4101
J. Phys. Chem. 1993, 97, 4101-4105
EPR Measurement of the Reaction of Atomic Hydrogen with Br- and I- in Aqueous Solution+ David M. Bartels' Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
Stephen P. Mezyk AECL Research, Whiteshell Laboratories, Pinawa, Manitoba ROE 1 LO,Canada Received: November 17, 1992; In Final Form: January 26, 1993
Electron paramagnetic resonance free induction decay (FID) attenuation measurements are reported for the reaction of H atoms with Br-, I-, and 2-propanol in aqueous solution. At 25 OC, we obtain scavenging rate constants of (1.7 f 0.3) X lo6 M-I s-* for B r , (2.8 f 0.4) X lo8 M-' s-l for I-, and (9.0 f 2.0) X lo7 M-I s-l for 2-propanol. Very small activation energies of 6.3 f 6.2 kJ/mol for the B r reaction, and 1.8 f 4.6 kJ/mol for the I- reaction, were found in the 10-58 OC temperature range. Anomalous chemically induced dynamic electron polarization (CIDEP) of the H atom in the presence of the halide ions suggests that a near-equilibrium H X-+ HX- is approached on the 5-bs time scale of the FID experiment, with the reverse rate on the order of 106 s-1. The proposed equilibrium resolves a number of apparent contradictions in previous work on these reaction systems.
+
TABLE I
I. Introduction The study of the radiation-induced chemistry of iodinecontaining species in the aqueous phase has been stimulated by the recognized safety hazard of radioactive iodine release following a nuclear reactor accident.' There now exists a significant body of information on the reactions of iodo-substituted species with a variety of free radicals in aqueous solution.2 However, the paucity of data for hydrogen atom reactionswith even thesimplest of these specieshas meant that models for predicting thevolatility of radioactive iodine under accident conditions3 are incomplete. Furthermore, even when such data are available, as for the hydrogen atom reaction with iodide ion,G8the large scatter in the reported rate constants has made their inclusion in these modeling schemes essentially futile. To begin to rectify this situation, we have undertaken measurement of rate constants for the reaction of H atoms with I- and B r . Table I lists the rate constants for the H + X- (X = I, Br) reactions presently available in the literature and our own results which are described below. References 5-7 report determinations based on the competition of the halide ions with other scavengers for the H atoms produced in steady-state y radiolysis. The smallest of these rate constants was reported recently by Deeble et ale7and would normally be considered the most reliable (as reaction with impurities would increase the measured value). However,the rate constantsrecorded in these competition studies are in marked contrast to the pulse radiolysis experiments of Elliot et a1.,8 who required a rate constant of 14 X lo8 M-l s-I for the H + iodide reaction to account for their data, but found "no evidence" for reaction with B r scavenger. Clearly some important chemistry has gone unrecognized in these previous studies. This paper reports our direct measurement of rate constants and activation energies for the reaction of H atoms with I- and Br-. Direct EPR detection of the decay of the hydrogen atoms followingpulse radiolysis9J0was the monitoring method of choice, because conventionalpulse radiolysis/optical transient absorption methodology cannot be applied in the absenceof convenient UV/ vis optical absorption of either the H atoms or the reaction
' Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE, under Contract W-31-109-ENG-38. Financial support for S.P.M.is provided by AECL Research. 0022-3654/93/2097-4101$04.00/0
H' + I-/M-l s-I (2.8
0.4)X 2 4 x 108 1 x 106 4 x 107 5.3 x 106
lo8
H' + Br-/M-I s-I (1.7 f 0.3)X no reaction 12 x 104 3.3 x 107
lo6
ref this work
8 7 6 5
product(s). The recently-developed pulsed-EPR-based free induction decay (FID) attenuation is particularly advantageous because of the simple pseudo-first-order scavenging kinetics generally obtained. Important insight into the nature of the products is also gained from H atom spin exchange and chemically induced dynamic electron polarization (CIDEP) in the presence of the halide ions. The halide systems are contrasted to the H atom reaction with 2-propanol, a system whose mechanism (H atom abstraction to form H2 and the 2-propyl01 radical) has been well established.2J5
