Pulse radiolysis study of I- oxidation with radical anions Cl2.- in an aqueous solution

Article abstract of DOI:10.1007/s11172-005-0413-2

Radiation-chemical transformations of chloride solutions in the presence of iodide additives were studied by pulse radiolysis. Radical anion Cl ₂^.- oxidize I^- ion, while in the secondary reactions Cl₂ reacts with I^- to form a mixed trihalide ion ICl₂^-. A reaction model that satisfactorily describes the experimental data was proposed.

Full text of DOI:10.1007/s11172-005-0413-2

1378
Russian Chemical Bulletin, International Edition, Vol. 54, No. 6, pp. 1378—1382, June, 2005
–
•–
Pulse radiolysis study of I oxidation with radical anions Cl
2
in an aqueous solution
B. G. Ershov,aꢀ E. Janata, and A. V. Gordeev
b
a
aInstitute of Physical Chemistry, Russian Academy of Sciences,
3
1 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 335 1778. Eꢀmail: ershov@ipc.rssi.ru
b
HahnꢀMeitner Institute,
1
00 Gliniker Str., 14109 Berlin, Germany*
Radiationꢀchemical transformations of chloride solutions in the presence of iodide addiꢀ
tives were studied by pulse radiolysis. Radical anion Cl2 oxidize I– ion, while in the secondꢀ
•–
ary reactions Cl reacts with I to form a mixed trihalide ion ICl2–. A reaction model that
–
2
satisfactorily describes the experimental data was proposed.
Key words: pulse radiolysis, redox reactions, halides, chlorine, iodine, ions, radicals, kinetꢀ
ics, reaction mechanism, reaction rate constants.
Radiationꢀchemical or photochemical oxidation of
this important reactions. Redox reactions involving chloꢀ
–
–
–
ions Cl , Br , and I in aqueous solutions gives rise to
rine, bromine, and iodine are widely abundant in the
nature and practical human activities. For example, water
decontamination by chlorination remains to be the main
process for preparation of drinking water. Technological
processes using halogens are widely used in industry. Takꢀ
ing into account the aforesaid, we continue to study the
kinetics and mechanism of redox reactions involving haꢀ
lide ions. In this work we studied the pulse radiolysis of
radical anions Cl2•–, Br2•–, and I2 , respectively. These
species have properties of rather strong oxidizing agents.
Rate constants of many oneꢀelectron oxidation reactions
of organic and inorganic compounds involving these radiꢀ
•–
1
2
cal anions were measured using pulse radiolysis and phoꢀ
tolysis techniques.
3
•–
It has recently been found that Cl2 reacts with the
–
•–
•–
–
Br ion to form a mixed radical anion ClBr . This speꢀ
reaction of the radical anions Cl2 with the I ions in an
aqueous solution.
cies is intermediate by its properties between the radical
anions Cl2•– and Br2 . For instance, the absorption
band maximum (λmax) in its spectrum lies at 350 nm,
•–
Experimental
3
and the molar absorption coefficient (ε) is 9.3•10
7
8
L mol–1 cm . The corresponding parameters for Cl
–1 3
•–
A pulse radiolysis technique and a software for the Van de
Graaf accelerator have been described earlier. The pulse duraꢀ
tion of electrons with an energy of 3.8 MeV was varied from 3
2
3
–1
–1 4
are 340 nm and 8.8•10 L mol cm , and those for
•
–
3
–1
–1 5
Br2
are 360 nm and 9.9•10 L mol cm . The stanꢀ
9
to 20 ns. Optical signals were obtained by the averaging of data
•
–
–
dard redox potentials (E°) for the Cl
/2 Cl and
2
of ∼ 10 pulses. The absorbed dose in a pulse was calculated from
•
–
–
6
Br2 /2 Br pairs are 2.09 and 1.66 V, respectively. The
potential of the ClBr /Cl , Br pair is 1.85 V. A comꢀ
parison of E°(Cl /2 Cl ) with E°(I /2 I ) = 1.03 V
and E°(I /I ) = 1.33 V suggests that the radical anions
10
the optical absorbance of a hydrated electron in water at 700 nm
•
–
•
–
3
4
–1
–1
and ε = 1.9•10 L mol cm . The radiationꢀchemical yield of
hydrated electrons in water at pH 7 after the reaction compleꢀ
tion was accepted to be 2.6 species per 100 eV of absorbed
•
–
–
•–
–
2
2
•
–
6
•
–
–
11
Cl2
can oxidize the I ions. However, this reaction
energy.
