In Situ Radiolysis Time-Resolved ESR Studies of Spin Trapping by

Previous determinations of spin trapping efficiency6-8 have been performed under steady-state ... using a 100 mA, 0.5 μs pulsed electron beam of 25 o...
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J. Phys. Chem. 1996, 100, 7511-7516

7511

In Situ Radiolysis Time-Resolved ESR Studies of Spin Trapping by DMPO: Reevaluation of Hydroxyl Radical and Hydrated Electron Trapping Rates and Spin Adduct Yields Keith P. Madden* and Hitoshi Taniguchi Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: NoVember 17, 1995; In Final Form: February 12, 1996X

The second-order rate constants for the reaction of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with radiolytically produced hydroxyl radicals and hydrated electrons have been measured in aqueous solution by direct observation of spin adduct initial yield using time-resolved electron spin resonance. The rate constants are 2.8 × 109 mol-1 dm3 s-1 for the DMPO-hydroxyl radical addition reaction and 3.2 × 109 mol-1 dm3 s-1 for the reaction of DMPO and hydrated electron, using sodium formate and chloroacetic acid as competitive scavengers of the hydroxyl radical and hydrated electron, respectively. The hydrated electron-DMPO competition study determined the fraction of DMPO-H produced directly from radiolytically produced hydrogen atoms as 0.082 of the total DMPO-H yield, indicating that approximately half of the hydrogen atoms react with DMPO to produce non-aminoxyl products. The fraction of the total hydroxyl radical yield leading to DMPO-OH spin adduct was determined to be 0.94, using the bleach of 2,2,6,6-tetramethylpiperidone-N-oxyl by carbon dioxide radical anion as a reference standard.

Introduction trapping1-3

is the conversion of a population of reactive, Spin transient radicals to persistent spin adduct radicals by reacting the transients with an unsaturated, diamagnetic scavenger. This conversion is advantageous in the cases where the transient radical is either too short-lived to be observed by conventional spectroscopic techniques, has weak and/or inaccessible optical transitions, or has a weak, broad ESR (electron spin resonance; equivalently EPR, electron paramagnetic resonance) absorption spectrum. Typically, the spin trap is a nitroso compound or a nitrone, so that the spin adduct formed is an aminoxyl (nitroxide) radical. This persistent species can then be examined using conventional ESR spectrophotometry, chromatography, or mass spectrometry to determine the identity of the reactive parent radical. Usually, qualitative information is desired concerning the presence of a given parent radical and some relative indication of its concentration. Quantitative spin trapping (the measurement of the parent radical concentrations), however, requires knowledge of four aspects of the system being investigated: the rate constant and order of parent radical termination reactions, the rate constant for the reaction of the parent radical with the spin trap, the order and rate constants for spin adduct termination, and the spin trapping efficiency. The spin trapping efficiency is the fraction of parent radicals reacting with the spin trap to produce the aminoxyl spin adduct. This may be less than unity if the parent radical can react with the spin trap by abstracting labile hydrogens or by adding to the unsaturated portion of the trap such that a persistent spin adduct is not formed. The performance of a given spin trap with respect to the above characteristics has been difficult to assess previously, since aminoxyl spin adducts typically possess low extinction coefficients in the UV-vis range, making them resistant to the conventional spectrophotometric techniques employed in the study of chemical kinetics. Radiation chemical methods and time-resolved ESR (TRESR), however, provide an ideal approach for testing the performance of spin-trapping systems. A large number of parent radicals can be produced X

Abstract published in AdVance ACS Abstracts, April 1, 1996.