II. Experimental Section The free induction decay attenuation experiment was carried out as described in several previous publication^.^^-'^ H atoms were generated in aqueous solution within an EPR cavity by radiolysis with 12-, 25-, or 55-ns pulses of 3-MeV electrons from a Van de Graaff accelerator. The pH = 2 perchloric acid stock solutions contained either 0.02 M MeOH or 5 X 10-3 M B r to scavenge O H radicals produced in the radiolysis. Hydrated electrons formed initially react with (H+)Bqwithin 10 ns to form H atoms. A total aqueous sample volume of approximately 190 mL was recirculated through a flat cell in the cavity at a high flow rate. The actual volume irradiated in each pulse was less than 0.1 mL. Approximate radiation dose to this volume was 1.5, 3.0, or 6.5 krad/pulse for the 12-, 2 5 , or 55-11s electron pulses, respectively. A 354s microwaveprobe pulse was applied to the sample immediately after radiolysis, and the resulting free induction decay of the H atom low-field ( m , = +'/2) EPR transition was recorded on a digital oscilloscope. Typically 2000 shots were averaged to measure each FID,at a repetition rate of 120 Hz. Scavenging experiments were performed by injecting 2.0 M NaI solution, 5.0 M NaBr, or neat 2-propanol into the initial known volume of solution. Accuracy of the concentrations is estimated to be ca. *S% or better. H atom longitudinal magnetization was also recorded as a function of time after the 0 1993 American Chemical Society
Bartels and Mezyk
4102 The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 IAdded 1.37 x 10" M 2-PrOHI
1 0 1
O H
>-
251-1s Pulse k, 5 1.13 e0 M"s"
i
I
I
I
I
I
0
2
4
6
0
Time (psec) Figure 1. Magnetization of the H atom high- and low-field lines in 2 X lo-? M MeOH, pH 2 perchloric acid solution, recorded in the presence (solid lines) and absence (dotted lines) of 2-propanol scavenger, following a 25-nsradiolysis pulse. Thegrowth of absorptive (high-field) and emissive (low-field) polarization is characteristic of the CIDEP phenomenon in randomized spin (F-pair) recombination reactons. The signal decay in the presence of 1.37 X M 2-propanol (ca. 800-ns time constant after the first microsecond) is very nearly the same as the (720-ns) chemical decay of the H atoms, deduced from the FID attenuation experiments.
0
2
6
4
8
10
12
[2-PrOH] / mM Figure 2. FID damping rate l/Tz(eff) as a fuction of 2-propanol concentration at 24.8 OC. The slopes give apparent scavinging rates of 1.01 X los M-I s-I and 1.12 X lo8 s-l for the 12- and 25-11s pulses, respectively. Extrapolation to zero dose gives (0.9 & 0.2) X lo8 M-I s-I as the limiting rate ~ 0 n s t a n t . I ~ 2.5
1
v)
:
2.0
radiolysis pulse for several different scavenger concentrations, by scanning the microwave pulse delay relative to the radiolysis pulse and integrating the first 100 ns of the FID signal in a gated integrator.10
r
Y
'O
x
Y
1.5
.
111. Results and Discussion
7-
In Figure 1 we show longitudinal magnetization of the H atom high- and low-field line transitions in the presence and absence of 2-PrOH scavenger. In the absence of scavenger, the low-field line develops a very strong emissive polarization (indicated as negative signal), while the high-field line develops strong absorptive polarization, typical of chemically induced dynamic electron polarization (CIDEP) generated primarily in the spindependent recombination reaction H H H2.9J0Addition of 2-propanol scavenger attenuates the H atom signal, but perhaps not as quickly as might be expected from the H atom chemical lifetime. The signals in the presence of scavenger have roughly an 800-ns decay constant (average), whereas the chemical lifetime of H atoms in this solution is 720 ns (calculated from the rate constant found below). Even as the H atom concentration is decaying, the polarization of the EPR transitions continues to increase because of spin-dependent reaction of H with the product 2-propyl01 radical, '(CH&COH. The observed magnetization is a product of the (decreasing) concentration and the (increasing) polarization,17 which accounts for the initial signal rise and a magnetization decay that may slightly lag the chemical decay. This behavior is typical for H atom scavenger systems in which the product radical has a spin relaxation time longer than a few nanoseconds, such that CIDEP can be generated in subsequent cross-recombination reactions. The H atom free induction decays recorded in these experiments were all well described by a pure exponentially-damped cosine. A plot of the fitted exponential damping rate vs 2-propanol concentration is shown in Figure 2 for two radiation doses, which differ by a factor of 2. As explained in previous publications,l1-I4 the effective damping rate of the FID in these experiments should be given by
+
-
where k,[S] is the H atom scavenging rate and C,kb,[R,] represents the spin-dephasing contribution of second-order spin exchange and recombination reactions between H and other free radicals R,. At the radiation doses typically used in our experiments, the latter term is not negligible, but on the ca. 5-ps time scale of the experiment the total radical concentration can
0
1
2
[r] I
3
4
5
mM
Figure 3. FID damping rate 1/ Tz(eff) as a function of I- concentration at 10 O C . With the 25-11s pulse, the apparent scavenging rate constant is 3.17 X lon M-I s-l, and with the 1 2 4 s pulse the slope is 2.86 X IOs M-I SKI, Extrapolation to zero dose gives (2.6 f 0.3) X IO8 M-1 s-I as the limiting rate constanti5at 10 OC.
