Reaction of Methyl Radicals with Iodine Species
J. Phys. Chem., Vol. 100, No. 22, 1996 9363
The reduction potential for the iodine atom is 1.33 V vs
NHE,29 much less than the ionization potential of DMSO (>2.0
V),30 and therefore electron abstraction reactions to give iodide
and the DMSO cation cannot occur. Therefore, the only other
pathway for this radical is recombination11
dm3 mol-1 s-1, much faster than our observed value of (2.75
( 0.23) × 109 dm3 mol-1 s-1. This discrepancy is possibly
due to the value of Φkd found for iodine reaction in hydrocar-
bons, which has been noted to be very high,22 greater even than
the diffusion-controlled rate constant. This was attributed to
the Φ value being almost unity in hydrocarbons, which may
not be correct for this reaction in water.
2I• f I2
2k ) 2.0 × 1010 dm3 mol-1 s-1 (12)
Other Reactions. Following the methodology established
for molecular iodine reaction with methyl radicals, similar
experiments were conducted to determine the extent of methyl
radical reaction with iodate, reaction 3, and iodide in aqueous
solution. In contrast to the iodine results, the addition of 10-2
mol dm-3 of either of these substances produced no significant
change in the second-order decay of the methyl radical. From
the fitting of a combined first- and second-order decay to these
curves, upper rate constants for both iodate and iodide reaction
with the methyl radical at 23 °C were estimated as k < 106
To determine the importance of this reaction on the monitored
methyl radical decay, modeling studies were performed. Using
reactions 1, 2, 7, and 12 with the rate constants listed above (k2
was assumed to be 1.0 × 1010 dm3 mol-1 s-1) and taking the
room temperature (22.8 °C) “worst case” experiment of [•OH]
) 2.0 × 10-5 mol dm-3 and [I2] ) 3.0 × 10-5 mol dm-3, it
was found that the inclusion of reaction 12 caused a 15%
increase in the final (50 µs) concentration of I2. From the
combined order fitting of this calculated methyl radical con-
centration time profile, again keeping the 2k7 value constant to
be consistent with the experimental analysis, this concentration
increase translated to a fitted k1 value that was about 35% higher.
However, when the initial iodine concentration was increased,
the calculated difference quickly became much smaller. By
modeling all of the iodine concentrations measured at this
temperature, an overall error of ∼15% in the second-order rate
constant for reaction 1 was obtained. This value is comparable
to the experimental scatter.
These calculations imply that our determined k1 ) 2.75 ×
109 dm3 mol-1 s-1 value is a lower bound rate constant;
however, given the uncertainty in the literature reaction rate
constants and the relatively large scatter in the data, we believe
that a correction to the experimental values is not warranted.
When the modeling was performed for the other temperatures
of this study, with diffusion-controlled activation energies being
assumed for the reactions without experimental values, similar
results were obtained. To make allowance for this potential
error in the k1 values summarized in Table 1, the errors stated
are simply the larger of the experimental scatter or the calculated
fitting differences.
dm3 mol-1 s-1
.
Conclusion
Direct ESR measurement of methyl radicals in aqueous
solution has been used to determine Arrhenius parameters for
the recombination reaction
2•CH3 f C2H6
and the reaction of the methyl radical with molecular iodine
(7)
•CH3 + I2 f CH3I + I•
(1)
in aqueous solution. Over the temperature range 5.7-39.2 °C,
the experimental data were found to be well fitted by the
Arrhenius equations
log 2k7 ) (11.86 ( 0.16) - [(14890 ( 870)/(2.303RT)] (8)
and
log k1 ) (11.75 ( 0.13) - [(13100 ( 710)/(2.303/RT)]
There is another potential pathway that could be important,
where the formed •CH3I2 complex dissociates to produce CH3I•+
and iodide,
(11)
respectively. The corresponding rate constant for methyl radical
reaction with iodate or iodide at room temperature was found
•CH3I2 f CH3I•+ + I-
(13)
to be <106 dm3 mol-1 s-1
.
The methyl iodide radical cation has been well characterized
in aqueous solution31,32 and is known to react with DMSO by
electron abstraction to give methyl iodide.33 This mechanism
would therefore give the anticipated products and also be
consistent with the experimental observations. Unfortunately,
the elucidation of the dominant mechanisms was beyond the
scope of this study.
Acknowledgment. We thank Drs. John Elliot and Joanne
Ball for their helpful comments on the analysis procedures used
in this study.
References and Notes
(1) Thompson, T. J.; Beckerley, J. G., Eds. The Technology of Nuclear
Reactor Safety; MIT Press: Cambridge, 1973.
(2) Campbell, D. O.; Malinauskas, A. P.; Stratton, W. R. Nucl. Technol.
1981, 53, 111.
(3) Cubicciotti, D.; Sehgal, B. R. Nucl. Technol. 1984, 65, 266.
(4) Proceedings of the 1st CNSI Workshop on Iodine Chemistry in
Reactor Safety; Deanne, A. M., Potter, P. E., Eds.; Harwell Research Report,
AERE-R 11974, 1986.
(5) Proceedings of the 2nd CNSI Workshop on Iodine Chemistry in
Reactor Safety; Vikis, A. C., Ed.; Atomic Energy of Canada Ltd Research
Report, AECL-9923, 1989.
(6) Proceedings of the 3rd CNSI Workshop on Iodine Chemistry in
Reactor Safety; Ishigure, K., Saeki, M., Soda, K., Sugimoto, J., Eds.; JAE
Research Report, JAERI-M 92-012, 1992.
(7) Paquette, J.; Ford, B. L. Radiat. Phys. Chem. 1990, 36, 353.
(8) Chuaqui, C. A.; Ball, J. M. Presented at the 4th Meeting of the
European Severe Accident Chemistry Group, ENEL/CRTN, Milano, Dec
1992.
The addition of iodine to elucidate radiation-induced reaction
mechanisms in hydrocarbons has been used for many years.34
In contrast, there has been only one previous, indirect, deter-
mination of the rate constant for hydrocarbon radical reaction
with iodine in aqueous solution.10 This study, which determined
the value for k1 by competition with peroxy radical formation
in aerated aqueous solutions, calculated an approximate rate
constant of 6 × 109 dm3 mol-1 s-1 at room temperature,
significantly faster than our measured value at 22.8 °C of k1 )
(2.75 ( 0.23) × 109 dm3 mol-1 s-1
.
The reaction of hydrocarbon radicals with iodine in cycloal-
kanes (C5 to C10) and alkanes (C6 to C17) was again found to
be accurately described by eq 9,22 with the parameters ka )
2.44 × 1010 dm3 mol-1 s-1 and Φkd ) 1.56 × 1010/η dm3 mol-1
mPa. This gives the predicted value in water as 1.02 × 1010
(9) Wren, J. C.; Ball, J. M.; Chuaqui, C. A. The Chemistry of Iodine
in Containment; CANDU Owners Group Report, COG-94-116, 1994.