The thermal charge-transfer reduction of uranyl UO22+(VI) to UO2+(V) by various functionalized organic compounds, and evidence for possible spin-spin interactions between UO2+(V) and hydroxymethyl ([rad]CH2OH) radical and between UO2+(V) and diphenyl sulfide radical cation (Ph2S[rad]+)

Article abstract of DOI:10.1016/j.ica.2018.07.049

The linear uranyl UO₂²⁺(VI) cation (D_∞h symmetry) exhibited strong and broad absorptions at 350–400 nm in anhydrous methanol and methanol-water mixtures in the UV-Vis spectra. The intensity of the absorptions (represented by absorbance at 375 nm) is directly proportional to molar concentrations of methanol and UO₂²⁺(VI), respectively. The linear relationships indicate formation of an electron-donor-acceptor (EDA) complex [UO₂²⁺, CH₃OH]. The absorptions at 350–400 nm originate from the charge-transfer (single-electron transfer) from CH₃OH (electron donor) to UO₂²⁺ (electron acceptor) within the [UO₂²⁺, CH₃OH] complex. Electron paramagnetic resonance (EPR) studies of various mixtures of UO₂²⁺-CH₃OH and UO₂²⁺-CH₃OH-H₂O have shown that the charge-transfer also took place slowly in the dark, resulting in thermal reduction of UO₂²⁺(VI) to UO₂⁺(V) (singlet, g = 2.08) by CH₃OH, and CH₃OH was oxidized to the hydroxymethyl ^[rad]CH₂OH radical (generating an axial signal). The charge-transfer oxidation-reduction reaction is believed to take place via the EDA [UO₂²⁺, CH₃OH] complex. EPR studies suggested spin-spin coupling between UO₂⁺(V) and ^[rad]CH₂OH in anhydrous methanol, supporting the formation of a [UO₂⁺, ^[rad]CH₂OH] ion-radical pair. The EPR studies have also shown that UO₂²⁺(VI) was reduced to UO₂⁺(V) thermally by other alcohols (ethanol, 2-propanol, and cyclohexanol), and by diphenyl sulfide (Ph₂S), L-ascorbic acid (AA), and 2-methyl-5-(propan-2-yl)phenol (carvacrol, ArOH), respectively. Ph₂S, AA, and ArOH were oxidized to the diphenyl sulfide Ph₂S^+[rad] radical cation (singlet, g = 2.00), ascorbic acid AA^[rad] radical (singlet, g = 2.00), and carvacrol ArO^[rad] radical (singlet, g = 1.98), respectively. Both EPR and UV-Vis studies indicate that the reactions followed the ground-state charge-transfer mechanisms similar to that of the UO₂²⁺/methanol reaction. EPR evidence supported formation of the [UO₂⁺, Ph₂S^+[rad]] ion-radical pair in the charge-transfer reaction of UO₂²⁺ and Ph₂S and spin-spin interactions within the ion-radical pair. The sulfuric-acid-catalyzed isomerization of ^[rad]CH₂OH to CH₃O^[rad] was found by EPR studies.

Full text of DOI:10.1016/j.ica.2018.07.049

Accepted Manuscript
Research paper
+
The Thermal Charge-Transfer Reduction of Uranyl UO2 2+(VI) to UO2 (V) by
Various Functionalized Organic Compounds, and Evidence for Possible Spin-
+
Spin Interactions between UO2 (V) and Hydroxymethyl (.CH2OH) Radical and
+
between UO2 (V) and Diphenyl Sulfide Radical Cation (Ph2S.+)
Xiaoping Sun, Derrick R.J. Kolling, Seth Deskins, Ethan Adkins
PII:
S0020-1693(18)30481-X
https://doi.org/10.1016/j.ica.2018.07.049
ICA 18395
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 March 2018
27 July 2018
28 July 2018
Please cite this article as: X. Sun, D.R.J. Kolling, S. Deskins, E. Adkins, The Thermal Charge-Transfer Reduction
+
of Uranyl UO2 2+(VI) to UO2 (V) by Various Functionalized Organic Compounds, and Evidence for Possible Spin-
+
+
Spin Interactions between UO2 (V) and Hydroxymethyl (.CH2OH) Radical and between UO2 (V) and Diphenyl
Sulfide Radical Cation (Ph2S.+), Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.07.049
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2+
+
The Thermal Charge-Transfer Reduction of Uranyl UO2 (VI) to UO2 (V) by Various
Functionalized Organic Compounds, and Evidence for Possible Spin-Spin Interactions
.
+
+
between UO2 (V) and Hydroxymethyl ( CH2OH) Radical and between UO2 (V) and
.
Diphenyl Sulfide Radical Cation (Ph2S +)
Xiaoping Sun,*1 Derrick R. J. Kolling,2 Seth Deskins,1 and Ethan Adkins2
1. Department of Natural Sciences and Mathematics, University of Charleston, Charleston,
West Virginia 25304, USA
2. Department of Chemistry, Marshall University, Huntington, West Virginia 25755, USA
* To whom all the correspondence should be addressed.
Phone: 1-304-357-4898
Fax: 1-304-357-4715
E-mail: xiaopingsun@ucwv.edu
1

