Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

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Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

A detailed chemical kinetic model for ethanol oxidation has been developed and validated against a variety of experimental data sets. Laminar flame speed data (obtained from a constant volume bomb and counterflow twin-flame), ignition delay data behind a reflected shock wave, and ethanol oxidation product profiles from a jet-stirred and turbulent flow reactor were used in this computational study. Good agreement was found in modeling of the data sets obtained from the five different experimental systems. The computational results show that high temperature ethanol oxidation exhibits strong sensitivity to the fall-off kinetics of ethanol decomposition, branching ratio selection for C2H5OH + OH Products, and reactions involving the hydroperoxyl (HO2) radical.



The multichanneled ethanol decomposition process is analyzed by RRKM/Master Equation theory, and the results are compared with those obtained from earlier studies. The ten-parameter Troe form is used to define the C2H5OH(+M) CH3 + CH2OH(+M) rate expression as 


k∞ = 5.94E23 T−1.68 exp(−45880 K/T) (s−1)




ko = 2.88E85 T−18.9 exp(−55317 K/T) (cm3/mol/sec)




Fcent = 0.5 exp(−T/200 K) + 0.5 exp(−T/890 K) + exp(−4600 K/T)



and the C2H5OH(+M) C2H4 + H2O(+M) rate expression as 


k∞ = 2.79E13 T0.09 exp(−33284 K/T) (s−1)




ko = 2.57E83 T−18.85 exp(−43509 K/T) (cm3/mol/sec)




F cent = 0.3 exp(−T/350 K) + 0.7 exp(−T/800 K) + exp(−3800 K/T)



with an applied energy transfer per collision value of = 500 cm−1.



An empirical branching ratio estimation procedure is presented which determines the temperature dependent branching ratios of the three distinct sites of hydrogen abstraction from ethanol. The calculated branching ratios for C2H5OH + OH, C2H5OH + O, C2H5OH + H, and C2H5OH + CH3 are compared to experimental data. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 183–220, 1999

A detailed chemical kinetic model for high temperature ethanol oxidation

Chemistry of Acyl Radicals

Kinetics of elementary reactions in low-temperature autoignition chemistry

This chapter is a critical survey of reaction rate coefficient data important in describing high-temperature combustion of H2, CO, and small hydrocarbons up to C4. A recommended reaction mechanism and rate coefficient set is presented. The approximate temperature range for this mechanism is from 1200 to 2500 K, which therefore excludes detailed consideration of cool flames, low-temperature ignition, or reactions of organic peroxides or peroxy radicals. Low-temperature rate-coefficient data are presented, however, when they contribute to defining or understanding high-temperature rate coefficients. Because our current knowledge of reaction kinetics is incomplete, this mechanism is inadequate for very fuel-rich conditions (see Warnatz et al., 1982). For the most part, reactions are considered only when their rates may be important for modeling combustion processes. This criterion eliminates considering many reactions among minor species present at concentrations so low that reactions of these species cannot play an essential part in combustion processes. The philosophy in evaluating the rate-coefficient data was to be selective rather than exhaustive: Recent results obtained with experimental methods capable of measuring isolated elementary reaction rate parameters directly were preferred, while results obtained using computer simulations of complex reacting systems were considered only when sensitivity to a particular elementary reaction was demonstrated or when direct measurements are not available. Theoretical results were not considered.