S0022-3654(95)03382-X CCC: $12.00

quickly and quantitatively using radiation chemical methods, while TRESR provides the spectral resolution and speed to measure the kinetics of many trapping reactions directly. Recent studies in these laboratories4,5 have elucidated the electronic and steric effects influencing the reaction of parent radicals with the nitroso spin trap 2-methyl-2-nitrosopropane (MNP) in aqueous solution. We are extending these studies to nitrone traps, starting with 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The design of these experiments requires knowledge of the rate constants for spin trap reaction with the hydroxyl radicals and hydrated electrons formed via water radiolysis; this allows the concentration of the spin trap and the parent compound to be set appropriately. Perusal of the available literature showed a wide variation in the observed rate constants for the reaction of the water transients with the nitrone spin trap DMPO. Preliminary experiments showed the literature rate constants for the reaction of DMPO with hydrated electron were inconsistent with ESR spectra recorded under steady-state conditions, motivating our reevaluation of the rate constants for the reaction of DMPO with hydroxyl radical and hydrated electron. These species, with the hydrogen atom, comprise the radical products from the homolysis and heterolysis of water and represent the strongest redox agents that can exist stably in aqueous solution. Therefore, the rate constants for their reaction with DMPO are of fundamental importance for the spin trapping of radicals produced by various methods in aqueous media. In the course of determining the rate constants, we discovered that the EPR data could also be used for quantitative measurement of the spin adduct (aminoxyl radical) initial yield in the reaction of DMPO and hydroxyl radical. Previous determinations of spin trapping efficiency6-8 have been performed under steady-state radiolysis or photolysis conditions, respectively, and therefore reflect not only aminoxyl production efficiency but also spin adduct stability. The measurement presented here determines spin trapping efficiency immediately after the formation of the spin adduct, separating the effect of adduct decay from that of trapping efficiency. The design of improved spin traps will involve optimizing both reactivity and efficiency of aminoxyl production in reactions with parent radicals, as well © 1996 American Chemical Society

7512 J. Phys. Chem., Vol. 100, No. 18, 1996

Madden and Taniguchi

as good spin adduct stability. The use of pulse techniques permits resolution of these three aspects of spin trap performance. Experimental Section In general, experimental procedures and techniques were the same as in preceding studies.4,5 Flowing, cooled (10-15 °C) aqueous solutions were irradiated within the microwave cavity of the ESR spectrometer with 2.8 MeV electrons from a Van de Graaff accelerator.9,10 ESR experiments measuring steadystate populations of radicals were carried out using a 6 µA dc electron beam. Kinetic experiments were performed using a 100 mA, 0.5 µs pulsed electron beam of 25 or 100 Hz repetition rate. The instantaneous electron beam intensity was monitored continuously using the collected beam current from the ESR cell. X-band (9.2 GHz) ESR experiments were performed using the apparatus described previously.11 All ESR experiments were performed at microwave powers well below saturation. Magnetic field measurements were by NMR methods and were measured as offsets from the central feature of the irradiated quartz flat cell, g ) 2.000 43. This field position was verified daily by calibration against the sulfite radical anion, g ) 2.003 16.12 ESR spectra and kinetic curves were analyzed using the Levenberg-Marquardt curve-fitting subroutines of the computer program Origin (MicroCal Software, Northampton, MA). Steady-state populations of radiolytically induced radicals were detected using field modulation at 100 kHz and 200 Hz, yielding a second-derivative presentation of the ESR spectrum. Time-resolved kinetic curves (13) were recorded at field positions corresponding to the line positions measured in the steady-state ESR spectra. ESR absorption spectra of transient radical populations present at a given postirradiation time were obtained by recording a family of ESR kinetic curves over the desired magnetic field range, while stepping the magnetic field uniformly for each kinetic curve. Plotting the average value of the kinetic curve during the time interval of interest as a function of magnetic field produced the ESR absorption spectrum. ESR absorption spectra of persistent 2,2,6,6-tetramethyl-4piperidone-N-oxyl (tempone) radicals were recorded using a similar approach. However, instead of using electron irradiation at the start of the kinetic curve recording interval to produce free radicals, and thereby measure their ESR absorption, the magnetic field was quickly stepped from the absorption line of the stable free radical to a base line magnetic field position, producing a bleach of the ESR absorption. A four-step, fourtrace data acquisition cycle was implemented to faithfully record the ESR bleach signal. In trace 1, the magnetic field is stepped to lower field off-resonance. Trace 2 is a base line trace, identical to trace 1, but without the field step. Trace 3 is similar to trace 1, with a field step of identical magnitude but opposite sense. Trace 4 is another base line curve, identical to trace 2. The base line traces were subtracted from the field-stepped traces, and the entire cycle was repeated 1024 times, summing the result. Dividing the accumulated trace by the number of field-stepped traces recorded yields a base line corrected microwave absorption bleach trace, which attains a steady plateau level within 250-400 µs after the field step. The plateau level represents the ESR absorption signal at the zero-shift magnetic field position. By evenly sampling the tempone line’s magnetic field domain, we could plot the plateau value as a function of field to create the tempone absorption spectrum. Solutions were freshly made using reagent-grade water from a Millipore Milli-Q water system. Ultrahigh-purity DMPO was