Abstract
The linear uranyl UO22+(VI) cation (D∞h symmetry) exhibited strong and broad
absorptions at 350-400 nm in anhydrous methanol and methanol-water mixtures in the UV-Vis
spectra. The intensity of the absorptions (represented by absorbance at 375 nm) is directly
proportional to molar concentrations of methanol and UO22+(VI), respectively. The linear
2+
relationships indicate formation of an electron-donor-acceptor (EDA) complex [UO2 , CH3OH].
The absorptions at 350-400 nm originate from the charge-transfer (single-electron transfer) from
2+
CH3OH (electron donor) to UO2 (electron acceptor) within the [UO22+, CH3OH] complex.
Electron paramagnetic resonance (EPR) studies of various mixtures of UO22+-CH3OH and
UO22+-CH3OH-H2O have shown that the charge-transfer also took place slowly in the dark,
+
resulting in thermal reduction of UO22+(VI) to UO2 (V) (singlet, g=2.08) by CH3OH, and
.
CH3OH was oxidized to the hydroxymethyl CH2OH radical (generating an axial signal). The
2+
charge-transfer oxidation-reduction reaction is believed to take place via the EDA [UO2 ,
.
+
CH3OH] complex. EPR studies suggested spin-spin coupling between UO2 (V) and CH2OH in
.
+
anhydrous methanol, supporting the formation of a [UO2 , CH2OH] ion-radical pair. The EPR
+
studies have also shown that UO22+(VI) was reduced to UO2 (V) thermally by other alcohols
(ethanol, 2-propanol, and cyclohexanol), and by diphenyl sulfide (Ph2S), L-ascorbic acid (AA),
and 2-methyl-5-(propan-2-yl)phenol (carvacrol, ArOH), respectively. Ph2S, AA, and ArOH were
.
.
oxidized to the diphenyl sulfide Ph2S+ radical cation (singlet, g=2.00), ascorbic acid AA radical
.
(singlet, g = 2.00), and carvacrol ArO radical (singlet, g = 1.98), respectively. Both EPR and
UV-Vis studies indicate that the reactions followed the ground-state charge-transfer mechanisms
2+
+
similar to that of the UO2 /methanol reaction. EPR evidence supported formation of the [UO2 ,
2

2+
Ph2S+.] ion-radical pair in the charge-transfer reaction of UO2 and Ph2S and spin-spin
.
interactions within the ion-radical pair. The sulfuric-acid-catalyzed isomerization of CH2OH to
.
CH3O was found by EPR studies.
Keywords: Uranyl / Alcohol / Charge-transfer / Oxidation-reduction / EPR / UV-vis / Radical
3