Concentrated solutions of NaCl were used. Therefore, the
salt (suprapur, Merck) was additionally recrystallized. The NaI
salt (Merck) was used as received. Solutions were prepared in
remained unstudied to date. It was of interest to reveal
whether the ability of chlorine to form a mixed radical
anion ClI•– with iodine is retained, as for bromine, or an
increase in the difference of oxidation potentials of chloꢀ
rine and iodine compared to that of chlorine and bromine
would change the reaction character. In addition to sciꢀ
entific reasons, practical problems stimulate the study of
triply distilled water and saturated with N O.
2
Results and Discussion
Weakly acidic solutions of NaCl (5•10–4 М HCl) conꢀ
*
HahnꢀMeitner Institut, 100 Gliniker Strasse, 14109 Berlin,
taining NaI additives were studied. The solutions were
–
Deutschland.
saturated with N O. Hydrated electrons e
formed in
2
aq
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1336—1340, June, 2005.
066ꢀ5285/05/5406ꢀ1378 © 2005 Springer Science+Business Media, Inc.
1

–
•–
Pulse radiolysis study of I oxidation with Cl2
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
1379
A•102
the primary processes of radiolysis of these solution along
with hydroxyl radicals OH by the reaction
•
0.6
3
2
1
eaq– + N O + H O
OH• + N + OH
–
(1)
2
2
2
transform into radicals OH ([N O] = 2.6•10 mol L–1
). Therefore, the presence of
N O makes it possible to generate predominantly the OH
radicals under water treatment with ionizing radiation.
The H ions catalyze the oxidation of the Cl ions with
•
–2
,
2
9
–1 –1 12
k = 9.1•10 L mol
s
0.4
1
•
2
+
–
•
1
the OH radicals in aqueous solutions.
0
.2
The absorption spectra of a 1 М solution of NaCl
(
(
рН 3.3) saturated with N O and containing NaI
5•10 mol L ) after accelerated electron pulse with a
2
–
5
–1
duration of 10 ns are shown in Fig. 1. The high concenꢀ
–
tration of Cl ions provides the almost complete capture
0
2
4
6
8
10
t/µs
•
of the ОН radicals formed by water radiolysis by these
ions rather than by the I ions. The characteristic absorpꢀ
tion spectrum of the Cl2• radical anion with a maximum
at 340 nm is observed 0.7 µs after a pulse. The concentraꢀ
tion of the formed radical anions Cl2
mined from the absorbance using the known value ε =
.8•10 L mol cm
data in Fig. 1 show that the intensity of the absorption
band of Cl2•– at 340 nm decreases with time, and simulꢀ
taneously the intensity of an absorption band appeared at
80 nm and caused by the radical anion I
The disappearance of Cl2
–
Fig. 2. Kinetic curves of the absorbance change at λ = 725 nm
for different absorbed doses: 6.6 (1), 15.2 (2), and 28.5 Gy (3).
For the solution composition, see Fig. 1.
–
•
–
can be deterꢀ
bance contributed by the I2•– radical anions at this waveꢀ
length for different absorbed doses is shown in Fig. 2. The
appearance of the I2 absorption at λ = 725 nm is preꢀ
ceded by the appearance and fast disappearance of the
absorption of hydrated electrons at this wavelength. It is
seen that the absorption (concentration) of the I2•– radiꢀ
cal anion increases with an increase in the absorbed dose.
However, the increase in the limiting concentrations is
not proportional to the absorbed doses because of the
3
–1
–1 4
•–
–6
–1
8
: [Cl2 ] = 1.4•10 mol L . The
•–
•
–
3
increases.
2
•
–
is also accompanied by a
decrease in the absorption of the solution at λ = 225 nm
corresponding to the absorption band of the I ions (λ
–
=
max
increasing contribution from the secondary reactions of
•–
4
–1
–1 13
•–
2
25 nm, ε = 1.1•10 L mol cm ). The Cl2 radical
I2
decay due to its recombination at high doses.
