The reduction of I2 by 1-hydroxyalkyl radicals in aqueous solution. A pulse radiolysis study

Article abstract of DOI:10.1016/S0009-2614(03)00401-9

The one electron reduction of iodine by various 1-hydroyalkyl radicals in aqueous solution is described, which leads first to the formation of I₂^-, and to I^- at longer times. Rate constants of 5×10⁹, 6.5×10⁹, or 8×10⁹dm³ mol^-1 s^-1 were obtained for the reaction between iodine and the radical of methanol, ethanol or 2-propanol, respectively.

Full text of DOI:10.1016/S0009-2614(03)00401-9

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Chemical Physics Letters 372 (2003) 195–198
www.elsevier.com/locate/cplett
The reduction of I2 by 1-hydroxyalkyl radicals in
aqueous solution. A pulse radiolysis study
1
B.G. Ershov ,E. Janata
*
Hahn-Meitner-Institut, Bereich Solarenergieforschung, Glienicker Str. 100, D-14109 Berlin, Germany
Received 16 December 2002; in final form 3 March 2003
Abstract
The one electron reduction of iodine by various 1-hydroyalkyl radicals in aqueous solution is described,which ꢀle1ads
3
ꢀ
first to the formation of Iꢀ2 ,and to I at longer times. Rate constants of 5 ꢁ 109, 6:5 ꢁ 109,or 8 ꢁ 109 dm mol sꢀ1
were obtained for the reaction between iodine and the radical of methanol,ethanol or 2-propanol,respectively.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
system [5,6] and the fundamental iodide equilibria
[7,8]. Basic radiation chemistry experiments show
that hydrogen atoms effectively reduce iodine with
a high rate constant of 3:5 ꢁ 1010 dm3 molꢀ1 sꢀ1
[9]. Furthermore,we report in the present Letter
on the one electron reduction of iodine by 1-hy-
droxyalkyl radicals in aqueous solution utilizing
pulse radiolysis. The reduction of iodine leads to
the formation of Iꢀ2 and demonstrates that 1-hy-
droxyalkyl radicals also have the power to reduce
iodine with high rate constants.
During the past several years,the interest in
cells for harvesting solar energy has increased
considerably. In aqueous solutions or organic
solvents,the redox couple iodine/iodide is a fa-
vorable system in photo-electrochemical solar cells
(PEC) [1]. An example would be the dye sensitized
nanostructured TiO2 PEC,where conversion effi-
ciencies of more than 10% are observed [2–4]. A
solar energy to electrical power conversion as ef-
ficient as this is only observed for a PEC con-
taining the iodine/iodide redox couple.
Considerable effort,therefore,has been spent to
understand the redox reactions in the iodine/iodide
2. Experimental
The pulse radiolysis apparatus [10] and the
computer software [11] of the 4 MeV van de Graaff
accelerator facility ELBENA at the Hahn–Meit-
ner-Institut were described previously. The dura-
tion of the electron pulses was 50 ns [12]. The
optical signals were derived from the average of 10
* Corresponding author. Fax: +49-30-8062-2434.
E-mail address: janata@hmi.de (E. Janata).
1
On leave of absence from the Institute of Physical Chem-
istry,Russian Academy of Sciences,31 Leninsky prospect,
119991 Moscow,Russian Federation.
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00401-9

