Formation of deuterium and hydrogen atoms in the pyrolysis of benzene-d6 and chlorobenzene behind shock waves

Abstract

provided that the products of the photoprocess are not strongly absorbing relative to the reactant, and provided that the total absorbance of all the nonreactive components S of the sample is not large. It is not surprising that the approximate equation works poorly in the cases where the products absorb strongly, since the approximation cannot account properly for the buildup of products that occurs near the surface of the sample closest to the light source. Owing to this concentration gradient, the products in a solid-phase photoreaction screen light more effectively from the reactant than in a well-stirred photoreaction. This leads to an overestimation of the rate of reactant photon absorption calculated from our approximate equation, which in turn causes an underestimation of the quantum yield. ConclusionWe have obtained the closed-form solution to the full kinetic equation describing a unimolecular, single-photon process conducted in a well-stirred medium, without assuming very high or very low absorbance and without assuming low conversion. The relatively simple equation obtained can be used to calculate the quantum yield for a photoprocess occurring in a system with any fractional absorption over the full range of 0-1. An approximation to this full kinetic expression has also been obtained. This approximate equation is sufficiently simple that it can be solved by hand and is better than 3% accurate over a wide range of conditions. Our results suggest that quantum yield calculations can be accurately and simply performed for a unimolecular photoprocess occurring in a stirred medium without restricting measurements to perfectly absorbing samples or to optically thin samples, and without limiting photolysis to very low conversion.We have also shown that our simple approximate equation is accurate over a relatively wide range of conditions for calculating quantum yields of photoprocesses conducted in solid media. This is important, since the kinetic equations for a solid-state photoprocess must otherwise be solved numerically by a lengthy iterative procedure, treating the quantum yield as an adjustable parameter. Acknowledgment. R.L.J. thanks Mr. John Wilkalis for help in formating and editing some of the computer codes used in this work.Dilute mixtures (3-20 ppm) of C6D6 (benZene-d6) were pyrolyzed behind reflected shock waves at temperatures of 1630-1940 K and total pressures of 2-3 atm. Progress of the reaction was followed by analysis for D atoms using resonance absorption spectroscopy. Appearance of D atoms was a first-order process with respect to benzene concentration, and with respect to time during the first part of each experiment. An Arrhenius equation for the formation of D atoms, based on 34 experiments, is kD = 9.7 X 10l2 exp(-87100 cal/RT) s-' with an estimated uncertainty of a factor of 1.5. From measurements of H atoms during pyrolysis of chlorobenzene under similar conditions at 1570-1790 K, the first-order rate constant for the dissociation of chlorobenzene to chlorine atoms and phenyl radic...

Kinetics and mechanism for the H‐for‐X exchange process in the H + C6H5X reactions: A computational study

“…The higher T values of Rao and Skinner [38] (T = 1200-1360 K, P = 2400 Torr) are slightly lower (within a factor of 2) than our theoretical prediction. The latter experimental rate constants were deduced from H and D concentration profiles.…”

“…[5]; (×) k 9 /6 from Ref. [38]; (solid line) k 12b (HPL); (dashed line) k TOT 12b (100 Torr); (dash-dotted line) k 12b (100 Torr). action at lower P (100 Torr) and T (612-832 K).…”

Kinetic modeling of benzene decomposition near 1000 K: The effects of toluene impurity

“…The removal of H by H ϩ C 6 H 5 CH 2 (5), negatively affects the yields of H 2 . For CH 4 , as revealed by Figure 6(b), reactions (8) and (11) have strong positive effects, whereas reactions (5) and (10) negatively affect the yield of CH 4 as can be expected.…”

“…Their estimated value is given in Table I and used in our modeling. The rate constant for reaction (8), which is more important than reaction (9), was obtained by adjusting k 8 to match the experimental and predicted concentrations of H 2 given in Figures 1 and 2 of Ref.…”

The decomposition of aromatic hydrocarbons during coal pyrolysis in hydrogen plasma: A density functional theory study

“…TS1 is the transition state of reaction (2), with activation energy barrier of 9.8 kcal/mol. Reaction (3) is a CeH bond breaking process [26,28,29], and the energy input required for this reaction is 110.2 kcal/mol. The energy required for breaking the CeH bond of benzene has been calculated to be 115.2 kcal/mol at the G2M level [30] and reported to be 112 AE 0.6 kcal/mol experimentally [31].…”