–
anions and I ions decay via the pseudoꢀfirstꢀorder reacꢀ
–
An increase in the concentration of the I ion accelerꢀ
ates the transformation of the Cl
the reaction with I to form radical anion I . The apꢀ
pearance of the absorption of I
different concentrations of the I ion and the same abꢀ
sorbed dose is illustrated in Fig. 3. It is seen that the time
of Cl2• transformation into I
tion. It is reasonable to relate these facts to the oxidation
of the I ion with the Cl2
formation of radical anion I2 . The formation of the
latter is especially pronounced at λ = 725 nm, where its
absorption is not impeded by the absorption of other shortꢀ
lived species.10 The kinetics of an increase in the absorꢀ
•
–
radical anion due to
–
•–
2
radical anion followed by the
–
•–
•
–
2
•–
at λ = 380 nm at
2
–
–
•–
shortens with an inꢀ
2
–
crease in the I concentration, while the contribution of
this process increases compared to the contribution of
A•102
1
2
3
•–
Cl2 decay due to recombination.
0
0
.4
.2
0
•–
and I2•– radical anꢀ
The recombination of the Cl
ions produces ions Cl3 and I3 , which are equilibrated
with molecular chlorine and iodine in solutions of chloꢀ
rides and iodides, respectively. As found for mixed soluꢀ
tions of chlorides and iodides in the presence of molecuꢀ
lar halogens, mixed trihalide ion ICl (λ
ε = 4.75•10 L mol cm , Keq = 6.0•10 mol L–1) and
I Cl (λ
.6 mol L–
20 nm, ε = 1.0•10 L mol cm , Keq = 5.5 mol L
and I3 (λmax = 285 and 355 nm, ε = 4.0•10 and 2.6•10
L mol cm , respectively, Keq = 1.35•10 mol L ).
The ratio of concentrations of the formed trihalide ions of
different types and equilibrated molecular halogens is deꢀ
2
–
–
4
5
1
1
2
3
300
400
λ/nm
–
–
–
0.2
0.4
0.6
–
= 225 nm,
2
max
–3
4
–1
–1
4
–
= 225 nm, ε = 4.75•10 L mol cm–1, Keq
4
–1
=
=
2
max
1 13
5
–
0
2
)
can be formed in addition to Cl (λmax
3
4
–1
–1
–1 1
)
Fig. 1. Absorption spectra of a 1 М solution of NaCl (рН 3.3)
containing NaI (5•10–5 mol L ) and saturated with N O 0.7 (1),
.0 (2), 3.0 (3), 5.0 (4), and 12.5 µs (5) after electron pulse
pulse duration 10 ns, absorbed dose 6.7 Gy, which corresponds
–
4
4
–1
2
–
1
–1
–3
–1
2
(
–
6
–1
to the formation of 1.8•10 mol L hydrated electrons).

1380
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Ershov et al.
A•102
A•102
5
3
2
1
6
4
2
4
1
4
3
2
2
3
1
3
2
4
5
1
3
00
400
λ/nm
0
2
4
6
8
10
t/µs
Fig. 4. Absorption spectra of a 1 М solution of NaCl (рН 3.3)
containing NaI (5•10– mol L ) and saturated with N O 1 (1),
6
–1
2
Fig. 3. Kinetic curves of the absorbance change at λ = 380 nm
for different concentrations of NaI: 0 (1), 5•10 (2), 1•10 (3),
and 2•10 mol L (4) (1 М solution of NaCl, рН 3.3, satuꢀ
1
2 (2), 50 (3), 100 (4), and 250 µs (5) after electron pulse (pulse
–
5
–4
duration 40 ns, absorbed dose 16.7 Gy).
–
4
–1
rated with N O; pulse duration 10 ns, absorbed dose 20 Gy).
2
constants: 8.8•109 19,20 and 8.0•109 14 L mol–1 s–1, reꢀ
•
–
spectively. The recombination rate constants of the Cl
and I2 radical anions were measured for 1 М solution of
2
termined by the corresponding equilibrium constants (Keq)
and reactant concentrations.
