J. Phys. Chem. 1994, 98, 10578-10583
10578
Rate Constant and Activation Energy Measurement for *he Reaction of Atomic Hydrogen with Methanol, Iodomethane, Iodoethane, and 1-Iodopropane in Aqueous Solutiont Stephen P. Mezyk* Research Chemistry Branch, AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, ROE 1 u ) Canada
David M. Bartels Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received: June 4, 1994; In Final Form: August 15, 1994@
The Arrhenius parameters for the reaction of hydrogen atoms with methanol in aqueous solution have been determined by use of pulse radiolysis and electron paramagnetic resonance free induction decay attenuation measurements. At 25.5 "C, an absolute scavenging rate constant of (2.84 f 0.07) x lo6 dm3 mol-' s-' has been measured, and over the temperature range 10.2-86.4 "C, an activation energy of 29.4 f 0.8 kJ mol-' has been determined. These values have been used to make minor corrections to rate constant and activation energy values for the reaction of aqueous hydrogen atoms with iodomethane, iodoethane, and 1-iodopropane. For these three compounds at 24.3 "C, rate constants of (1.17 f 0.07) x lolo, (1.39 f 0.05) x lolo, and (1.42 f 0.06) x 1Olo dm3 mol-' s-' and corresponding activation energies of 10.4 f 0.4, 11.8 f 0.4, and 11.9 f 0.4 kJ mol-' have been respectively calculated over the temperature range 9-52 "C. These fast reaction rate constants and the similarity of the activation energies suggest that the mechanism of reaction for all these iodoalkane scavengers is dominated by halogen abstraction, 'H RI H+ I- 'R. Comparison of the methanol scavenging rate to gas phase results suggests that tunneling which contributes to the gas phase reaction may be quenched in aqueous solution.
+
1. Introduction Radioactive iodine is generally recognized as one of the most hazardous fission products that could be released to the environment following a nuclear reactor accident in which fuel failure occurs.1 This estimate is based on a combination of its total fuel inventory, a short half-life, and a diverse chemistry which may produce volatile species and high biological activity. The initial release of iodine from fuel is expected to be in the form of C S I , ~and , ~ because of the large quantities of water also assumed present in most accidents at water-cooled reactors, a good knowledge of the aqueous chemistry of iodide is necessary to understand its subsequent behavior. These considerations have stimulated a resurgence of interest in the radiation-induced chemistry of iodine-containing c o m p o ~ n d s . ~ - ~ The high radiation fields present after a reactor accident have been shown to significantly affect the iodine chemistry that can occur.6 Reactions of nonvolatile iodide that lead to the formation of volatile are of particular importance. The formation of volatile, low molecular weight iodoalkanes, from the radiolysis of aqueous iodine species in the presence of organic impurities, has also been demon~trated.~-IOAlthough some radiolytic yields are e~tablished,~ very little rate constant or mechanistic information is available. Such information is necessary for an understanding of these radiolytic processes, and for the confirmation of proposed multistep mechanisms by computer modeling. The overall concentration of volatile iodine species is also dependent on the further radiation-induced chemistry of iodoalkanes under reactor containment conditions. The reactions of
* To whom correspondence is to be addressed.
' Work performed at Argonne under the auspices of the Office of Basic
Energy Sciences, Division of Chemical Science, US-DOE, under Contract W-3 1-109-ENG-38. @Abstractpublished in Advance ACS Abstracts, September 15, 1994.