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B.G. Ershov, E. Janata / Chemical Physics Letters 372 (2003) 195–198
individual experiments. The optical absorption
was calculated by dividing the absorptivity (A) by
the concentration of the radicals (c) generated in a
pulse and by the length of the optical cell (l ¼ 1:5
cm),and thus was expressed in units of molar
absorptivity (e). The absorbed dose per pulse was
calibrated by using e ¼ 1:9 ꢁ 104 dm3 molꢀ1 cmꢀ1
for the eꢀaq absorption at 700 nm [13]. The yield
of electrons in water at pH 7 was taken to be
2.6 species per 100 eV of absorbed energy (0:269
lmol Jꢀ1) [14]. The software for the simulation of
kinetic data was described elsewhere [15]. The so-
lutions were prepared daily using triply distilled
water and were deaerated with argon. All experi-
ments were performed at room temperature,i.e.,at
around 22 °C.
Fig. 1. Absorption vs. time curves at 725 nm for aqueous so-
lutions of 5 ꢁ 10ꢀ5 mol dmꢀ3 I2 at pH
2 and containing
1 ꢁ 10ꢀ3 mol dmꢀ3 2-PrOH (curve a),0 :2 mol dmꢀ3 2-PrOH
(curve b),or 2 :4 mol dmꢀ3 MeOH (curve c). Solutions were
purged with Ar. Dose per pulse corresponds to 1:2 ꢁ
10ꢀ6 mol dmꢀ3 of eꢀaq
.
3. Results and discussion
result of reaction (1). This is followed by the build-
up of an absorption ascribed to Iꢀ2 which exhibits
absorption bands at 725 and 385 nm [13]. After the
absorption of the hydrated electron vanishes,the
Iꢀ2 can be expected to be the only absorbing species
at this wavelength. The absorption vs. time curves
for two different concentrations of 2-propanol
concentrations are also shown in Fig. 1. At first
glance,it may be surprising that the build-up of
the absorption is faster the lower the alcohol
concentration. This can be attributed to the com-
petition between the alcohol,reaction (2),and the
iodine for hydrogen atoms according to
Aqueous solutions containing iodine and an
alcohol,i.e.,methanol,ethanol or 2-propanol,are
irradiated. In a solution at pH 2 and purged with
argon,hydrated electrons produced during the
radiolysis of water are converted into hydrogen
atoms according to
eꢀaq þ Hþ ) HÅ k ¼ 2:3 ꢁ 1010 dm3 molꢀ1 sꢀ1
ð1Þ
hydrogen atoms and hydroxyl radicals subse-
quently react with the alcohol (RH) to form 1-
hydroxyalkyl radicals (RÅ)
HÅ þ I2 ) Iꢀ2 þ Hþ
HÅ þ RH ) RÅ þ H2
ÅOH þ RH ) RÅ þ H2O
ð2Þ
ð3Þ
k ¼ 3:5 ꢁ 1010 dm3 molꢀ1 sꢀ1
The 1-hydroxyalkyl radical,in turn,reacts with
iodine according to
ð4Þ
where RH is methanol,ethanol or 2-propanol,and
Å
R is CH2OH,CH 3ÅCHOH or ðCH3Þ COH,and
Å
Å
RÅ þ I2 ) Iꢀ2 þ X þ Hþ;
ð5Þ
2
k2 ¼ 1:1 ꢁ 106, 1:8ꢁ107 or 5:3ꢁ107 dm3 molꢀ1 sꢀ1
,
and, k3 ¼ 9:0 ꢁ 108, 2:2 ꢁ 109 or 2:0 ꢁ 109 dm3
where X is an oxidation product of the alcohol
radical: formaldehyde,acetaldehyde,or acetone,
for methanol,ethanol,or 2-propanol,respectively.
At low alcohol concentration,both reducing spe-
molꢀ1 ꢀ1, respectively [14,16,17]. Thus, the main
s
products of the water radiolysis are converted into
1-hydroxyalkyl radicals.
Å
cies,H and RÅ,participate in the reduction of
Optical absorption vs. time curves at 725 nm
are depicted in Fig. 1 for solutions containing ei-
ther methanol or two different concentrations of 2-
propanol. During the pulse,the absorption of
hydrated electrons appears as a short spike as a
iodine,while at high alcohol concentration most of
the hydrogen atoms are scavenged by the alcohol,
and the 1-hydroxyalkyl radical is practically the
only reducing agent. The reaction between hy-

