OH Decay Research Articles

Comprehensive calculations on the “variable size” spur diffusion model of aqueous radiolysis

Spur diffusion model calculations had been done by us earlier incorporating the assumption that the size of a spur for a given number of dissociations varied in a Gaussian manner, obtaining agreement not only with experimental e - aq and OH decay results in pico- and nanosecond time scales, but also with the variation of molecular product yields with increasing scavenger concentration. However, in these calculations only a single spur containing 6 dissociations was considered. In the present work an entire range of spurs, from a single dissociation spur to a 30 dissociation spur have been taken into account, the final result being obtained by calculating a probability weighted average. The computed results are still in agreement with experimental results, suggesting that our new approach closely models the experimental system.

Temperature Dependence of the Reaction OH + OH → H2O + O

AbstractRate constants for the disproportionation reaction (1) OH + OH → H2O + O in the range 250–580 K have been measured by flash photolyzing H2O/N2 mixtures and monitoring the decay of OH using quantitative UV resonance spectrometry. The result k1 = (3.2 · 0.8) 10−12 exp (‐242/T) cm3/molecule · s, does not correlate with existing high temperature data and confirms previously suggested non‐Arrhenius behaviour for this reaction. On the basis of all available data the temperature variation of k1 over the range 250–2000 K is best represented by the empirical expression k1(T) = exp (‐27.73 + 1.49 · 10−3T) cm3/molecule · s. The extent of the Arrhenius graph curvature can be reconciled by a “close‐collision” model incorporating the energy variation of accessible product states.

OH radical nonequilibrium in methane-air flat flames

The OH radical concentration and rotational temperature were measured in atmospheric pressure methane-air flat flames using absorption spectra obtained with a tunable dye laser. The OH radical nonequilibrium and the associated recombination in the post flame gases were observed for flue-air equivalence ratios of .8, .9, 1.0, 1.1, 1.2. Predicted peak OH concentrations from one-dimensional laminar flame models 1–3 were found to be in good agreement with those observed in the experiment. Both the measured and laminar flame model 4 predicted OH decay profiles were consistent with a second order dependence of the decay on the OH concentration, i.e., d [OH]/ dt =−α[OH] 2 [M]. For the fuel-rich flames this second order dependence can be derived analytically from the recombination chemistry and partial exquilibrium arguments 5 . An integration of the chemical rate equations 6 governing the chemistry in the recombination dominated post flame region of the fuel-rich flames also leads to this second order dependence. For the fuel-lean flames, the integration of the chemical rate equations yields OH decay profiles which are not second order in [OH]. For these flames, it appears that diffusional transport effects are responsible for the observed second order behavior of the OH decay profiles. The decay rate constant, α, characterizing the measured OH decay was found to remain relatively constant for the lean and stoichiometric flames and to increase substantially for fuel-rich conditions. This result agrees, with the laminar flame models, and is consistent with the integration of the chemical rate equations and an analytic derivation for the fuel-rich flames. Values of the decay rate constant, α, measured in this experiment are comparable to those obtained from other experimental observations 7 at similar temperatures. Decay rate constants obtained at somewhat lower temperature 8 are two orders of magnitude larger than those found in the present work. This increase in the decay rate constant cannot be attributed to the temperature dependence of the recombination rate constants. Recent measurements 9 duplicating the conditions of ref. 8 indicate that the (OH) is a factor of 100 larger than was reported. This error accounts for the unexpectedly large decay rate constants which were reported.

Theoretical prediction of hydrated electron behaviour in pico-second time scales in the presence of scavengers in aqueous radiolysis

The present work involves the prediction of the variation of [e - aq] and [OH] in the subnanosecond and nanosecond time scales in aqueous solutions containing high concentrations of ethanol and OH -. In our earlier paper, spur diffusion model calculations had been done assuming that the size of a spur for a given number of dissociations varies in a Gaussian manner, obtaining excellent agreement with experimental e - aq and OH decay in pico and nanosecond time scales and also predicting accurately the variation of molecular product yield with increasing scavenger concentration. The same model has been used in the present work. Comparison with experimental data, where available, gives good agreement.