0022-365419412098-10578$04.50/0
-
+ +
the hydrated electron with primary iodoalkanes in aqueous solution, to give nonvolatile iodide and alkyl radicals, are known to have1'-13 room temperature reaction rate constants of -1.5 x 1O1O dm3 mol-' s-'. However, a survey of the literature showed that such rate constants for reaction of hydrogen atoms or hydroxyl radicals have only been studied for diiod~methane.'~J~ Values for the reactions *OH 4- CH,I,
*OH -tCHJ,
-
H,O
+ 'CHI,
k = 6.3 x lo9 dm3 mol-' s-' (1)
CH,I,('OH)
k = 2.1 x lo9dm3 mol-' s-' ( 2 )
were directly measured by monitoring the product radical absorption^,'^ and for hydrogen atom reaction 'H -t CH,I,
-
I-
+ 'CH,I + H+
k = 1.2 x 10" dm3 mol-' s-' (3)
by using a conductometric te~hnique.'~For hydrogen atom reaction with CH31 a lower limit reaction rate constant of '2 x lo9 dm3 mol-' s-' was obtained. This is the only information which exists for monohalogenated iodoalkanes,16 which have been shown to be the dominant species produced under reactor accident conditions.I7 In the present study, rate constants and activation energies for the reaction of hydrogen atoms with iodomethane, iodoethane, and 1-iodopropane have been determined in aqueous solution. Direct EPR detection of the decay of the 'H atom following pulse radiolysis was the monitoring method of choice.'*Jg The pulsed EPR-based free induction decay (FID) attenuation meth~d,O-~~ has been shown to be particularly
Published 1994 by the American Chemical Society
J. Phys. Chem., Vol. 98, No. 41, 1994 10579
Reactions of H with Methanol and Iodoalkanes advantageous for rate constant measurements because of the pseudo-first-order scavenging kinetics generally obtained. Although these iodoalkanes are relatively soluble in water,25 their rate of dissolution was found to be slow. In order to enhance this process, mixed iodoalkane/methanol solutions were used. These mixtures dissolved quickly in solution; however, this also changed the concentration of methanol with each addition. The reaction rate of hydrogen atoms with methanol is known to be slow at room temperature, -2.5 x lo6dm3mol-' s - ' , , ~ but an accurate value was necessary for correcting the iodoalkane data. Moreover, no temperature dependence of the methanol reaction rate constant had been determined. Consequently, the Arrhenius parameters for methanol reaction with the hydrogen atom were also measured in this study.
I
I/
1 .o
~
2
000
004
0.02
2. Experimental Section
+
-
'H
k = 2.3 x 10" dm3 mol-' s-'
(4)
28
and lo-, mol dm-3 methanol was initially added to scavenge all the hydroxyl radicals,
'OH
+ CH30H -H,O + 'CH,OH
~
006
008
010
12 ns pulse
~ 012
" 014
016
~ 018
[Methanol] (dm3 mor')
The experimental procedure used for the methanol experiments was essentially the same as has been described in detail in several previous publication^,^^-^^ and thus only a brief description shall be given here. Hydrogen atoms were generated in aqueous solution within an EPR cavity by pulse radiolysis with 3 MeV electrons from a Van de Graaff accelerator. Stock solutions of pH 2.0 were prepared by addition of HC104 to Millipore-filtered water. The exact acid concentration was determined by standardization of the concentrated HC104 against 1.029 N HCl (Aldrich, Volumetric Standard). As the vapor pressure of methanol," and also of the i o d o a l k a n e ~is , ~high, ~ no head spaces could be tolerated. Thus, the standard recirculating system was completely filled with Ar-saturated stock solution (203.5 f 1.0 mL) and then sealed. This solution was flowed through a flat cell in the cavity at a rate sufficient to ensure that each cell volume was completely replaced between pulses. The actual volume irradiated in each pulse was less than 0.10 mL. The approximate average radiation dose for this cell volume was 0.3, 1.5, and 3.0 krad/pulse for the 5, 12, and 25 ns pulses used, respectively. For extrapolationto obtain the limiting, zero dose rate constants (see later), the relative dose values used were simply the average beam currents measured on a shutter positioned before the irradiation cell for the three pulse widths. These currents were checked frequently to allow for any small drift in the beam. A 35 ns microwave probe pulse was applied to the sample immediately after irradiation, and the resulting free induction decay of the 'H atom low-field (mI = l/2) EPR transition was recorded on a digital oscilloscope. Typically, 500-2000 shots were averaged to measure each FID,at a repetition rate of 120 Hz. Scavenging experiments were performed by successive injection of methanol (Aldrich, HPLC Grade, 99.9%+), which had also been saturated with argon. Accuracy of these concentrations is estimated at better than 2%. For the iodoalkane experiments, a stock solution of pH 1.0 was used to ensure complete scavenging of the hydrated electrons to form hydrogen atoms,
eaq- H+
~
J
25nspulse
k = 9.7 x lo8 dm3 mol-' s-l
29
(5)
Standard 1.OO% iodoalkane solutions (CH31, Aldrich, 99.5%; C2H5I, Aldrich, 99%; C3H71, Aldrich, 99%) in methanol were
Figure 1. Dose dependence of the aqueous hydrogen atom scavenging rate constant determination for methanol reaction at pH 2.0 and 25.5 "C.
made, and successive aliquots were injected into the recirculating stock acid solution. The accuracy of these concentrations is estimated at better than 3%. In order to minimize the iodoalkane degradation, the dose per pulse was lowered, the repetition rate was reduced to 60 Hz, and only 2000 shots were averaged per measurement. By a slight adjustment of the flow rate, this ensured that the entire sample was irradiated only once for each FID collected. The smaller signals led to reduced precision, with -5% uncertainty in each measured rate constant, but within this experimental error no systematic effect due to scavenger depletion could be observed in successive FID determinations.