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B.G. Ershov, E. Janata / Chemical Physics Letters 372 (2003) 195–198
197
ð10Þ
ð11Þ
I2 þ Iꢀ () Iꢀ3
drogen atoms and iodine,reaction (4),seems to be
faster than the one between 2-propanol radicals
and iodine,reaction (5). Similar results were
obtained for solutions containing methanol or
ethanol.
Fig. 2 shows the spectra at various times after
the impingement of the irradiation pulse. After the
pulse the Iꢀ2 absorption at 725 and 385 nm can be
observed. As this absorption decreases,another
band at around 230 nm grows in intensity and a
bleaching at 480 nm occurs. The 230 nm band is
ascribed to the Iꢀ ion [7],while the bleaching is due
to the consumption of iodine.
kf ¼ 5:6 ꢁ 109 dm3 molꢀ1 sꢀ1
kb ¼ 7:5 ꢁ 106 sꢀ1
I þ I ) I2 2k ¼ 3 ꢁ 1010 dm3 molꢀ1 sꢀ1
The rate constants for the recombination of the
alcohol radicals are: k6 ¼ 2:4 ꢁ 109, 1:5 ꢁ 109 or
1:1 ꢁ 109 dm3 molꢀ1 sꢀ1 for methanol,ethanol,or
2-propanol,respectively [16]. The computer simu-
lation of this reaction mechanism can be carried
out unambiguously,since,with the exception of
the unknown rate constant of reaction (5),all other
rate constants and the molar absorption coeffi-
cients are well known [9,14,16–18]. Fig. 3 depicts
the experimental trace of the Iꢀ absorption at 230
nm as well as the simulated curves for the overall
absorption and for the contributions of Iꢀ, I2ꢀ and
Iꢀ3 . The reaction of the 1-hydroxyalkyl radical with
iodine results in the formation of Iꢀ2 ions,which
only contribute to the overall absorption during
the initial phase. Also included in Fig. 3 are the
overall absorption curves for a lower and a higher
value for k5 in order to show the extend to which
the overall absorption depends on this reaction.
The transient absorption curves were analyzed
using computer simulation for a reaction mecha-
nism which,in addition to reactions (1)–(5),also
consists of the following reactions
RÅ þ RÅ ) prod
HÅ þ HÅ ) H2
ð6Þ
ð7Þ
ð8Þ
ð9Þ
2k ¼ 1:55 ꢁ 1010 dm3 molꢀ1 sꢀ1
Iꢀ2 þ Iꢀ2 ) Iꢀ3 þ Iꢀ
2k ¼ 4:6 ꢁ 109 dm3 molꢀ1 sꢀ1
I þ Iꢀ () Iꢀ2
kf ¼ 8:8 ꢁ 109 dm3 molꢀ1 sꢀ1
kb ¼ 7 ꢁ 104 sꢀ1
Fig. 3. Absorption vs. time curve at 230 nm for an aqueous
solution of 5 ꢁ 10ꢀ5 mol dmꢀ3 I2 and 1 ꢁ 10ꢀ3 mol dmꢀ3 2-
PrOH at pH 2 (thin solid line). Solution was purged with Ar.
Dose per pulse corresponds to 1:1 ꢁ 10ꢀ6 mol dmꢀ3 of eaꢀq. The
result of the computer simulation (thick solid line) and the
contributions of Iꢀ (thick dashed line),I (thick dotted line),
ꢀ
2
and Iꢀ3 (dash-dotted line) are also shown for k5 ¼
Fig. 2. Spectra of a solution containing 5 ꢁ 10ꢀ5 mol dmꢀ3 I2
and 1 ꢁ 10ꢀ3 mol dmꢀ3 2-PrOH at 3 ls (solid line),10 ls
(dotted line),and 50 ls (dashed line) after irradiation. Solution
at pH 2 was purged with Ar. Dose per pulse corresponds to
8 ꢁ 109 dm3 molꢀ1
s
ꢀ1. The results of computer simulation
using 50% and 200% of k5 are also included (thin dashed lines).
Computer simulation used eðIꢀÞ ¼ 12000 dm3 molꢀ1 cmꢀ1
,
eðIꢀ2 Þ¼3800 dm3 molꢀ1 cmꢀ1 and eðI3ꢀÞ¼5000 dm3 molꢀ1 cmꢀ1
.
1:2 ꢁ 10ꢀ6 mol dmꢀ3 of eaꢀq
.

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B.G. Ershov, E. Janata / Chemical Physics Letters 372 (2003) 195–198
The simulation also indicates that at the end of the
measurement Iꢀ is the main product while the
contributions of I2ꢀ or Iꢀ3 are almost negligible.
This is also demonstrated in Fig. 2 where at longer
times there is no absorption of Iꢀ2 or I3ꢀ,the latter
with its characteristic absorption bands at 280 nm
and 350 nm [7]. The lack of Iꢀ2 and I3ꢀ indicates that
equilibria (9) and (10) have shifted to the left.
From the computer simulation of the kinetic
curves at 725 nm and 230 nm,rate constants for
the reaction between iodine and the radical of
methanol,ethanol or 2-propanol were obtained to
therefore influence the stability of the iodide/iodine
system.
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4. Conclusions
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The extremely high rate constants for the reac-
tions of some 1-hydroxyalkyl radicals with iodine
show that they are diffusion controlled. These
values are higher the larger the reduction potential
of the 1-hydroxyalkyl radicals, )1.18, )1.25,or
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