•–
NaCl with pH 3.3 saturated with N O. The recombinaꢀ
2
Our experiments showed that the pulse irradiation of a
•–
•–
tion of the Cl2 and I2 radical anions affords an equiliꢀ
brated mixture of the corresponding halogens and trihalide
ions. This fact is taken into account in the scheme. Since
the calculations were predominantly performed for low
–
5
–1
1
М solution of NaCl containing NaI (<1•10 mol L )
–
produces the mixed trihalide ion ICl2 . This process is
–
illustrated in Fig. 4. The chosen conditions favor ICl2
formation to the greatest extent. The low concentration
–
concentrations of the I ion, the formation of the mixed
of NaI (5•10– mol L ) and a relatively high concentraꢀ
tion of the Cl2•– radical anion formed upon the radiolysis
of a 1 М solution of NaCl provide the predominant disapꢀ
pearance of the latter due to recombination. It is seen that
the disappearance of the absorption at λ = 340 nm, which
belongs to the Cl2• radical anion, is accompanied by the
6
–1
–
trihalide ion ICl2 via reactions (2)—(4), which is imporꢀ
tant for this case, was taken into account in the scheme.
9
–1 –1
The rate constant of reaction (4) (3.5•10 L mol
s )
Table 1. Reaction rate constants used for the simulation of the
–
kinetics of oxidation of the I– ion with the radical anion Cl2
•–
appearance of an intense band with a maximum at λ =
–
2
25 nm caused by the trihalide ion ICl2 formed in reacꢀ
Reaction
Rate constant References
tions (2)—(4)
OH• + Cl → ClOH
–
•–
–
4.3•10 L mol s–1
9
–1
4
4
4
14
4
•
–
•
9
–1
Cl2•– + Cl2•–
Cl + 2 Cl ,
–
(2)
(3)
ClOH → OH + Cl
6.1•10 s
2
+
+ ClOH → Cl + H O 2.1•10 L mol s–1
•–
•
10
–1
H
2
–
Cl + Cl
2
Cl3–,
–
•
•–
9
–1 –1
Cl + Cl → Cl2
8.0•10 L mol
s
•
–
→ Cl• + Cl–
5
–1
Cl2
1.1•10 s
–
Cl + I
2
ICl2–.
(4)
•–
•–
–
9
–1 –1
Cl2 + Cl
→ Cl + Cl
2.6•10 L mol
s
3
2
2
–
–
2.0•10 L mol s–1
4
–1
Cl + Cl → Cl3
15, 16
15, 16
17
18
—*
19, 20
19, 20
19, 20
—*
19, 20
19, 20
—*
2
All the experimental facts indicate that the reaction of
–
–
5
–1
Cl → Cl + Cl
1.1•10 s
3
•
2
Cl2•– with I affords the I2 radical anion.
–
•–
•–
→ Cl– + HCl
9
–1 –1
H
H
+ Cl
7.0•10 L mol
s
s
s
2
•
•
9
–1 –1
+ H → H2
•–
7.8•10 L mol
Cl2•– + I–
I• + I–
2 Cl + I
I2•–
–
•
(5)
(6)
+ I– → 2 Cl– + I•
9
–1 –1
Cl
4.5•10 L mol
2
•
+ I → I2•–
–
9
–1 –1
I
8.8•10 L mol
s
I2 → I• + I–
•
–
7.0•10 s
4
–1
•
+ I• → I
1.5•10 L mol s–1
10
–1
The computer simulation of the process was performed
to describe the oxidation of the I ion with the Cl2
radical anion. The scheme of reactions presented by the
equations with the known rate constants, which were used
for matching calculations with experimental data, are
I
2
–
•–
•–
+ I2 → I3– + I–
•–
9
–1 –1
I2
4.5•10 L mol
s
–
–
6
–1
I3 → I + I
I2 + I → I3
7.5•10 s
2
–
–
9
–1 –1
5.6•10 L mol
s
–
–
9
–1 –1
Cl + I → ICl
3.5•10 L mol
s
2
2
ICl2 → ICl + Cl•
–
•–
3.6•10 s
7
–1
13
13
•
given in Table 1. The interaction of the I atom with the
•
–
•
•
6.0•10 L mol s–1
9
–1
ICl + Cl → ICl2
–
•–
I
ion producing the I2
radical anion and a similar
•
reaction for the Cl atom proceed with very high rate
* Data of this work.

–
•–
Pulse radiolysis study of I oxidation with Cl2
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
1381
was calculated from the kinetics of absorbance appeared
at λ = 225 nm.