generally be assumed constant (note that H atoms react with scavenger to form another free radical). The effect of spin exchange and of radiation dose is then just represented by a shift in the intercept of the scavenging plot, as is evident in the data of Figure 2. From the slope of the scavenging plots were obtain the apparent rate constants of 1.12 X 108 M-I s-I with the 25-11s pulse and 1.01 X IO8 M-I S-I with the 1 2 4 s pulse. Linear extrapolation to the zero-dose limit gives (9.0 f 2.0) X 107 M-1 s-l as our best rate constant estimate,I5 which is in agreement with previous reports9J6 to within their probable error limits. The measurable effect of dose on the slope of the 2-propanol scavenging plot15 is a phenomenon we have not encountered in previous FID attenuation experiments under these irradiation conditions. A similar dose effect in the 10-30'35 range is observed in scavenging experiments with I-and B r , as illustrated in Figures 3 and 4, respectively. These observations clearly represent a breakdown of the simplifying assumptions used to derive eq 1, and a reanalysis is now necessary. Given the known radiation chemistry of aqueous solutions,lo we expect three important second-order spin-exchange/reaction contributions to the sum in eq 1. O H radicals created in the water radiolysis will be scavenged quickly by the alcohol to give a radical R'. (Alternatively, at higher X- concentrations, OH reacts to form the XOH*- radical.) Hydrated electrons immediately react with H30+ in acid solution to give more H atoms, which thereby constitute half the free radical yield. Finally, H atoms react with scavenger S to give a product radical P'. We
The Journal of Physical Chemistry, Vol.97, No. 16, 1993 4103
Reaction of Hydrogen with Br and I-
r
Z
17
n + BrE,
0.00
I
I
0.05
0.10
I 0.15
I 0.20
rate 6.3 f 6.2 kJlmole
14
I
3.1
0.25
[Bf]/M Figure4. FID damping rate 1/ Tl(eff) as a function of Br concentration at 25 O C . The fitted slopes increase slightly with applied radiation dose. Correction for this effect by extrapolation to zerodose's gives a scavenging rate of (1.7 f 0.3) X lo6 M-I s-I.