Sensitivity analysis of a model for the radical recombination region of hydrocarbon-air flames

The concentration of radicals such as OH, O, and H above a flame are determined in part by three body recombination reactions. These radicals are responsible for CO oxidation and also NO production via the Zeldovich mechanism. The chemistry of the recombination region was modeled on a computer using a mechanism with nine species and nineteen reactions. A sensitivity analysis of the model was carried out to determine the effects of errors in rate constants, diffusion coefficients, and initial concentrations on the model predictions. Similar results were obtained using both the FAST (Fourier Amplitude Sensitivity Test) algorithm developed by Cukier and coworkers and a simpler brute force method. The sensitivity was large for only a few parameters. In particular, for fuel lean flames, the OH decay rate is mainly sensitive to the rate constant for H+O2+M→HO2+M. The hydroxyl concentration profiles above atmospheric, premixed, laminar methane-air flames were measured using laser absorption spectroscopy. Measurements were made for both fuel lean and stoichiometric flames. Predictions from the computer model were in reasonable agreement with the experimental profiles. The rate constant for the H+O2+H2O→HO2+H2O reaction which gives the best fit between the experiment and the model is 8±3×10−32 cm6/molecule2 at 1500 K. This is larger than that derived from earlier flame measurements, but is in reasonable agreement with shock tube results and the theoretical predictions of R. J. Blint. The model shows that the hydrogen-oxygen system stays in partial equilibrium throughout, most of the recombination zone. However, CO and CO2 do not stay in partial equilibrium.

Pulse Radiolysis of Monophenyl Phosphate in Aqueous Solution

Pulse radiolysis of aqueous monophenyl phosphate was carried out at pH values between 0.3 and 13.5. The rate constants with the primary products of water radiolysis were measured, and kinetic absorption spectroscopy was used to investigate the spectra and reactions of the transients. The primary reactions and the spectroscopic characteristics of the radicals produced by the attack of H and OH are mainly determined by the aromatic moiety of the phenyl phosphate molecule. The subsequent reactions, however, are markedly influenced by the phosphate group. The rate of bimolecular decay of OH- and H-adducts is exceptionally low at pH > 6; in neutral and alkaline solutions, no unimolecular elimination of either H2O or inorganic phosphate from adducts occurs on the time scale of pulse radiolysis measurements. Rapid elimination processes are only possible from OH-adducts in acid medium (pH ( 3). Under these conditions, phenoxyl radicals and a previously unreported transient absorption with Xmax = 420 nm and first-order decay are observed. The latter is tentatively assigned to the phenyl phosphate radical formed from OH-adducts by H20 elimination.

A Combined Flash Photolysis/Shock‐Tube Study of the Absolute Rate Constants for Reactions of the Hydroxyl Radical with CH4 and CF3H around 1300 K

AbstractA combined flash photolysis/shock‐tube technique for the direct investigation of the rates of bimolecular reactions of the OH radical at temperatures between 1000‐1500 K is described. In this technique shock heated mixtures of approximately 0.5% of H2O in Ar and containing small amounts of a selected reactant, are subjected to flash photolysis as an instantaneous source of OH. The shock wave only serves to heat the gas mixture; thermal decomposition does not occur. Formation and subsequent decay of OH are monitored by time resolved UV resonance absorption. – First results with this technique have been obtained for the reactions (1) OH + CH4 r̊ CH3 + H2O and (2) OH + CF3H r̊ CF3 + H2O. Rate constants were found to be k1 (1300 K) = (2.5 ± 0.8) 1012 and k2 (1350 K) = (4.0 ± 1.0) 1011 cm3/mol · s. The result for reaction (1) is shown to confirm previously suggested non‐Arrhenius behaviour for this reaction, and in combination with low temperature data yields k1(T) = 106.19 (T/K)2.13 exp(‐1234 K/T) cm3/mol · s. The dependence of k1, on temperature is described in terms of transition state theory.