3. Results and Discussion 'H Reaction with Methanol. The overall hydrogen atom scavenging rate constant at pH 2.0 and 25.5 "C for the reaction 'H
+ CH30H -H, + 'CH,OH
(6)
was determined at three different pulse widths (doses), with these typical values being shown in Figure 1. Although excellent linearity for these scavenging plots is observed, a slight dose dependence is evident from the slopes of (3.58 f 0.03) x lo6, (3.19 f 0.02) x lo6, and (2.94 f 0.03) x lo6 dm3 mol-' s-' for the 25, 12, and 5 ns pulse widths, respectively. The general expression for the effective damping rate of the FID in these experiments is given by20-22 (7) where k,[S] is the 'H atom scavenging rate and CkL,[Ri] represents the spin-dephasing contribution of second-order spin exchange and recombination reactions between 'H atoms and other free radicals. The observed dose dependence occurs when the latter term is not approximately constant over the 5 ps experimental time scale.u To correct the rate constants for this dose dependence, limiting values were calculated by extrapolation to zero dose, as shown in Figure 2. An excellent linear relationship was obtained, and a limiting value of (2.84 f 0.07) x IO6 dm3 mol-' s-l was determined. This procedure was then repeated over the temperature range 10.2-70.9 "C. (Extrapolation to zero dose for 70.9 "C is also shown in Figure 2.) The calculated Arrhenius plot is shown in Figure 3 with all the extrapolated values given in Table 1.
~
10580 J. Phys. Chem., Vol. 98, No. 41, 1994 20.0
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Mezyk and Bartels
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TABLE 1: Summary of the Temperature-DependentRate Constant Data for Hydrogen Atom Reaction with Methanol, Iodomethane, Iodoethane, and 1-Iodopropanein Aqueous Solution scavenging rate constant, species temp, "C dm3 molTi s-l
1
~~~
methanol
10.2 16.7 25.5 30.3 43.0 57.7 70.9 74.8 86.4 8.5 17.4 24.1 24.2 40.4 50.5 9.5 17.5 24.4 40.4 52.6 11.5 17.5 24.3 40.3 52.8
(1.67 & 0.07) x (2.21 f 0.06) x (2.84 f 0.07) x (3.64 i0.11) x (6.22 f 0.19) x (1.01 f 0.16) x (1.32 i 0.17) x (1.80 f 0.27) x (2.26 f 0.45) x (9.44 f 0.13) x (1.06 f 0.03) x (1.17 f 0.07) x (1.21 f 0.05) x (1.44 f 0.06) x (1.69 f 0.13) x (1.07 f 0.01) x (1.28 & 0.02) x (1.39 & 0.05) x (1.79 f 0.04) x (2.11 f 0.12) x (1.11 f 0.02) x (1.29 f 0.02) x (1.42 f 0.06) x (1.81 f 0.04) x (2.14 f 0.29) x
lo6 lo6 lo6 lo6 lo6 107 lo7 lo7 lo7 109 1Olo 1Olo 1O'O 1Olo 1OIo 1O'O 1O'O loio 1O'O 10'0 10'0 1O'O 1O'O lolo 1Olo
4 0 F iodomethane 20
00
40
60
80
12 0
100
Relative dose
Figure 2. Rate constant extrapolations to zero dose for aqueous hydrogen atom reaction with methanol at pH 2.0 and 25.5 "C (N) and 70.9 "C (0).Error bars shown correspond to one standard deviation obtained from the linear fit to the FID scavenging plots.