Figure 1. Steady-state X-band (9.2 GHz) second-derivative ESR spectrum of neutral aqueous solutions containing 2 mM DMPO: top trace, solution deoxygenated with nitrogen gas; lower trace, solution deoxygenated with nitrous oxide.

supplied by the OMRF Spin Trap Source (Oklahoma City, OK), stored at dry ice temperatures, and used as received. Chloroacetic acid (99+%, Aldrich), sodium formate (99%, Aldrich), and tempone (Eastman Kodak) were used without further purification. Solution pH was adjusted using KOH or perchloric acid (Fisher) and measured using a Radiometer-Copenhagen Model PHM84 pH meter. Scavenging studies using chloroacetate ion as the scavenger were performed at pH 11.0; studies using formate were performed at pH 6.5. For hydroxyl radical studies solutions were deoxygenated with nitrous oxide (U.S.P. grade, Mittler); for hydrated electron studies, nitrogen (ultrahigh purity, Mittler) was used as the purging gas. Results Steady-State Experiments. The protonated DMPOelectron spin adduct (DMPO-H) and the DMPO-OH spin adduct were observed during continuous electron irradiation of neutral, aqueous solutions containing 2-5 mM DMPO. In nitrogen-saturated solutions, ESR absorption lines from both species were observed in comparable intensity (Figure 1). In nitrous oxide-saturated solutions, the hydrated electron is quantitatively converted to hydroxyl radical with a rate constant of 9.1 × 109 mol-1 dm3 s-1,14 doubling the initial yield of hydroxyl radical and as a consequence doubling the initial yield of DMPO-OH if the concentration of DMPO is not limiting. By scaling and subtracting ESR spectra recorded in nitrogensaturated and nitrous oxide-saturated solutions, the two ESR spectra for DMPO-OH and DMPO-H could be resolved, as in Figure 2. The spin Hamiltonian parameters observed for both spin adducts are in good agreement with the literature values. Harbour et al.15 observed the DMPO-OH radical during continuous photolysis of an aqueous 1% hydrogen peroxide solution containing 0.01 M DMPO. The 1:2:2:1 quartet pattern was centered at g ) 2.0060, with a(N,NO) ) a(H,C(OH)H) ) 15.3 G. Measurements in deuterium oxide permitted the observation of two smaller proton couplings of a(H,CHγ1) ) 0.61 G and a(H,CHγ2) ) 0.25 G. Sargent and Gardy16 observed both DMPO-OH and DMPO-H during the continuous electron radiolysis of argon-saturated aqueous solution of DMPO, finding a(N,NO) ) a(H,C(OH)H) ) 15.0 G for DMPO-OH. They reported hyperfine splittings of a(N,NO) ) 16.7 G and a(H,CH2) ) 22.6 G for DMPO-H. Unambiguous confirmation of the DMPO-OH assignment was provided by Mottley et al.,17 who produced the oxygen-17-labeled DMPO-OH adduct via

TRESR Studies of Spin Trapping by DMPO

Figure 2. Steady-state X-band (9.2 GHz) second-derivative ESR spectrum of the DMPO-OH spin adduct (bottom trace) and DMPO-H spin adduct (top trace) resolved by spectral subtraction of the two traces in Figure 1.