Yield and Decay of the Hydroxyl Radical from 200ps to 3ns

The yield of OH at 200 ps and the decay of OH from 200 ps to 3 ns have been measured. The yield of OH was 5.9 +- 0.2 mol/100 eV (error is statistical, systematic error could be larger). The OH radical decays to 0.73 +- 0.05 of its initial value from 200 ps to 3 ns. The implications of these results are compared with theoretical calculations.

Determination of Rate Constants for the Reactions of OH with Propylene and Ethylene by a Pulsed Photolysis‐Resonance Fluorescence Method

AbstractAbsolute rate constants for the reactions of OH radicals with CH3CHCH2, CD3CHCH2, CH3CDCD2, CD3CDCD2, and CH2CH2 were determined at 298°K. Hydroxyl radicals were generated in the pulsed vacuum‐uv photolysis of H2O. Decays of OH were monitored by resonance fluorescence in an excess of hydrocarbons. The rate constants for the reactions with propylene and ethylene were determined to be 1.45 (±0.22)·10−11 and 3 (±1)·10−12 cm3 molecule−1 sec−1, respectively. The kinetic isotope effect for deuterated propylene was found to be small, thus supporting the proposed addition mechanism.

Rate Measurements on OH + NO + M and OH + NO2 + M

Measurements of the termolecular rate constants for OH+NO+M→ NHO2+M(k1) and OH+NO2+M→ HNO3+M(k2) in a fast flow reactor with ESR detection of OH decay are reported. The method was to add large excesses of either NO or NO2 to OH generated by the reaction H+NO2→ NO+OH so that pseudo-first-order OH decay resulted. Experiments over a pressure range of 0.5–5 torr for M = He gave the results k1 = 4.7, 2.9, and 1.3×1017, and k2=7.3, 5.7, and 2.1× 1017 cm6 mole−2 · sec−1 at 273, 298, and 395°K, respectively. Data at 298°K for M = Ar were about a factor of 2 lower for both reactions.

Reactions of OH Radicals in the H–NO2 and H–NO2–CO Systems

A mass-spectrometric study has been made of the stoichiometry of the reactions of OH radicals generated by the reaction H+NO2→OH+NO, under conditions similar to those used in some previous studies of the kinetics of the reactions of OH radicals with each other and with CO. Contrary to previous assumptions, when H atoms are initially in excess over NO2, the reactions 2OH→H2O+O, O+OH→O2+H, OH+CO→CO2+H, do not account completely for the decay of OH. An additional reaction occurs which produces water and is probably the first-order heterogeneous reaction recently postulated on purely kinetic evidence by Breen and Glass [J. Chem. Phys. 52, 1082 (1970)]. When NO2 is initially in excess over H, a fast reaction takes place involving OH radicals and NO2 {as first observed by Wilson and O'Donovan [J. Chem. Phys. 47, 5455, (1967)]}. This is probably the reaction OH+NO2→HNO3.

Kinetics of OH Radicals as Determined by Their Absorption Spectrum I. The Electric Discharge Through Water Vapor

(1) The order of the reaction causing the rapid disappearance of OH radicals after interrupting an electric discharge through H2O vapor is measured by photometry of absorption spectra taken in snapshots. (2) These spectra are taken in time intervals of a few tenths of a second with a 21-foot grating and rotating interrupter and sector. (3) Relative concentrations of OH radicals are derived by comparing intensities of rotational lines within one branch of the 0→0 band of OH, the relative intensities being given by the theory of Hill and Van Vleck. (4) The decay of OH with time for various H2O pressures and addition of He is given in diagrams and a table of rate constants. (5) A discussion of the kinetics leads to triple collisions consuming OH. (6) KCl covering the surface of the absorption tube materially increases the rate of the reaction. (7) The absorption spectrum of H2O2 investigated simultaneously showed that no appreciable H2O2 is formed in the reaction. (8) In the processes consuming OH radicals after interrupting the discharge through H2O vapor in clean glass tubes, the triple collision H+OH+M→H2O+M plays a predominant part.