P) + H 2 + M f H 2 S + M (-1), has been investigated. The experiments have been conducted with an excimer-laser photolysis (248 nm) in a shock tube at a lower temperature range, 900-1050 K, and a higher pressure range up to 4 atm, than previous studies, where the insertion process (-1) was estimated to be dominant over the simple hydrogen-atom-transfer reaction, S( 3 P) + H 2 f H + HS (2). The decay rate of S( 3 P) atoms has been measured by using an atomic resonance absorption spectrometry technique. The measured rate constant agreed well with the extrapolation of the previous measurements for reaction 2, showing that the recombination channel (-1) is still minor at these experimental conditions. The upper limit of the rate constant for reaction 1 derived in the present study was shown to be consistent with the theoretical rate constant calculated by the Troe's formula with the ab initio threshold energy for the intersystem crossing and with reasonable weak collision factors derived from the rate constant for reaction 1 reported by Shiina et al. [J.

Thermal decomposition of COS was investigated by shock tubes between 1140—3230 K. The decay of COS and S was monitored by IR emissions and atomic resonance absorption spectrometry (ARAS) coupled with laser flash photolysis technique, respectively. The rate constants for the reactions COS + M → CO + S + M (1) and COS + S → CO + S2 (2) were determined as k1 = (4.07 ± 1.83) × 10−10exp (−257 ± 24kJ/RT), T: 1900—3230 K and k2 = (3.91 ± 1.18) × 10−11exp (−28.3 ± 0.9 kJ/RT) cm3 molecule−1 s−1, T: 1140—1680 K.

1,1,1-Trifluoroethane (CH3CF3) has been frequently used as a chemical thermometer or an internal standard in shock tube studies to determine relative rates of chemical reactions. The rate constants for the thermal decomposition of CH3CF3 were recently reported to have anomalous pressure dependence in the high-temperature falloff region. In the present study, the kinetics of the CH3CF3 decomposition were reinvestigated using shock tube/laser absorption (ST/LA) spectroscopy and single-pulse shock tube (SPST) methods over the temperature range 1163-1831 K at pressures from 95 to 290 kPa. The present rate constants are 2-3 times smaller than those reported in previous single-pulse experiments performed at near high-pressure limit conditions. The recommended rate constant expression, k = 5.71 × 10(46)T(-9.341) exp(-47073 K/T) s(-1), was obtained over the temperature range 1000-1600 K with uncertainties of ±40% at temperatures below 1300 K and ±32% at 1600 K. The rate constants at the high-temperature region showed clear falloff behavior and were in good agreement with recent high-temperature experiments. The falloff rate constants could not be reproduced by a standard RRKM/master-equation model; this study provides additional evidence for the unusual pressure dependence previously reported for this reaction. Additionally, the rate constants for the decomposition of 1,1-difluoroethylene (CH2CF2) were determined over the temperature range 1650-2059 K at pressures of 100 and 205 kPa, and were reproduced by the RRKM/master-equation calculation with an average downward energy transfer of 900 cm(-1).

Hydrogen abstraction reactions from hydrofluorocarbons are important reaction steps in both atmospheric and combustion chemistry. In this study, kinetics of the H-abstraction reactions from methane, ethane, fluoromethanes, and fluoroethanes by H, O, and OH radicals are investigated using the CBS-QB3//ωB97X-D quantum chemical method and transition state theory, and the site-specific reactivity of the fluoroalkanes systematically examined. The calculated rate constants for these reactions are found to accurately reproduce available experimental data for a wide temperature range, with only minor differences between experimental and theoretical barrier heights. Analysis of site-specific reactivity indicates that fluorine substitution at the β-carbon site of the fluoroalkanes systematically increases the barrier heights of H-abstraction, and the substitution effect at the α-carbon site can be interpreted by electron-withdrawing and steric effects. Fluorine substitution also decreases the pre-exponential factor for the H-abstraction reactions by O and OH radicals due to steric and electrostatic interactions between these radicals and the F atoms.

The thermal decomposition of gaseous nitromethane and the subsequent bimolecular reaction between CH and NO have been experimentally studied using time-resolved cavity-enhanced absorption spectroscopy behind reflected shock waves in the temperature range 1336-1827 K and at a pressure of 100 kPa. Temporal evolution of NO was observed following the pyrolysis of nitromethane (diluted to 80-140 ppm in argon) by monitoring the absorption around 400 nm. The primary objectives of the current work were to evaluate the rate constant for the CH + NO reaction (k) and to examine the contribution of the roaming isomerization pathway in nitromethane decomposition. The resultant rate constant can be expressed as k = (9.3 ± 1.8) × 10(T/K) cm molecule s, which is in reasonable agreement with available literature data. The decomposition of nitromethane was found to predominantly proceed with the C-N bond fission process with the branching fraction of 0.97 ± 0.06. Therefore, the upper limit of the branching fraction for the roaming pathway was evaluated to be 0.09.

Motivated by recent shock tube studies on the thermal unimolecular decomposition of fluoroethanes, in which unusual trends have been reported for collisional energy-transfer parameters, the rate constants for the thermal decomposition of fluoroethane were investigated using a shock tube/laser absorption spectroscopy technique. The rate constants were measured behind reflected shock waves by monitoring the formation of HF by IR absorption at the R(1) transition in the fundamental vibrational band near 2476 nm using a distributed-feedback diode laser. The peak absorption cross sections of this absorption line have also been determined and parametrized using the Rautian-Sobel'man line shape function. The rate constant measurements covered a wide temperature range of 1018-1710 K at pressures from 100 to 290 kPa, and the derived rate constants were successfully reproduced by the master equation calculation with an average downward energy transfer, ⟨ΔEdown⟩, of 400 cm(-1).