17.0
1
I
'
I
L\+
'
I
I
At the higher temperatures of this study, several single-point determinations, using the 12 ns pulse, were also performed. These measured rate constants were also corrected for dose effects, by calculation of their expected dose dependence. These corrections were based on the temperature dependence of the extrapolations performed at lower temperatures (see Figure 2). Although these calculated, zero-dose rate constants are believed less accurate than the extrapolated data (all data points shown in Figure 3 show lo error bars), they are seen to be in very good agreement with the extrapolated values. From a weighted linear fit on these measurements over the temperature range 10.2-86.4 "C, the temperature-dependent rate constant is well described by In k6 = (26.8 f 0.4) - (3540 f 95)/T
iodoethane
(8)
1-iodopropane
measured a value of 2.4 x lo6 dm3 mol-' s-l at 281 K; our calculated value of 1.44 x lo6 dm3 mol-' s-' is much slower. The rate constant of Neta et a1.16 at 15 "C is 1.6 x lo6 dm3 mol-' s-', within error of our calculated value of 1.96 x lo6 dm3 mol-' s-'. However, their other determination at 30 "C (-2.0 x lo6 dm3 mol-' s-l) is much slower than our measured value of (3.64 f 0.11) x lo6 dm3 mol-' s-' at the same temperature. The activation energies measured in this work and the several preceding studies in our l a b o r a t ~ r y ~are~ nearly - ~ ~ ~the ~ ~first available for 'H atom reactions in solution. The results for 'H abstraction reactions to date are too sparse to establish any general pattems. Nevertheless, comparison of the present results for methanol scavenging with the previous result for addition to benzene23provides an interesting contrast. In the benzene scavenging reaction, 'H atoms add to the benzene ring, forming the cyclohexadienyl radical. The average activation energy of 19.1 kJ mol-' in water is virtually identical to the gas phase value, but the preexponential factor is some 54 times larger, and some slight curvature (concave downward) of the Arrhenius plot is visible in the 5-90 "C temperature range. These properties of the reaction were correlated in a nearly quantitative fashion by application of the transition state theory.23 It is straightforward to show that the ratio of liquid and (high pressure) gas phase reaction rates can be given in terms of the free energies of solvation of the reactants and transition state (t),as long as tunneling is not a significant component of the reaction rate:
with kg and T in units of dm3 mol-' s-' and K, respectively. This corresponds to an activation energy of 29.4 f 0.8 kJ mol-'. There have been many previous determinations, using a variety of techniques, of the rate constant for this r e a c t i ~ n . ~ ~ . ~ ~ For the particular case of the benzene addition reaction, the Our room temperature calculated value of 2.64 x lo6 dm3 mol-' transition state and benzene itself are very similar in size and s-' (T = 22 "C) is in excellent agreement with the recommended shape; the solutiodgas phase ratio reduces to just value of 2.6 x lo6 dm3 mol-' s - ' . ~ ~However, there is very exp[hC&~(.H)], which is simply the (inverse) Ostwald solubility little rate constant data available at other temperatures for coefficient for the hydrophobic 'H atom. On the basis of the comparison of the activation energy. Smaller et aL3' directly near equivalence of 'H and H2 polarizability and size, and their
Reactions of H with Methanol and Iodoalkanes
J. Phys. Chem., Vol. 98, No. 41, 1994 10581
necessary similar solubilities, it was demonstrated that the solvent rate enhancement and the curvature in the Arrhenius plot stem almost entirely from the free energy associated with collapse of the ‘H atom hydrophobic solvation shell.23 That is, “hydrophobic attraction” of the ‘H and benzene in water induces many more attempts of the barrier crossing and a correspondingly larger rate constant. The solvent rate enhancement was much smaller (k(aq)/k(gm)= 3.7) for the light muonium atom (p+-e-). Although the hydrophobic attraction between the muonium atom and benzene should still be present, the large tunneling component present in the gas phase reaction33 is quenched in the One might predict a similar rate enhancement for the ‘H atom reaction with other organic molecules where a hydrophobic “attraction” may be expected. However, no enhancement is found for the reaction of methanol. At room temperature, the gas phase reaction rate is352.3 x lo6 dm3 mol-’ s-l compared with the aqueous rate of 2.84 x lo6 dm3 mol-‘ s-l found in this study. The liquid phase activation energy is 6.5 kJ mol-’ larger than the gas phase result.35 One could argue that the water will solvate the transition state differently from the methanol, so that the aqueous activation energy should be different from the gas phase value. Also contributing to this difference may be solvent quenching of tunneling. Using a simple theoretical treatment of the abstraction reaction, Roduner and F i s ~ h e Sshowed ~ , ~ ~ that tunneling through the barrier is very important for the ‘H atom reaction in the gas phase, with an estimated tunneling factor of 6.8 at room temperature. Removal of this gas phase tunneling enhancement by the solvent “friction”34would counteract the hydrophobic