This scheme (see Table 1) well describes the kinetics
of changes in the absorption generated by an electron
pulse at wavelengths that characterize a change in the
concentration of reactants and intermediates decaying and
forming during the radiationꢀchemical process. These are,
first of all, the wavelengths corresponding to absorption
The following values were accepted for the initial
1
1,21
–
yields of water radiolysis products
: G(eaq ) = 2.7,
+
•
G(H ) = 0.6, and G(OH ) = 2.9. According to known
data,22 the presence of a 1 М solution of NaCl or soluꢀ
tions of other halides exerts no significant effect on the
initial yields of water radiolysis products. We also took
into account that hydrated electrons formed in reaction (1)
are transformed into hydroxyl radicals.
–
band maxima of the ion I (λ = 225 nm), radical anions
•
–
•–
Cl2 (λ = 340 nm) and I
(λ = 380 and 725 nm), and
2
–
trihalide ion ICl2 (λ = 225 nm). The known molar abꢀ
sorption coefficients of intermediates involved in the reꢀ
actions were used for each wavelength.
A•103
6
A•103
2
4
6
8
10
t/µs
4
a
2
–
–
4
8
a
–
–
12
16
0
2
4
6
8
10
t/µs
A•103
A•102
8
6
4
2
5
4
3
2
1
b
b
0
2
4
6
8
10
t/µs
0
0
.04
0.08
0.12
0.16 0.20
t/µs
Fig. 5. Comparison of experiment and calculation (smooth line).
a. Kinetics of the absorbance change at λ = 380 nm of a 1 М
solution of NaCl (рН 3.3) containing NaI (5•10–5 mol L ) and
Fig. 6. Comparison of experiment with calculation (smooth line).
а. Kinetics of the absorbance change at λ = 225 nm of a 1 М
solution of NaCl (рН 3.3) containing NaI (5•10–5 mol L ) and
–1
–1
saturated with N O after electron pulse (pulse duration 5 ns,
2
absorbed dose 3.3 Gy). The reaction scheme, rate constant valꢀ
saturated with N O after electron pulse (pulse duration 10 ns,
2
ues presented in Table 1, and molar absorption coefficients (ε)
absorbed dose 7.3 Gy). b. Kinetics of the absorbance change at
λ = 225 nm of a 1 М solution of NaCl (рН 3.3) containing NaI
–
1
–1
•–
equal to 5500, 9400, 3300, and 1000 L mol cm for Cl2
,
I2 , I3 , and ICl2–, respectively, were used for computer simuꢀ
•
–
–
(2•10 mol L ) and saturated with N O after electron pulse
–6
–1
2
lation. b. Kinetics of the absorbance change at λ = 725 nm of a
(pulse duration 40 ns, absorbed dose 26.7 Gy). In both cases, the
reaction scheme, rate constant values given in Table 1, and
molar absorption coefficients (ε) equal to 400, 10000, 11000,
–
5
–1
1
М solution of NaCl (рН 3.3) containing NaI (5•10 mol L
)
and saturated with N O after electron pulse (pulse duration
2
•–
–1
–1
•–
–
–
•
–
2
2
0 ns, absorbed dose 20 Gy). The ε value for I
500 L mol cm was used for computation.
equal to
250, and 48000 L mol cm for Cl2 , Cl3 , I , I , and ICl
respectively, were used for computation.
,
2
2
–
1
–1

1382
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Ershov et al.
Figures 5, а, b and 6, a illustrate the agreement beꢀ
tween the calculations and experimentally measured
2. P. Neta, R. E. Huie, and A. B. Ross, J. Phys. Chem. Ref.
Data, 1988, 17, 1027.
3
4
5
. B. G. Ershov, M. Kelm, A. V. Gordeev, and E. Janata, Phys.
Chem. Chem. Phys., 2002, 4, 1872.
. G. G. Jayson, B. J. Parsons, and A. J. Swallow, J. Chem.
Soc., Faraday Trans. 1, 1973, 69, 1579.
change in the absorbance of a 1 М solution of NaCl conꢀ
–
4
–1
taining NaI (5.0•10 mol L ) at λ = 380 (see Fig. 5, а),
25 (see Fig. 5, b), and 225 nm (see Fig. 6, а). The
contributions of absorbances from the radical anions
7
. D. Zehahi and J. Rabany, J. Phys. Chem., 1972, 76, 312.