the xanthine/xanthine oxidase/Fe2+ system in the presence of DMPO and 17O oxygen gas. The appearance of an additional 4.66 G splitting from a spin 5/2 nucleus in the DMPO-17OH spectrum confirmed the assignment of Harbor et al. The parameters observed here for DMPO-OH are g factor ) 2.005 58 ( 0.000 03, a(N,NO) ) 14.93 ( 0.01 G, and a(H,(OH)CH) ) 14.91 ( 0.03 G. The parameters for DMPO-H are g factor ) 2.005 36 ( 0.000 03, a(N,NO) ) 16.52 ( 0.01 G, and a(H,CH2) ) 22.60 ( 0.01 G. Of particular importance concerning the quantitative aspects of hydrated electron and hydroxyl radical trapping by DMPO is the clean nature of the base line segments shown in Figures 1 and 2. Under the irradiation and detection conditions used here, any significant concentration of alkyl radicals formed by hydrogen abstraction from the C-H bonds of the spin trap would be visible in these steady-state spectra. The absence of extraneous ESR absorption features in these spectra strongly suggests the vast majority of water transients react to produce DMPO-OH or DMPO-H, in accord with the kinetic results (see below). Kinetic Experiments (Rate Constants for Radical Addition). In order to measure the reaction rate constant of the hydrated electron with DMPO, competition kinetics using chloroacetic acid as an electron scavenger was employed, since the direct reaction of hydrated electron and DMPO at reasonable spin trap concentrations is faster than the response time of the ESR instrument (0.3 µs). The accepted18 rate constant for the reaction of chloroacetate and hydrated electron is 1.0 × 109 mol-1 dm3 s-1. Kinetic traces for the formation of DMPO-H as a function of chloroacetate concentration are given in Figure 3. At the lowest concentrations of chloroacetate, the shape of the curve was a fast instrument-response-limited rise followed by a plateau representing the amplitude of the DMPO-H spin adduct ESR line. At higher chloroacetate concentrations, a decay component is observed superimposed on the plateau, due to spectral overlap of the carboxymethyl radical low-field line with the DMPO-H ESR absorption. This overlap was corrected by recording kinetic curves from the central line of the carboxymethyl radical for each concentration of chloroacetate studied and then subtracting the decaying carboxymethyl component from the experimental DMPO-H kinetic curve using a nonlinear least-squares fitting procedure. The decaying component of the kinetic curve was 2-3% the intensity of the carboxymethyl central feature for all concentrations of chloro-

J. Phys. Chem., Vol. 100, No. 18, 1996 7513

Figure 3. TRESR kinetic traces showing the formation of DMPO-H as a function of chloroacetate concentration: 0.05 mM (open triangles), 10 mM (solid triangles), 30 mM (open circles), and 200 mM (solid squares). The electron beam pulse occurs at 6.5 µs on the kinetic trace, indicated by the downward-pointing arrow.

Figure 4. Competition plot of the reciprocal yield of DMPO-H as a function of the concentration ratio of chloroacetate and DMPO. The solid line is the least-squares fit to the data points.

acetate employed. The corrected plateau level was proportional to the total yield of DMPO-H. This yield of DMPO-H results from direct hydrogen atom addition to DMPO, as well as electron reaction with DMPO and subsequent protonation. The direct reaction of DMPO with radiolytically produced hydrogen atoms is manifest as a constant additional yield of DMPO-H over that due to hydrated electron reaction. At the chloroacetate ion concentration used in this study (200 mM), DMPO reacts with 96% of the hydrogen atoms, since chloroacetate ion and hydrogen atoms react slowly (3.6 × 106 mol-1 dm3 s-1).18-20 The fraction of DMPO-H due to hydrogen atoms was resolved by determining the constant offset in DMPO-H yield during the competition plot fitting process. From conventional competition kinetics,21 plotting the reciprocal yield of DMPO-H against the concentration ratio of chloroacetate to DMPO gives the standard form of the competition plot (Figure 4). Linear least-squares fitting of the data gives a line with slope 0.346 and a y-intercept of 1.116, yielding a calculated rate constant (3.2 ( 0.4) × 109 mol-1 dm3 s-1 for the reaction between DMPO and hydrated electron. The DMPO-H yield offset due to hydrogen atoms is 0.080, compared to a yield of (1.116)-1 ) 0.896 for DMPO-H produced by hydrated electrons. The fractional yield of DMPO-H originating from hydrogen atom is therefore 0.080/(0.080 + 0.896) ) 0.082. Since the relative yield of H atoms to the total yield of hydrated electrons and H atoms is 0.182, this indicates that roughly half (0.45) of the radiolytically produced hydrogen atoms react with DMPO to give non-aminoxyl products.