attraction effect and also yield a somewhat higher activation energy (Le., in the absence of tunneling, the system must always cross over the barrier, so that the activation energy under these conditions reflects the “true” potential). However, the curvature of the Arrhenius plot characteristic of the ‘H atom solubility coefficient seems to be missing in the methanol reaction. Neither is any curvature apparent in our recent measurements of ‘H abstractions from acetone, 2-butanone, propionaldehyde, and b ~ t y r a l d e h y d e .It~ ~is difficult to see how the solvation free energy of the hydrophobic ‘H atom can fail to be recovered at the transition state in its reaction with another (larger) hydrophobic group or species. As we noted above, this “hydrophobic effect” should simply result in a larger number of attempts to cross the barrier, regardless of its nature. We can speculate that, for the case of ‘H abstraction reactions yielding H:!, the transition is “late”, with the H:! molecule largely formed. It may be that the hydrophobic cage is nearly re-formed about the H:! product; if this were true, then the free energy gained in the hydrophobic attraction of ‘H to the organic scavenger would be lost again to the escaping H2 molecule. The result would be no net “hydrophobic” curvature of the Arrhenius plot. However, one should be careful not to overinterpret this point, as the predicted curvature may yet be distinguished by more points and better signal-to-noise data. ‘H Reaction with CHJI. Initial experiments on CH31showed that the FID attenuation was measurable over the concentration range (30-150) x mol dm-3. However, the measurement of the scavenging rate constants under the standard dose conditions was not possible, as major degradation at the lower CH31 concentrations occurred. This was evidenced by the nonlinear scavenging plots obtained (see Figure 4). This curvature was attributed to a reaction analogous to hydrogen atom reaction with C H ~ I ~i.e. ,’~
‘H iCH31
-
I-
+ ‘CH, + Hf
(10)
t 2.4
-
2.0
-
y,,,
1.6
-
’Pro
1.2
-
-2
m
i
”
0.4 o’8
i
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
lo5 (CH31](mol d
12.0
140
16.0
18.0
d )
Figure 4. Scavenging rate constant measurements for aqueous hydrogen atom reaction with iodomethane in pH 1.0 solution at 24.4 “C for high (0)and low (m) dose conditions (see text for actual values).
and because the subsequent hydrogen atom reaction with the product iodide is much slower,
+
31 I-
-
HI’-
k = 2.4 x 10’ dm3 mol-’ s-l
24
(11)
an overall decrease in the measured rate constant is observed with increasing dose. Moreover, the 25 and 12 ns pulses produce hydrogen atom concentrations of -10 x and 5 x mol dm-3, respectively, under these experimental conditions, which does not give pseudo-first-order conditions at the lower iodoalkane concentrations. This problem was remedied by lowering the dose until linearity was observed, and there was no further change in the measured scavenging rate constant. This occurred at a dose of -0.7 h a d , which corresponds to hydrogen atom concentrations of -2.5 x mol dm-3. Under these reduced dose conditions, the measured scavenging curve for hydrogen atom reaction with methyl iodide at 24.1 “C is shown in Figure 4. After making the small correction due to the hydrogen atom reaction with the changing methanol concentration (this correction was done for all the iodoalkane systems studied), a calculated rate constant of (1.17 f 0.07) x lolo dm3 mol-’ s-l was obtained. This rate constant is about an order of magnitude faster than the lower limit value of 2 x lo9 dm3 mol-l s-l determined previously16 and is much faster than typically found for ‘H atom abstraction reactions.26 The excellent agreement with the room temperature value for hydrogen atom reaction with CH212, 1.2 x 1O’O dm3mol-’ s-l,15 however, supports our belief that the hydrogen atom reaction with iodomethane also consists of a iodine atom abstraction by the hydrogen atom (eq 10). These experiments were repeated over the temperature range 8.5-50.5 “C, with the measured rate constants again given in Table 1, and the Arrhenius plot is shown in Figure 5 . The temperature-dependent rate constant is well characterized by the equation Ink,, = (27.4 f 0.2) - (1250 f 50)/T
(12)
which corresponds to an activation energy of 10.4 & 0.4 kJ mol- l. The upper temperature of this study was limited by the minimum concentration of CH31required to maintain pseudofirst-order conditions (-2.0 x mol dm-3). For all experiments, the thermal hydrolysis reaction
10582 J. Phys. Chem., Vol. 98, No. 41, 1994
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236
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"
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"
'
Mezyk and Bartels
I
1
Ink,, = (28.1 f 0.2) - (1420 k 40)/T
-
E
Y
-C 2 3 4 -
232
-
230
-
30
values again listed in Table 1 and the Arrhenius plot also shown in Figure 5. For this system, the temperature-dependent rate constant is given by
31
33
32
34
35
36
l o 3 / Temperature (K)
Figure 5. Arrhenius plots of In krmvs 1IT for aqueous hydrogen atom reaction with iodomethane (W), iodoethane (O),and l-iodopropane (A). Note that the latter values for C3H7I have been offset by 0.20 In unit to facilitate comparison in the plot.