•
–
•–
•
•–
Cl2 , ClOH , I , and I2 involved in chemical transꢀ
formations are also given in Figs 5 and 6. We can conꢀ
clude that this scheme agrees satisfactorily with experiꢀ
6. P. Wardman, J. Phys. Chem. Ref. Data, 1989, 18, 1711.
7. E. Janata, Radiat. Phys. Chem., 1992, 40, 437.
8. E. Janata, Radiat. Phys. Chem., 1994, 44, 449.
9. E. Janata and W. Gutsch, Radiat. Phys. Chem.,
1998, 51, 65.W
10. G. L. Hug, Optical Spectra of Nonmetallic Inorganic Tranꢀ
sient Species in Aqueous Solution, NSRDS—NBS, Washingꢀ
ton, 1981.
1. G. V. Buxton, C. L. Greenstock, W. P. Helman, and A. B.
Ross, J. Phys. Chem. Ref. Data, 1988, 17, 513.
2. E. Janata and R. H. Schuler, J. Phys. Chem., 1982,
86, 2078.W
–
ment in the time interval of oxidation of the I ion with
the Cl2•– radical anion and formation of I2 (∼ 12 µs).
The calculated rate constant of reaction (5) is 4.5•10
L mol–1
•–
9
–1
s .
The results of matching of the experimental data for
1
–
the formation of mixed trihalide ion ICl2 in a time interꢀ
val of ∼ 300 µs are shown in Fig. 6, а. Just these times are
1
characteristic of recombination of the Cl2• and I2
–
•–
radical anions and formation of the trihalide ions and
molecular halogens. The calculated ε value is 4.75•10
13. D. W. Margerum, P. N. Dickson, J. C. Nagy, K. Kumar,
C. P. Bowers, and K. D. Fogelman, Inorg. Chem., 1986,
25, 4900.
4
–
1
–1
L mol cm , which coincides with the earlier meaꢀ
sured13 magnitude.
14. V. Nagarajan and R. W. Fessenden, J. Phys. Chem., 1985,
8
9, 2330.
•
–
The results of this work show that the Cl2 radical
1
5. B. G. Ershov, A. V. Gordeev, E. Janata, and M. Kelm,
–
anion in a 1 М solution of NaCl oxidize the I ion to form
Mendeleev Commun., 2001, 4, 149.
16. B. G. Ershov, M. Kelm, E. Janata, and A. V. Gordeev,
Radiochem. Acta, 2002, 90, 617.
17. S. Navaratnam, B. Parsons, and A. J. Swallow, Radiat. Phys.
Chem., 1980, 15, 159.
18. P. Pagsberg, G. Fenger, and S. O. Nielsen, J. Phys. Chem.,
1969, 73, 1029.
•
•–
the I atom, which is further transformed into the I
2
•
–
radical anion. However, a mixed radical anion ClBr is
formed when the Br ion is oxidized under the same
conditions. In our opinion, this is a consequence of the
–
3
•
–
–
differences between E°(Cl
/2 Cl ) = 2.09 V and
2
•
–
•
–
E°(Br /Br ) = 1.93 V and E°(I /I ) = 1.33 V. In the first
1
9. H. A. Schwarz and B. H. J. Bielski, J. Phys. Chem., 1986,
0, 1445.
case, the potential difference is only 0.16 V, which is
9
favorable for the organization of a mixed orbital in ClBr•
–
.
2
2
0. A. J. Elliot, Can. J. Chem., 1992, 70, 1658.
In the second case, the difference reaches 0.76 V, and the
1. I. G. Draganic and Z. D. Draganic, The Radiation Chemistry
of Water, Academic Press, New York—London, 1971.
2. E. Peled, D. Meisel, and G. Czapski, J. Phys. Chem., 1972,
–
•–
electron transfer from I to Cl2 becomes more enerꢀ
getically favorable.
2
7
6, 3677.
References
1
. B. G. Ershov, Usp. Khim., 2004, 73, 107 [Russ. Chem. Rev.,
004, 73 (Engl. Transl.)].
Received September 23, 2004;
in revised form January 14, 2005
2