7514 J. Phys. Chem., Vol. 100, No. 18, 1996 Similarly, sodium formate was used as the competitor for the hydroxyl radical to study the rate of hydroxyl radicalDMPO spin adduct formation. The rate constant for the reaction of formate with hydroxyl radical is 3.2 × 109 mol-1 dm3 s-1.18 Due to the greater line width of the DMPO-OH spin adduct, the line intensity was lower, but the ESR kinetic curves showed a rapid rise, followed by a flat plateau at all concentrations of sodium formate used in this study. The magnitude of the DMPO-OH kinetic curve plateau was proportional to the yield. Plotting the reciprocal yield of DMPO-OH against the concentration ratio of formate to DMPO, the standard form of the competition plot was obtained (Figure 5). Linear least-squares fitting gives a slope of 1.052 and a y-intercept of 0.924, resulting in a calculated rate constant of (2.8 ( 0.5) × 109 mol-1 dm3 s-1 for the reaction of DMPO and hydroxyl radical. Kinetic Experiments (Hydroxyl Radical Spin Adduct Yield Determination). The number of spins in an ESR microwave cavity, and therefore the radical concentration, is given by the total intensity of the ESR spectrum. In the in situ radiolysis ESR experiment, the total free radical concentration produced in the path of the electron beam can be related to the change in the integrated ESR spectrum immediately after the delivery of the electron pulse. If a reaction quantitatively converts the radiolytically produced free radical population to a single free radical, the integrated ESR signal represents the yield of radicals produced by radiolysis; conversely, a quantitative reaction of radiolytically produced radicals with a stable radical would bleach its ESR signal. This integrated bleach represents the total yield of radiolytically produced radicals as well. This change in spectral intensity can then be used to correlate the yield of a particular radical as a fraction of the total radical yield. This method has been recently used in a flash photolysis/TRESR study of the C60 fullerene triplet.22 In the current study, the stable aminoxyl radical tempone serves as the indicator for the total yield of radiolytically produced hydrogen atom, hydroxyl radical, and hydrated electron. These water transients react in nitrous oxide-saturated aqueous formate solution to give the carbon dioxide anion radical, which in turn reduces the tempone, bleaching the tempone ESR signal. The radiation chemical mechanism for the destruction of the tempone follows.23,24

H2O f •OH (45%) + H• (10%) + e-(hyd) (45%) N2O + e-(hyd) f N2 + OH- + •OH •

OH + HCO2- f •CO2- + H2O H• + HCO2- f •CO2- + H2

CO2- + •tempone + H+ f tempone-H + CO2

•

The tempone solution composition (20 mM sodium formate and 400 µM tempone) was chosen such that the reaction of the hydroxyl radical with the formate and the subsequent reaction of carbon dioxide anion with tempone are both fast: 3.2 × 109 mol-1 dm3 s-1 18 for the former reaction and 7.0 × 108 mol-1 dm3 s-1 25 for the latter. The high concentration of the aminoxyl radical assured that there would be no appreciable line width change with the radical concentration (typically 40 µM) produced by radiolysis in this experiment. Thus, essentially all the radiation-induced free radical chemistry participates in the bleach of the tempone ESR signal, which is then a standard

Madden and Taniguchi

Figure 5. Competition plot for the reciprocal yield of DMPO-OH as a function of the concentration ratio of sodium formate and DMPO. The solid line is the least-squares fit to the data points.