3.2
3.3
3.4
x k L x [ R i ] = k:['R]
is
i
The *R radicals usually recombine slowly relative to our experimental time scale, so that their spin-exchangecontribution should be nearly constant."-I4 Obviously, the H' and P' concentrations are changing rapidly during the experiment, but if the sum ([H']+ [ P ' ] )is nearly constant, the FID decay will be sensitive to the change in species only if the rate constants k: and kLx differ significantly. Experience to date1I-l4suggests that often the exchange rate constants k: and k:. do not differ sufficiently to affect the FID attenuation measurement. Spin exchange is a diffusion-limited spin relaxation process, but requires that the interacting radicals be nonidentical, at least insofar as their nuclear hyperfine states during an encounter.I8 Thus, even though H atoms diffuse at a cm2 s-I)I9 which is 4 times faster than the rate (ca. 8 X typical product radical P',only half of the H-H encounters can be effective in producing spin relaxation. Assuming similar effective encounter radii for H spin exchange with other H atoms or the product P', one can expect perhaps a 50% difference in the spin-dephasing efficiency of the H atoms and the P' radicals. Any experimental effect of this difference for the FID attenuation will be further diluted by the "constant" H-'R exchange contribution (especially in neutral solution where H atoms represent only 10% of the total radical yield),'O and by the use of low-dose conditions to minimize the second-order exchange term relative to the primary scavenging process. This accounts for the very linear scavenging plot for 2-propanol in Figure 2 and similar results found for other scavengers in the Nevertheless, for the scavengersunder discussion,the difference k: - k:x seems to be quite significant, as indicated by the effect of radiation dose on the slopes of Figures 2 4 . Assuming that the sum Ho = [H'] + [P']is nearly constant, we can treat the time-dependent concentrations [H'] and [PI as a first-order approach to equilibrium:
- [H']
+ kfx[P'lM (4)
+ kE[H'] + k:x[P']
[PO] = Hoexp(-kct)
3.5
I/T (K-' ~ 1 0 ~ ) Figure 5. Arrhenius plots of ln(krxn)vs 1/T for the reaction of H atoms with I- and B r in aqueous solution.
dM/dt = - M / T ; - k,[S]M - kz[H']M
have
-
where k-, is the rate constant for a possible back reaction P' H* S. The first-order "chemical" decay constant, k,, is included to mimic the minor effect of second-order recombination reactions in reducing the overall radical concentration. The effectiveBloch equation for decay of transverse magnetization (phase coherence)
+
5
where the H-'R exchangecontributionhas been included in 1/ T2O. Solution of the differential equation gives ln(M/M,) = -t/T:
+ (Hok~/kc)[exp(-kct)- 11 k,[Slt - E ( t ) ( 5 )
exp[-(k, + k-, kc + k-,
+ k,[S])t] - 1 + k,[S]
where A = k: - kx: and HOis the initial concentration of H atoms. The first two terms of eq 5 represent the signal in the absence of scavenger, the third term is the primary scavenging effect, and the function E ( t ) corrects for the difference k: k,Px. As expected, the rate of magnetization decay is a function of time after thecreationof the radical population, andin principle each FID should be fitted by least squares to function 5 for the damping envelope. In practice, this is not necessary because all of the FIDs closely resemble pure exponential decays. We note that the individual fitted values of T2(eff) represent for each scavenger concentration the time t at which ln(M/Mo) = -1. Substitution of this relationshipintoeq 5 gives an implicit function for the FID decay rate l/Tz(eff) which can easily be solved by iteration at each value of [SI. Simulations of the FID decay time using eq 5 show that fairly large values of the ratio HoA/(Hok: 1/ T:), on the order of unity, are required to produce a significant change in the slope of the scavengingplot, and larger values still (ca. 3-4) are required to produce noticeable curvature. The direction of the dose effect found in our experiments, where the apparent scavenging rate increases with applied dose, is consistent with a negative value of 4i.e. k: < k,Px. Variation of the effective chemical decay rate constant, k,, within reasonable values has little effect on the slope of the scavenging plot. The effect of any back-reaction k-, simply to reduce the magnitude of E ( t ) , because the H population does not convert entirelyto P. The procedure introduced for 2-propanol above, where the apparent scavenging rate is plotted vs relative applied dose and then extrapolated to zero, appears to be justified at least when curvature is minor. This will always be true in our experiments if :k and kx: differ by less than a factor of 2. Rate constants for the reaction of H + I- were measured by FID attenuation at several temperatures in the range 10-57 OC, and results are plotted in Arrhenius form in Figure 5. At 25 O C , the rate constant (extrapolated to zero dose) is (2.8 f 0.4) X lo8 M-I s-1. The reaction rate is found to be remarkably insensitive to the temperature, with fitted activation energy of only 1.8 f
+
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993
4104
Bartels and Mezyk
4
P,
P,
LL - 4
U.
-10
0
i
4
6
8
Time (psec) Figure 6. Effect of I- on H atom CIDEP in pH 3 perchloric acid solution at 25 "C following a 25-11s radiolysis pulse. In the presence of 5.45 X lo-' M iodide the chemical lifetime of the H atoms should be only 680 ns, but magnetization persists beyond 10 f i s .
