Richard B. Timmons

ESR Study of the Kinetics of the Reaction of H Atoms with Methane

A wide‐temperature‐range study using ESR atom detection has been employed to measure the rate of the reaction H + CH4→H2 + CH3. Over the temperature range 426°–747°K, we obtain a specific rate constant for the above reaction of k1 = 6.9 ± 0.6 × 1013exp[(−11 800 ± 200) / RT] expressed in units of cubic centimeters per mole per second. The value obtained in the present work is compared to a number of other results obtained by various workers. The activation energy we observe is considerably higher than previous values obtained in this temperature range. However, our results coupled with the heat of the reaction predict an activation energy for the reverse reaction which agrees well with experimental values. In addition, the pre‐exponential factor we obtain agrees with absolute rate theory predictions as well as with entropy considerations. We do not agree with literature results which give very low pre‐exponential factors suggesting steric factors of the order of 10−3–10−5 for this reaction.

Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide

Absolute rate constants for the reactions of O(3P) atoms with dimethyl sulfide and episulfide have been measured over the temperature range 268 to 424 °K. The kinetic data were obtained using the flash photolysis–resonance fluorescence method. The O(3P)+CH3SCH3 reaction exhibited a negative temperature dependence and the Arrhenius expression obtained was k1= (1.42±0.07) ×10−11 exp(727±31/RT) cm3 molecule−1⋅sec−1. The episulfide reaction was essentially temperature independent, the corresponding Arrhenius expression being k2= (1.34±0.09) ×10−11 exp(−35±40/RT) cm3 molecule−1⋅sec−1. Use of a fast flow system with direct mass spectrometric analysis of reaction products provides evidence that the reaction mechanisms involved are O(3P)+CH3SCH3⇄[CH3SOCH3]*→CH3SO+CH3, O(3P)+CH2–SCH2⇄[CH2–SOCH2]*→CH2=CH2+SO. The very great difference in reaction rates and mechanisms between these sulfides and the corresponding oxygen analogs CH3OCH3 and ethylene oxide is discussed.

Kinetics of the Reaction of O(3P) Atoms with Benzene

The kinetics of the gas phase reaction of O(3P) atoms with benzene was investigated using fast flow techniques. Specific rate constants were obtained using two separate systems, one with ESR detection to monitor the decrease in O(3P) atoms and the other with mass spectrometry to follow the change in benzene concentration. The rate constant computed with respect to O(3P) removal is much larger than that obtained in the mass spectrometer studies and this is believed to result from further reactions of oxygen atoms with radical species produced in the initial O(3P)+C6H6 interaction. On the basis of the mass spectrometric studies a specific rate constant for the reaction O(3P)+C6H6→ [C6H6O] of (3.8± 1.5) × 1013 exp(−4,400± 500/RT) cm3 mole−1· sec−1 was obtained over the temperature range of 255–305°K. A negligibly small kinetic isotope effect was observed in competitive experiments with C6H6 and C6D6. The results obtained are interpreted in terms of a rate determining step involving the addition of oxygen ato...

Absolute rate of the reaction of O(3P) with hydrogen sulfide over the temperature range 263 to 495 K

The technique of flash photolysis coupled with time resolved detection of O via resonance fluorescence has been used to obtain rate constants for the reaction of O(3P) with H2S at temperatures from 263 to 495 K and at pressures in the range 10–400 torr. Under conditions where secondary reactions are avoided, the measured rate constants for the primary step obey the Arrhenius equation k= (7.24±1.07) ×10−12 exp(−3300±100/1.987 T) cm3 molecule−1 sec−1. The results are discussed and comparisons are made with previous work and theoretical predictions. Experiments with D2S show that the reaction exhibits a primary isotope effect, in support of a hydrogen abstraction mechanism.