Figure 6. Absorption spectrum of low-field line of nitrous oxidesaturated aqueous solution containing 20 mM sodium formate and 400 µM tempone. The solid curve is a nonlinear least-squares fit to the data using a composite Lorentzian/Gaussian line shape (see text).

for the comparison of the spin adduct radical yields. Using the direct-detection signal channel of our ESR spectrometer, we recorded the absorption spectrum of the low-field line of tempone (Figure 6). The peak amplitude of this line was 0.0784 V. The experimental line shape was modeled using the linear combination of a Gaussian and Lorentzian line shape26 as an approximation to the Voight line shape (ref 27 and references therein). Nonlinear least-squares fitting to these data resulted in a peak of 0.69 Lorentzian character, with a half-width at halfmaximum (hwhm) of 0.22 G, and an area of 4.92 × 10-2 V G. The ratio of the line area to the line height was therefore 0.627 G, and the ratio of the total tempone spectral area to the lowfield line height was 1.88 G. To determine analogous line shape parameters for the lowfield line of DMPO-OH, an ESR experiment was performed using a pulsed electron beam to produce DMPO-OH, using field modulation at 100 kHz and 200 Hz to obtain a secondderivative presentation of the ESR line. The result is presented in Figure 7. This experimental line shape could be well reproduced by the superposition of two Gaussian lines of width 0.49 G (hwhm) separated by 0.56 G. This lineshape was used to fit the absorption line shape determined from the ESR kinetic curves. ESR kinetic curves of a nitrous oxide-saturated aqueous solution containing 20 mM sodium formate and 400 µM tempone were recorded at a number of beam current levels at the tempone low-field line position. The kinetic curves showed a rapid, instrument-response-limited bleach, followed by a steady

TRESR Studies of Spin Trapping by DMPO

J. Phys. Chem., Vol. 100, No. 18, 1996 7515 TABLE 1: Observed Rate Constants for the Reaction of the Water Transients with DMPO water radical hydrated electron hydroxyl radical

Figure 7. (bottom) Second-derivative X-band (9.2 GHz) ESR spectrum of the low-field line of DMPO-OH recorded during radiolysis with a pulsed electron beam. Superimposed upon the experimental spectrum is an simulation based upon two Gaussian lines (0.49 G hwhm) separated by 0.56 G. (top) The absorption line generated from the fitting parameters above.

rate constant, mol-1 dm3 s-1

ratio value/ present value

ref

2.0 × 1010 a 1.0 × 1010 b 3.2 × 109 c 4.3 × 109 d 4.3 × 109 e 3.6 × 109 f 3.4 × 109 g 2.8 × 109 h 2.7 × 109 i 2.6 × 109 j 2.6 × 109 k 2.1 × 109 l 2.0 × 109 m

6.3 3.1 1.0 1.5 1.5 1.3 1.2 1.0 1.0 0.9 0.9 0.8 0.7

32 33 this work 34 32 33 35 this work 36 36 31 35 36

a Radiolysis, pH 11.0. b Radiolysis. c Radiolysis, pH 11.0. d Radiolysis. e Radiolysis, pH 11.0. f Radiolysis. g Fenton system, pH 7.4. h Radiolysis, pH 6.5. i Photolysis of H O , pH 7. j Photolysis of H O , 2 2 2 2 pH 8. k Photolysis of H2O2. l Photolysis of H2O2, pH 7.4. m Photolysis of H2O2, pH 6.

to hydrogen atom reaction with tempone), the total hydroxyl radical yield is 7.563 × 10-3 V G. The fraction of hydroxyl radicals reacting with DMPO is therefore 0.935, a near quantitative reaction. Discussion and Conclusions

Figure 8. Open square: plot of average plateau level of DMPO-OH as a function of magnetic field. Solid line: constrained nonlinear leastsquares fit to the data using the line shape parameters from the field modulation study of Figure 7 (see text).

