356
GORSHKOV et al.
Figure 1 shows that, in our case, enrichment of
2+
18
UO in O weakly affects the absorption of the
2
UO F solution. Therefore, the difference in the re-
2 2
duction rates cannot be explained by different absorp-
tion coefficients of the isotopic species of uranyl. This
fact suggests that other factors must be responsible for
16
18 2+
accelerated escape of the [ O U O] groups from
solution.
As known, UO F is virtually nondissociated in
aqueous solutions and tends to dimerize [14]. In non-
2+
2 2
Fig. 4. Rate of photoreduction of UO in isopropanol in
2
relation to the isotope composition of oxygen in UO F .
(1) Natural isotope composition of oxygen in UO F and
O. Regression equations:
(1) y = 0.0056x + 6.6292, R = 0.9879 and (2) y =
2 2
aqueous and aqueous-organic solutions, UO F often
2 2
2 2
18
(2) 42% enrichment in
exhibits higher degrees of polymerization even in the
presence of ligands forming strong complexes, such as
phosphine oxides, amides, etc. [15 19]. For example,
2
2
0.0051x + 6.4756, R = 0.9583.
the benzene-soluble complex of UO F with tributyl-
phosphine oxide is a hexamer [15]; a complex of
2 2
Here K , K , and K are the photoreduction rate
1
2
3
constants, and K and K , the oxygen exchange rate
constants for the corresponding isotopic species of
uranyl.
UO F with trioctylphosphine oxide also has a poly-
2 2
meric structure [16]. A complex with dimethyl sulfox-
ide is also a coordination polymer [17]. Antipyrine
Photoreduction is a first-order reaction, and for
solutions of unenriched uranyl its rate is described by
the equation
and dimethylformamide form polymers with UO F
[17]. Urea [18] and hexamethylphosphoric triamide
[19] form dimeric complexes.
2 2
We think that saturated isopropanol solutions also
d[UO22+]/dt = K3[UO22+];
dln [UO22+]/dt = K3.
contain polymers (dimers) of UO F . In this case, the
2 2
time dependence of the logarithm of the total concen-
tration of U(VI) remains linear, while the changes in
the concentrations of each isotopic species will be de-
scribed by more complex equations than schemes (I)
and (II). Since excitation causes reduction of only one
The total reaction time is proportional to the num-
ber of flashes, since the time between the flashes is
much longer than the reaction time. Thus, the slope of
the plot of the logarithm of the uranium concentration
vs. the number of flashes yields a nominal K value.
3
2+
UO group, the excitation energy is eventually loca-
2
Figure 4 presents these dependences for uranyl both
18
lized on one of the molecules constituting the poly-
mer. Our experiments suggest that the photoreduction
enriched and unenriched in O. It is seen that the
slopes of both plots are close, which suggests that the
16
18 2+
18
more often involves specifically the [ O U O]
group. Such behavior of the latter can be explained by
rate constants of the photoreduction of O-enriched
and unenriched uranyl are close as well. This means
that the isotope effect in photoreduction proper is,
16
the fact that, when one O atom is replaced by an
O atom in uranyl, the symmetry of the vibrational
18
evidently, lacking [i.e., in schemes (I) and (II) K =
1
Hamiltonian of the uranyl group decreases from D
h
K
= K ].
2
3
to C . Therefore, such group predominantly accepts
v
Rofer-DePoorter and DePoorter [13] studied the
excitation during nonradiative energy exchange within
the polymer.
18
photochemical behavior of O-enriched uranyl fluo-
ride in methanol. They also observed no differences in
the quantum efficiency of photoreduction for different
isotopic species of uranyl upon illumination of the
solutions with a xenon pulse lamp and a laser at the
generation wavelengths of 455 and 448 nm. They
could not monitor the behavior of individual uranyl
species, but observed a general decrease in the content
ACKNOWLEDGMENTS
The authors are grateful to L.G. Mashirov for as-
sistance in carrying out the spectrometric analysis.
18
REFERENCES
of O in UO F . They explained the depletion of
2 2
18
UO F in O in photoreduction under the conditions
2 2
1. Burrows, Y.D. and Kemp, T.J., Chem. Soc. Rev.,
of completely suppressed, in their opinion, oxygen
exchange by a difference in the extinctions of the
1974, vol. 3, no. 2, pp. 139 165.
UO F solutions differing in the isotope compositions
of oxygen.
2. Matsushima, R. and Sakuraba, Sh., J. Am. Chem. Soc.,
2 2
1971, vol. 93, p. 5421.
RADIOCHEMISTRY Vol. 48 No. 4 2006