CH,I
+ H,O -CH30H + I- + H+
(13)
was not important, as even at 50 "C, the half-life for this reaction under these conditions is greater than 71 h.38 Comparison of the Arrhenius values for haloalkane reactions in water with gas phase results is difficult because there are very few numbers for the gas and because the reactions fall into the "partially" diffusion-limited category. The overall measured rate constant can be decomposed using 1 - 1 kobs
kdiff
+- 1 kact
is the diffusion-limited rate of encounters, and llkactis the thermally activated rate constant obtained from transition state theory. The latter quantity may be usefully compared with gas phase numbers, as was done for the methanol reaction in the previous section. kdiff is approximately given by the Smoluchowski equation, which predicts a rate of encounters proportional to the relative diffusion coefficients of the reactants. For 'H atom reactions in water, the activation energy for a diffusionlimited process should be very close to 14 W The present reaction of 'H with CH31has activation energy of only 10.4 kJ mol-', which suggests that kdiff and kaCtare of similar magnitude, but k,,, has activation energy less than 10 W mol-'. The only gas phase activation data for this reaction40 deduced a value of E, < 13.4 kJ mol-', consistent with our result in water. The room temperature rate constant is (5.86 f 0.29) x lo9 dm3 mol-' s-' in the gas:' about a factor of 2 slower than found here for water. The slightly larger rate in water seems to be typical for halogen atom abstraction by 'H atoms!2 'H Reaction with C2H5I. Identical experiments were performed for the measurement of hydrogen atom reaction with C&I, using the same low dose and pH conditions. At 24.4 "C a scavenging rate constant of (1.39 & 0.05) x 1O1O dm3 mol-' s-' was obtained. Although slightly higher than measured for CH31, its similarity with the CH31and CH2Iz values again suggests that the reaction mechanism is dominated by a halogen atom abstraction reaction to give kdiff
'H
+ C,H,I
-
H+
+ I- + 'C,H,
(15)
These scavenging rate constant measurements were repeated over the temperature range 9.5-52.6 "C, with the individual
(16)
corresponding to an activation energy of reaction of 11.8 f 0.4 W mol-'. Again the effects of hydrolysis were negligible on the experimental time scale.43 The measured rate constant at room temperature is consistent with the trend observed for aqueous hydrogen atom reactions with CZH~CI and C2H5Br, 1.7 x 10, and 1.6 x lo8 dm3 mol-' s-', respectively.'6 The slight increase in rate constant over CH3I is consistent with the larger value obtained for iodoethane reaction in the gas phase,44where a value of -7.5 x lo9 dm3 mol-' s-' was measured, and also in qualitative agreement with the gas phase activation energy40 of '13.4 kJ mol-'. 'H Reaction with C3H71. The small increases in the Arrhenius parameters observed between hydrogen atom reaction with iodomethane and iodoethane in aqueous solution did not continue for l-iodopropane. The measured value for the reaction
'H
+ C,H,I -H+ + I- + T,H,
(17)
at 24.3 "C, under the same experimental conditions, was (1.42 f 0.06) x 1O1O dm3 mol-' s-l, within error of the value for CzH51. This observed equality between rate constants for hydrogen atom reaction with iodoethane and l-iodopropane differs from that observed in the gas phase:' where for l-iodopropane a slightly faster rate constant of -1 x 1011dm3 mol-' s-' was obtained. Similar concordance in this investigation was found over the entire temperature range studied (1 1.552.8 "C). The individual scavenging rate constants obtained for C3H7I are listed in Table 1, and the corresponding Arrhenius plot is also shown in Figure 5. Note that to distinguish this system from iodoethane in this plot, the values for C3H71have been offset by 0.20 unit. The temperature dependence of this rate constant is given by
In k17 = (28.2 & 0.2) - (1440 f 50)/T
(18)
which gives an activation energy of 11.9 f 0.4 kJ mol-'. Such close agreement to iodoethane indicates that its reaction mechanism also occurs for C3H71.