plateau at longer times. The level of this plateau was taken as the amplitude of the bleach of the tempone low-field line. A plot of the plateau level as a function of collected electron beam current yielded a straight line, which was used as the standard curve for radiation-produced free radical yield as a function of collected beam current. ESR kinetic curves of nitrous oxide-saturated neutral aqueous solutions containing 2 mM DMPO were recorded at 70.5 mG intervals centered about the resonance position of the low-field line of DMPO-OH. As before, a rapid rise in the ESR signal was followed by a steady plateau for 80 µs. The average level of this plateau is plotted as a function of field in Figure 8, yielding the absorption line shape for the DMPO-OH spin adduct measured in the period from 8 to 88 µs after aminoxyl radical formation. Using the line shape model employed in the pulse radiolysis/field modulation ESR study and the base line value derived from a blank experiment, a constrained nonlinear least-squares fit to the absorption yielded line area of 1.179 × 10-3 V G.26,27 Since this line represents one-sixth of the spectral area enclosed by the 1:2:2:1 quartet, the total intensity is 7.074 × 10-3 V G. At the corresponding electron beam intensity, the total radical yield given by the tempone ESR bleach is 8.404 × 10-3 V G. Since only 90% of this bleach is due to hydroxyl radicals in nitrous oxide-saturated solution (the remainder due

Rate constants for the reaction of DMPO with hydroxyl radical and hydrated electron were measured for the first time by direct observation of the spin-adduct yield within several microseconds of spin-trap parent radical reaction. The comparison with the literature values appears in Table 1. The previously reported hydroxyl-DMPO rate constants are in fair agreement with our measurements, ranging from 30% lower to 50% higher than the value found in this study. The agreement with the range of literature values demonstrates the essential soundness of the measurement approach used here. The literature reaction rate constants of DMPO with hydrated electron are 3.1 and 6.3 times higher than our measurement, an appreciable discrepancy. These latter rates were measured by observing the decay of the electron instead of the growth of the spin adduct. Since the electron is capable of quickly reducing aminoxyl radicals to hydroxylamines, the reaction of the electron with residual aminoxyl radical may contribute to the rapid rate constant observed in the optical studies. The lower rate measured here is consistent with the ESR spectra observed using 10 mM DMPO in nitrous oxide-saturated solutions, where little DMPO-H is observed in addition to the spin adduct of the parent radical.28 The fraction of radiolytically produced hydrogen atoms reacting with DMPO to produce DMPO-H spin adduct is calculated as 0.45, indicating roughly half of the H atoms produce non-aminoxyl products. Carmichael et al.6 using the spin trap PBN (R-phenyl-N-tert-butylnitrone) also found a low (0.14) aminoxyl yield for the reaction of H atoms and PBN, suggesting that the reactive H atom could add not only to the nitrone CdN bond but also to the aromatic ring. DMPO lacks the aromatic substituent of PBN, but the addition of the reactive hydrogen atom to the CdN bond may be bipolar, giving a shortlived carbon-centered radical at C-2 in addition to the aminoxyl spin adduct. The low yield of this carbon-centered radical, in this case, precludes its detection in the in situ radiolysis experiment. The measured initial yield of the DMPO-hydroxyl radical spin adduct, 0.94, is appreciably higher than the DMPO-OH

7516 J. Phys. Chem., Vol. 100, No. 18, 1996 yield in the longer time domain reported in previous studies. The reported values are 0.33 and 0.35, which might reflect the effect of termination reactions.6-8 There is another work,29 however, claiming that almost all OH radicals produced eventually react with DMPO, in agreement with the current results. The quantitative result measured here using timeresolved ESR is consistent with the clean base lines observed in the steady-state ESR spectra observed in nitrous oxidesaturated solution. Since both the R-phenyl-N-tert-butylnitrone-OH radical30 and DMPO-OH radical31 are known to participate in both first- and second-order termination reactions, it would be reasonable to assume that such processes are operative in DMPO-OH as well. It was reported31 that the decay of the DMPO-OH follows second-order kinetics above 4 × 10-5 mol dm-3 and first-order kinetics at lower concentrations. The continuous radiolysis and photolysis methods used in previous studies include the effect of first- and second-order decay of the spin adduct, leading to lower yields under these steady-state conditions; therefore, the numbers are not directly comparable with the initial yield reported here. The effect of decay processes can be isolated and analyzed by pulse radiolysis combined with time-resolved ESR. These results will appear in a subsequent report. Acknowledgment. The work described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. This is Contribution NDRL-3873 from the Notre Dame Radiation Laboratory. References and Notes (1) Janzen, E. G.; Blackburn, B. J. J. Am. Chem. Soc. 1968, 90, 5909. (2) Lagercrantz, C.; Forshult, S. Nature 1968, 218, 1247. (3) Chalfont, G. R.; Perkins, M. J.; Horsfield, A. J. Am. Chem. Soc. 1968, 90, 7141. (4) Madden, K. P.; Taniguchi, H. J. Am. Chem. Soc. 1991, 113, 5541. (5) Madden, K. P.; Taniguchi, H. J. Chem. Soc., Perkin Trans. 2 1993, 2095. (6) Carmichael, A. J.; Makino, K.; Riesz, P. Radiat. Res. 1984, 100, 222.