4.6 kJ/mol. Rate constants for the reaction of H + Br- were also measured over the 10-58 OC temperature range, and the results are also included in Figure 5. At 25 OC the rate constant is (1.7 f 0.3) X lo6 M-I s-!. Above room temperature, bromide scavenging plots showed significant curvature for all of the applied radiation doses, as though the scavenging rate decreases with increasing bromide concentration. An attempt was made to fit these effective T2 values for three different doses to eq 5, using the constraints that HokE, HoA, and k, must be directly proportional to the applied dose. Much of the curvature in the plots could be accounted for by this treatment, provided that k:x is 4-5 times larger than k:. However, the trends in the auxiliary second-order rate parameters were not found to be selfconsistent as a function of temperature. More likely explanations for the observed curvature are the following: (a) the reaction rate depends on the (fairly high) NaBr salt concentration, which might significantly increase the free energy barrier; or (b) an impurity is injected along with the NaBr which has a much higher rate (and activation energy) for reaction with H atoms, and which is partially consumed by the radiolysis during each measurement. In the former case, the linear extrapolation method will underestimate the (dilute solution) reaction rate, while in the latter case, the presence of the impurity will cause us to overestimate the rate constant. We have simply included these points in Figure 5 with generous error bars, pending future investigation. As in the iodide case, the reaction of H with B r seems to have only a small activation energy of 6.3 f 6.2 kJ/mol. The CIDEP of the H atom in the presence of the halide ions was also recorded as a function of time, and the results were quite curious in light of the rate constants determined by FID attenuation. In Figure 6 we show the H atom magnetization in the presence of 5.45 X M iodide. The decay of H magnetization is anomalously slow, given the expected 1/(2.8 X 108 M-1 s-I X 5.45 X M) = 660 ns chemical lifetime and the measured FID decay time of 480 ns. Observable H atom magnetization persists beyond 10 ps. Similar behavior is found in the Br- scavenging systems. Figure 7 shows the H atom magnetization in the presenceof 0.26 M NaBr, where thechemical lifetime should be 2.3 ps, yet significant H atom magnetization is still clearly visible at 10 ps after the radiolysis pulse. Qualitatively similar results, where the magnetizationdecay time lagged the calculated chemical decay by factors of 2-4, were obtained in the presence of both I- and B r in solutions of pH 1, 2, and 3. The same phenomenon was also found in iodidecontaining solutions at neutral pH. No indication was found that the FID attenuation rate might be a function of pH. The H atom magnetization decay may slightly lag thechemical decay if polarization is generated in subsequent H + P crossrecombinations, as already noted in discussing the "normal" 2-PrOH scavenging system. However, it is unreasonable to expect this behavior when P is H B r or HI-. Small linear free radicals
0
2
6
4
8
Time (psec) Figure 7. Effect of Br- on H atom CIDEP in pH 2 perchloric acid solution at 25 OC following a 25-11sradiolysis pulse. In the presence of 0.26 M bromide the chemical lifetime should be only 2.2 bs, but a signal is still clearly visible 10 f i s after the radiolysis pulse.