THE PHOTOCHEMICALLY INDUCED REACTIONS OF SULFUR DIOXIDE WITH ALKANES AND CARBON MONOXIDE

Abstract— The gas phase photochemical reactions of SO2 induced by 3130 Å radiation have been studied in the presence of added alkanes or added CO. The quantum yields obtained in the reactions with the low molecular weight alkanes employed are lower than those obtained by previous workers. The quantum yields were found to be pressure dependent increasing slowly with increasing pressure. A stoichiometric ratio of one SO2 removed per molecule of hydrocarbon consumed was observed only under experimental conditions of [SO2] < [RH]. For reaction mixtures where [SO2] < [RH] the ratio of [SO2]/[RH] reacted always exceeded unity. The quantum yields decreased slightly with increasing temperature. In all the alkane reaction systems studied, the deposition of viscous, nonvolatile reaction products was observed. In the experiments with added CO, the quantum yields were computed with respect to the rate of CO2 formation. At 25°C and equal pressures of SO2 and CO, φco2 was observed to be 0.005 and it decreased slightly with increasing temperature. The results obtained are interpreted in terms of the sulfoxidation of the alkanes and the oxidation of CO proceeding by way of a 3SO2 reaction intermediate.

ESR Study of the Kinetics of the Reactions of D Atoms and O Atoms with NH3

A wide temperature‐range study using ESR atom detection has beem employed to measure the rates of the reactions of D atoms and O atoms with NH3. In the case of the deuterium reaction, evidence is presented which shows that the interaction with NH3 proceeds via an exchange process of the type: D+NH3→NH3D+H. This reaction was studied over the temperature range of 423–741°K. The specific rate constant for this exchange reaction was found to be 1.9 ± 0.2 × 1013exp(− 10 100 ± 150 / RT) in units of cubic centimeters mole−1·second−1. The reaction of O atoms with NH3 was studied over the temperature interval of 361–677°K. The specific rate constant obtained for the reaction O+NH3→OH+NH2 was found to be 4.0 ± 0.9 × 1012exp(− 6600 ± 130 / RT) in units of cubic centimeters mole−1·second−1. This Arrhenius pre‐exponential factor depends on a stoichiometric factor of 3.50 atoms per NH3 molecule assumed from other work.

Absolute rate parameters for the reaction of atomic hydrogen with carbonyl sulfide and ethylene episulfide

Absolute rate constants for the reaction of atomic hydrogen with carbonyl sulfide H+OCS→SH+CO (1) and ethylene episulfide H+C2H4S→SH+C2H4 (2) have been determined using the flash photolysis–resonance fluorescence technique. Under conditions where secondary reactions are avoided, rate constants for the carbonyl sulfide reaction over the temperature range 261 to 500 K gave the Arrhenius expression k1= (9.06±1.53) ×10−12 exp(−3850±110/1.987T) cm3 molecule−1 s−1. The corresponding Arrhenius expression for the ethylene episulfide reaction over the temperature range 223 to 423 K is k2= (2.87±0.12) ×10−11 exp(−1880±24/1.987T) cm3 molecule−1 s−1. These results are discussed, comparisons made with previous work, and implications considered for a model of the sulfuric acid aerosol clouds on the planet Venus.

The kinetics and mechanisms of the reactions of O(3P) atoms with CH3CN and CF3CN

Absolute rate constants for the reactions of O(3P) atoms with CH3CN and CF3CN were measured using the flash photolysis–resonance fluorescence method under conditions which minimized complications from secondary reactions. These reactions were studied over the temperature range of 383–500 K. The Arrhenius expression obtained for O(3P)+CH3CN was (7.27±1.75) ×10−13 exp(−4770±200/RT) cm3 molecule−1⋅sec−1, while that for the O(3P)+CF3CN reaction was (14.2±5.2) ×10−13 exp(−6130±310/RT) cm3 molecule−1⋅sec−1. Mechanistic information concerning these reactions was obtained from kinetic isotope effect studies using CD3CN in place of CH3CN and from fast flow tube studies of both the CH3CN and CF3CN reactions using direct mass spectrometric and ESR detection methods. The lack of an isotope effect in the CD3CN/CH3CN experiments coupled with the reaction products identified mass spectrometrically leads to the conclusion that the major reaction channel in both systems involves displacement of CH3 and CF3 by O(3P). In ad...