Rate Coefficients in the C/H/O System

The reactions of alkyl radicals (R = CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, and t-C{sub 4}H{sub 9}) with HBr have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry. R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetics of reverse reactions, Br + RH (R = C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, n-C{sub 4}H{sub 10}, i-C{sub 4}H{sub 10}), have also been investigated using laser flash photolysis/resonance fluorescence methods. These results, combined with previously obtained kinetic information, were used in second- and third-law thermochemical calculations to obtain accurate determinations of the heats of formation of the C{sub 2}-C{sub 4} alkyl radicals involved. Second- and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol{sup {minus}1}). The heats of formation of the radicals thus obtained are in excellent agreement with values obtained from studies of dissociation/association equilibria, within 2.6 kJ mol{sup {minus}1}. Recommended alkyl-radical heats of formation (with uncertainties) at 298 K are provided that are based on an assessment of all the results of the current study and a review of other recent determination (kJ mol{sup {minus}1}): C{sub 2}H{sub 5},more » 121.0 {+-} 1.5; i-C{sub 3}H{sub 7}, 90.0 {+-} 1.7; sec-C{sub 4}H{sub 9}, 67.5 {+-}2.2; t-C{sub 4}H{sub 9}, 51.3 {+-} 1.8. Accurate determination of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol{sup {minus}1}): primary C-H in C{sub 2}H{sub 6} (422.8 {+-} 1.5); secondary C-H in C{sub 3}H{sub 8} (412.7 {+-} 1.7) and n-C{sub 4}H{sub 10} (411.1 {+-} 2.2); tertiary C-H in i-C{sub 4}H{sub 10} (403.5 {+-} 1.8). 44 refs., 4 figs., 8 tabs.« less

Kinetics and thermochemistry of R + hydrogen bromide .dblarw. RH + bromine atom reactions: determinations of the heat of formation of ethyl, isopropyl, sec-butyl and tert-butyl radicals

Laser flash photolysis measurements on the kinetics of the title system in a He bath gas are reported for the temperatures 290, 473, and 700 K and a pressure range of 7.6−678 Torr. CH3 and OH radicals were generated simultaneously by 193 nm photolysis of acetone containing traces of water, with CH3 in large excess. The course of reaction was followed by monitoring the concentration of OH using laser-induced fluorescence (LIF), while CH3 was monitored by absorption at 216.4 nm. The experimental data were analyzed in the first instance by a simple model based on the decay of OH and CH3, which was verified by more comprehensive numerical simulations. Further analysis of the data, using a combined master equation/inverse Laplace transform/RRKM procedure, yielded estimates of the association rate coefficient of CH3 and OH ( = (8.0 ± 0.3) × 10-11 (T/300 K-0.79±0.09 cm3 molecule-1 s-1) and for the enthalpy change for the CH3 + OH → 1CH2 + H2O channel (Δ = 1.6 ± 2.0 kJ mol-1). Pressure-dependent rate coefficients...

Temperature and Pressure Dependence of the Multichannel Rate Coefficients for the CH3 + OH System

A method is presented for calculating the time-dependent G values and product yields formed in a fast electron track. The calculations are based on an efficient stochastic simulation technique--the independent reaction times (IRT) method--which has previously been developed and validated in detail. The IRT method combines the reactants in pairs and selects reaction times randomly from the appropriate marginal reaction time distributions. Reactions are counted sequentially, starting with the shortest reaction time and limiting subsequent consideration to pairs in the set of remaining particles. The technique is efficient and accurate and is capable of dealing with ionic species, partially diffusion-controlled reactions, and reactive products. Our previous calculations based on the IRT method have considered only reactions of small numbers of particles in isolated spurs. A new simulation protocol is presented that efficiently handles the extended distribution and the large number of reactive particles encountered in a radiation track while making use of any isolation that exists on appropriate timescales. The simulated G values at zero time and 0.1 {mu}s for a model 22-MeV electron track differ significantly from experimental values, and the discrepancy is ascribed to deficiencies in the initial distribution.

M. J. Pilling

Papers

Kinetics and thermochemistry of R + hydrogen bromide .dblarw. RH + bromine atom reactions: determinations of the heat of formation of ethyl, isopropyl, sec-butyl and tert-butyl radicals

Temperature and Pressure Dependence of the Multichannel Rate Coefficients for the CH3 + OH System

Stochastic modeling of fast kinetics in a radiation track

Fitting of pressure-dependent kinetic rate data by master equation/inverse Laplace transform analysis

The testing of models for unimolecular decomposition via inverse Laplace transformation of experimental recombination rate data