tend to havevery fast spin relaxation timesdue to thespin-rotation coupling mechanism.'*J7 The presence of the heavy atom in the HX- molecules should enhance the spin relaxation even further, to the point that spin coherence does not persist between reencounters of HX- and H radicals. In this limit, CIDEP will not be generated.20,21Even if H atom CIDEP could be efficiently generated in such encounters, the chemical conversion of H to the product radical will ensure that the H magnetization decay only slightly lags (10-20%) the chemical decay, as illustrated in Figure 1 for the 2-propanol case. The CIDEP decay times observed in the halide solutions differ dramatically from the rate constants deduced in the FID attenuation experiments. We suggest that this anomalous CIDEP result in the halide scavenging systems is consistent with an equilibrium H + XHX- which is established on a microsecond time scale. In this case the FID decay time will be controlled primarily by loss of spin coherence in the forward reaction, but additional CIDEP can be generated by self-recombination of the residual H atoms. Back-reaction rates on the order of lo6 s-I would be required to maintain a sufficient H atom concentration to qualitatively explain the CIDEP results. We have already noted that such a backreaction would change the FID attenuation rate measurement merely by reducing the effect of the spin exchange A in term in eq 5, thereby improving the linearity of the scavenging plots. In their pulse radiolysis/transient absorption study of the production of 1,- in iodide-containing acid solutions, Elliot et ala8 demonstrated that the observed formation kinetics wereconsistent with the following mechanism:
+ I- + HI-
(6)
HI- + I- + HIZ2-
(7)
H HI,'-
+ (H'),,
-
H,
+ 1;
(8)
Under the conditions of their experiments [large I-, (H+)as concentrations], reaction 8 was shown to be rate-limiting [ks = (2.3 f 0.4) X lo7 M-' s-l], with the H atoms, HI-, and HIz2reaching preequilibrium on a submicrosecond time scale (K7 = 0.82 f 0.13 M-I). They considered only the forward reaction of (6), whose rate they deduced must be greater than or equal to about 4 X lo8M-I s-I, in good agreement with our FID attenuation measurement (2.7 X 108 M-I s-I). With the inclusion of the back-reaction (A),the observed formation rate of 12- should be given by
where K6 and K7 denote equilibrium constants for the corresponding reactions. This equation reduces to the form used by Elliot et a].* when 1 + K7[I-] >> 1/K6[1-]. It appears that the
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4105
Reaction of Hydrogen with B r and Idata collected by Elliot, et al. demand K6 > 25 M-I. This means, given our forward rate constant of k6 = 2.8 X 1Os, that the reverse rate k-6 < 10' SKI. Values of k-6 on the order of several times lo6 s-I, as required to explain our CIDEP results, are perfectly compatible with the mechanism shown above. Moreover, with the relatively small I- concentrations used in our study, the "residual" H atom concentration is largely controlled by the first equilibrium, reaction 6, so that qualitatively the CIDEP decay should not depend strongly on pH, as observed. The discovery of reaction -6 can also resolve the discrepancy between the kinetic study of Elliot et a1.8 and the steady-state radiolysis/product analysis study of Deeble et al.' In the reaction scheme assumed by Deeble et al., the H atoms may react irreversibly with either the halide ion or 2-propanol-& to give HD product. This competition should, in principle, reduce the amount of HD product. However, rate constants some 2 orders of magnitude smaller than those determined in our study (see Table I) were deduced for both ions. Clearly if the HX- product dissociates again on a short time scale, the liberated H atoms will all eventually react with 2-propanol-d~,and the added halide ion will produce essentially no change in the HD product yield. The effective rate for kg found in the competition experiment is naturally much smaller than the correct value. Nevertheless a significant depression in HD yield (ca. 5070%)was obtained in ref 7, which is much too large to be explained by impurities. We must postulate an irreversible reaction of the HX- which can compete with the back-reaction (-6). Furthermore, this reaction may not give H2 as a product, because H2 was specifically searched for and not found. The reaction system of ref 7 included lo4 M acetone as an electron scavenger. Given the experimental constraints, reduction of acetone by HX- seems to be the only plausible explanation for the result of Deeble et al.:
HI- + (CH,),CO or
HI-
-
(CH,),CO'-
+ (CH,),CO
+ I- + H+
(CH,),COH'
(10)
+ I-
Assuming that reactions 7 and 8 are unimportant, one can find an effective rate constant in the competition kinetics experiment in the form
where [A] represents the acetone concentration. Given our value for k6 and our ke6estimate of roughly lo6s-I, the results of Deeble et al. would suggest klo is on the order of lo8 M-l s-I for both HI- and the analogous H B r reaction. Reaction rates for both I- and Br- were also reported by Draganic and Draganic,6 based on a similar competition scheme using the formate ion. In these experiments, 2.5 X lo4 M N03ion was added to scavenge electrons. If HI- reduces nitrate ion to N032-,
HI- + NO;
+
-
NO,*-+ I- + H+
(12)
then the reported H I- rate constant could be explained if klz z 7 X 108 M-I s-1. However, the rate constant reported for the Br- reaction was actually higher than found with the direct FID attenuation measurement (see Table I). This result cannot be reconciled with the mechanism proposed above, and we can only attribute the result to the reaction of H atoms with impurities or some error in analysis. Finally, we note that Elliot et a1.8 found "no evidence" for any reaction of Br- with H. This does not conflict with our results, so long as bromide reactions analogous to (7) and (8) do not
readily form a Br2- product with a strong visible absorption spectrum.