Madden and Taniguchi (7) Sun, L. Ph.D. Dissertation, The University of Western Ontario, 1994. (8) Sun, L.; Hoy, A. R.; Bolton, J. R. AdV. Oxid. Tech. preprint, 1995. (9) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1963, 39, 2147. (10) Eiben, K.; Fessenden, R. W. J. Phys. Chem. 1971, 75, 1186. (11) Madden, K. P.; McManus, H. J. D.; Fessenden, R. W. ReV. Sci. Instrum. 1994, 65, 49. (12) Jeevarajan, A. S.; Fessenden, R. W. J. Phys. Chem. 1989, 93, 3511. (13) Fessenden, R. W. J. Chem. Phys. 1973, 58, 2489. (14) Janata, E.; Schuler, R. H. J. Phys. Chem. 1982, 86, 2078. (15) Harbour, J. R.; Chow, V.; Bolton, J. R. Can. J. Chem. 1974, 52, 3549. (16) Sargent, F. P.; Gardy, E. M. Can. J. Chem. 1976, 54, 275. (17) Mottley, C.; Connor, H. D.; Mason, R. P. Biochem. Biophys. Res. Commun. 1986, 141, 622. (18) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (19) Jortner, J.; Rabani, J. J. Phys. Chem. 1962, 66, 2078. (20) Scholes, G.; Simic, M. J. Phys. Chem. 1964, 68, 1738. (21) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 2nd ed.; John Wiley and Sons: New York, 1976; p 180. (22) Veselov, A.; Fessenden, R. W. Manuscript in preparation. (23) Madden, K. P. Unpublished results. (24) Asmus, K.-D.; Nigam, S.; Willson, R. L. Int. J. Radiat. Biol. 1976, 29, 211. (25) Willson, R. L. Trans. Faraday Soc. 1971, 67, 3008. (26) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance: Elementary Theory and Practical Applications, McGraw-Hill: New York, 1972; p 32. (27) Halpern, H. J.; Peric, M.; Yu, C.; Bales, B. L. J. Magn. Reson., Ser. A 1993, 103, 13. (28) Madden, K. P.; Taniguchi, H. Unpublished results. (29) Wolfrum, E. J.; Ollis, D. F.; Lim, P. K.; Fox, M. A. J. Photochem. Photobiol. A: Chem. 1994, 78, 259. (30) Kotake, Y.; Janzen, E. G. J. Am. Chem. Soc. 1991, 113, 9503. (31) Castelhano, A. L.; Perkins, M. J.; Griller, D. Can. J. Chem. 1983, 61, 298. (32) Faraggi, M.; Carmichael, A.; Riesz, P. Int. J. Radiat. Biol. Relat. Stud.Phys., Chem. Med. 1984, 46, 703. (33) Sridhar, R.; Beaumont, P. C.; Powers, E. L. J. Radioanal. Nucl. Chem. 1986, 101, 227. (34) Neta, P.; Steenken, S.; Janzen, E. G.; Shetty, R. V. J. Phys. Chem. 1980, 84, 532. (35) Finkelstein, E.; Rosen, G. M.; Rauchman, E. J. J. Am. Chem. Soc. 1980, 102, 4994. (36) Marriott, P. R.; Perkins, M. J.; Griller, D. Can. J. Chem. 1980, 58, 803.

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