4. Conclusion The Arrhenius parameters for aqueous hydrogen atom reaction with methanol, iodomethane, iodoethane, and l-iodopropane have been determined. The direct measurements of the scavenging rate over the temperature range 10.2-86.4 "C have shown that the rate constant for 'H atom abstraction from methanol, 'H
+ CH,OH - H, + 'CH,OH
(6)
is described by Ink, = (26.8 f 0.4) - (3540 f 95)/T
(8)
where the stated uncertainties are la and the calculated rate constants are in units of dm3 mol-' s-l. Comparison with gas phase data shows no evidence of rate constant enhancement due to hydrophobic caging. A hydrophobic enhancement may be masked by quenching of the tunneling effect present in the gas phase.
Reactions of H with Methanol and Iodoalkanes
J. Phys. Chem., Vol. 98, No. 41, 1994 10583
The Anhenius expressions for CH3I, C2H5I, and C3H71, over the temperature range 9-52 "C, are Ink,, = (27.4 f 0.2) - (1250 f 50)/T
(12)
Ink,, = (28.1 f 0.2) - (1420 f40)/T
(16)
Ink,, = (28.2 f 0.2) - (1440 f 50)/T
(18)
respectively. The similarity of these parameters indicates that the reaction mechanism is the same for all these iodoalkanes, consisting of iodine atom abstraction to produce I-, H+, and the corresponding alkyl radical. Acknowledgment. The authors thank Dr. David Werst for his assistance in operating and maintaining the Van de Graaff accelerator. References and Notes (1) Thompson, T. J.; Beckerley, J. G. The Technology of Nuclear Reactor Safety; MlT Press: Cambridge, 1973. (2) Garisto, F. Thermodynamics of lodine, Cesium and Tellurium in the Primary Heat Transport System Under Accident Conditions; Atomic Energy of Canada Limited Report, AECL-7782, 1982. (3) Cubicciotti, D.; Sanecki, J. E. J . Nucl. Mater. 1W8, 78, 96. (4) Proceedings of the 1st CSNI Workshop on Iodine Chemistry in Reactor Safety; Deane, A. M., Potter, P. E., Eds.; Harwell Research Report, AERE-R 11974, 1986. (5) Proceedings of the Second CSNI Workshop on Iodine Chemistry in Reactor Safety; Vikis, A. C., Ed.; Atomic Energy of Canada Limited Research Report, AECL-9923, 1989. (6) Proceedings of the 3rd CSNI Workshop on Iodine Chemistry in Reactor Safety; Ishigure, K., Saeki,M., Soda, K., Sugimoto, J., Us.; JAE Research Report, JAERI-M 92-012, 1992. (7) Paquette, J.; Ford, B. L. Radiat. Phys. Chem. 1990, 36, 353. (8) Beahm, E. C.; Wang, Y.-M.; Wisbey, S. J.; Shockley, W. E. Nucl. Technol. 1987, 78, 34. (9) Lutz, J. B.; Kelly, J. L. Nucl. Technol. 1987, 80, 431. (10) Ono, S.; Fujita, N.; Fujiwara, K.; Mochizuki, H.; Shiraishi, H.; Ishigure, K. Proceedings of the International Conference on Water Chemistry of Nuclear Reactor Systems 4; British Nuclear Energy Society: London, 1986. (11) Bullock, G.; Cooper, R. Trans. Faraday Soc. 1970, 66, 2055. (12) Thomas, J. K. J . Phys. Chem. 1967, 71, 1919. (13) Szutka, A.; Thomas, J. K.; Gordon, S.; Hart, E. J. J. Phys. Chem. 1965, 69, 289. (14) Mohan, H.; Moorthy, P. N. J . Chem. Soc., Perkin Trans. 2 1990, 277.
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