IV. Conclusion The direct measurement of the rate constants for H atom reaction with I- and B r gives values much larger than previously determined by steady-state competition methods. To reconcile thisdiscrepancy,and toalsoexplain the anomalous H atom CIDEP observed in these systems, we believe that the halide reaction must be an equilibrium of the form
H'+X-*HXwith a back-reaction rate constant of -lo6 s-I near room temperature. The measured activation energies for the forward reaction are seen to be far smaller than generally determined for reactions of small free radicals in aqueous solution. However, given the large errors associated with these values, resulting from the difficulties in the data analysis and the few temperatures investigated, a quantitative understanding of this process is not possible at this time. Further investigationof these and other analogous systems is presently being performed.
Acknowledgment. The authors would like to thank Dr. David Werst for his assistance in operating and maintaining the Van de Graaff accelerator. We also wish to thank Dr. John Elliot for his interest and useful advice throughout the course of the work. References and Notes ( I ) Thompson, T. J., Beckerley, J. G., Eds. The Technology of Nuclear Reactor Safety; MIT Press: Cambridge, 1973. (2) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. ReJ Data 1988, 17, 513. (3) Wren, J. C.; Sagert, N. H.; Sims, H. E. Proceedings of the Third CSNI Workshop on Iodine Chemistry in Reactor Safety: Ishigure, K., Saeki, M.; Soda, K.; Sugimoto, J., Eds.; Japan Atomic Energy Research Institute Report, JAERI-M 92-012, 1992, pp 381-395. (4) Czapski, G.; Jortner, J.; Stein, G. J . Phys. Chem. 1961, 66, 960. (5) Hentz, R. R.; Johnson, C. G. J . Chem. Phys. 1969, 51, 1236. (6) Draganic, Z. D.; Draganic, I. G . J . Phys. Chem. 1972, 76, 2733. (7) Deeble, D. J.; Parsons, B. J.; Johnson, G. R. A. Rodiot. Phys. Chem. 1990, 36,487. (8) Elliot, A. J.; Geertsen, S.;Buxton, G. V. J . Chem. SOC.,Faradoy Trans. I 1988, 84 (4), 1101. (9) Fessenden, R. W.; Verma, N. C. Forodoy Discuss. Chem. SOC.1977, 63, 104. (10) Bartels, D. M.;Craw,M.T.; Han,P.;Trifunac,A. D.J. Phys. Chem. 1989, 93, 2412. (11) Han, P.; Bartels, D. M. Chem. Phys. Leu. 1989, 159, 538. (12) Han, P.; Bartels, D. M. J . Phys. Chem. 1990, 94, 7294. (13) Han, P.; Bartels, D. M. J . Phys. Chem. 1992, 96,4899. (14) Roduner, E.; Bartels, D. M. Ber. Bunsen-Ges. Phys. Chem. 1992,96, 1037. (1 5) T2(eff) measurements on individual solutions are very precise, with standard errors (and actual reproducibility) on the order of 1-2%. The dose or radical concentration dependence discussed here is determined at each scavenger concentration for FIDs collected back-to-back for two or three different pulse widths.Consequently, the relative slopes of the scavenging plots for different pulse sizes can be accurately measured to well within 10%. The larger absolute errors estimated for these rate constant measurements arise primarily from imprecisedetermination of the actual concentration of scavenger in the solutions, as well as from the systematic errrors which we discuss. (16) Smaller, B.; Avery, E. C.; Remko, J. R. J. Chem. Phys. 1971, 55, 2414. (17) Bartels, D. M.; Trifunac, A. D.; Lawler, R. G. J . Chem. Phys. 1985, 83, 2686. (18) Bartels, D. M.; Trifunac, A. D.; Lawler, R. G. Chem. Phys. Lett. 1988, 152, 109. (19) Benderskii, W. A.; Krivenko, A. G.; Rukin, A. N. Khim. Vys. Energ. 1980, 14, 400. [English translation: High Energy Chem. 1980, 14, 3031. (20) Adrian, F. J. Reu. Chem. Intermed. 1979, 3, 3. (21) Freed, J. H.; Pedersen, J. B. Adu. Magn. Reson. 1976, 8, 1.