1. Introduction
A particularly interesting chemical property of the linear uranyl UO22+(VI) ion (D∞
2+
symmetry) is that it possesses a high reduction potential and thus, UO2 can oxidize many
+
substances [1]. Since uranium (V) (in the form of UO2 ) is another readily accessible and
2+
relatively stable oxidation state of uranium, the oxidation-reduction chemistry of UO2
(diamagnetic) often takes place via a single-electron transfer (charge-transfer) process from a
2+
reductant to the valence shell of uranium in UO2 [2]. This leads to the formation of
+
paramagnetic UO2 (V), another linear ion, and the reductant (diamagnetic) upon losing an
electron transforms into a radical (Eq. 1).
e
CT
2+
+
UO2
+ Reductant
UO2 + Oxidant (radical)
(1)
Photochemical reduction of uranyl UO22+(VI) by many reducing agents have been
studied. For example, the UO22+-doped polyvinyl alcohol (PVA, –CH2–CH(OH)–CH2–) film
was irradiated by -ray (dose 4 kGy). Then an electron paramagnetic resonance (EPR) spectrum
was recorded on the -irradiated UO22+-PVA film, exhibiting a strong broad singlet signal (g =
+
2.037) attributable to UO2 and a weak triplet signal (g = 2.001, aH = 33 G) attributable to a
.
.
PVA radical (–CH2–C (OH)–CH2–, an -hydroxyalkyl radical) [3]. Another example is the UV-
irradiation (using a mercury lamp) of a 1:2 adduct of uranyl (VI) nitrate and tributylphosphate
(TBP, (BuO)3PO), UO2(NO3)2-2TBP, in n-dodecane [4]. In many photochemical reactions of
2+
UO2 with other reducing agents, such as halides (Br- and I-), phenols, and alcohols (methanol,
ethanol, and 2-propanol), only radicals (the oxidation products) have been observed by EPR
and/or electronic absorption spectroscopy from the reaction mixtures, but the expected reduction
+
product UO2 was not identified [5-8]. The reactions were thought to take place via a charge-
4

transfer from the reductant to the excited state of uranyl(VI) (UO22+)*. It has been proposed that
+
+
2+
the lack of UO2 was due to the facile disproportionation of the initially formed UO2 to UO2
and U4+ at certain photochemical conditions [9].
In a previous paper [2], we reported our discovery of thermal charge-transfer reduction of
2+
UO2 by halides (Br- and I-), DMSO (CH3SOCH3), and phenol (C6H5OH) in the dark. Both
.
+
UO2 and oxidized radicals (e.g. C6H5O ) were observed by EPR and/or electronic absorption
2+
spectroscopy. Our work represented investigations of thermal charge-transfer reduction of UO2
by any reducing agents for the first time.
In the present paper, we extend our studies to the thermal charge-transfer reduction of
2+
UO2 by other functionalized organic compounds, including alcohols (methanol, ethanol, 2-
propanol, and cyclohexanol), 2-methyl-5-(propan-2-yl)phenol (carvacrol, a biologically relevant
substance), diphenyl sulfide, and ascorbic acid. This is the first study of the thermal reduction of
2+
+
UO2 by these compounds. For each of the reducing agents, both UO2 and an oxidized radical
.
(e.g., hydroxymethyl, CH2OH) have been observed by EPR spectroscopy from the thermal
+
reaction, while only the radical, but not UO2 , was observable in the previously studied
photochemical reactions [5-8].
.
+
Direct observation of both UO2 and an oxidized radical (e.g. CH2OH) by EPR in our
thermal redox reactions is a significant advance. This way, along with UV-Vis spectroscopic
studies, we have demonstrated a charge-transfer mechanism for the reactions via an electron-
2+
donor-acceptor (EDA) complex between UO2 and a reductant in the ground state. Such a
2+
ground-state charge-transfer mechanism for UO2 was previously unknown. In addition, we
+
have obtained EPR evidence for the spin-spin interactions between UO2 and the hydroxymethyl
5

.
.
+
CH2OH radical and between UO2 and diphenyl sulfide Ph2S+ radical cation in ion-radical pairs.
Such interactions have not been reported before. In the course of our study, we have found
.
thermal as well as photochemical isomerization of -hydroxyalkyl radical (e.g. CH2OH) to
.
alkoxy radical (e.g. CH3O ). The formations of two different alcohol radicals in reactions of
2+
UO2 with methanol, ethanol, and 2-propanol have been confirmed in this work by variable
power EPR measurements and computer-aided EPR spectra simulations. We now present our
2+
new advances in the studies of thermal charge-transfer reduction of UO2
.
2. Experimental
The electronic absorption (UV-visible) spectra were recorded throughout this work using
a UV-1601 Shimadzu spectrophotometer which is connected to a Dell computer equipped with
Shimadzu UV Probe (Ver. 2.60) software. The data were processed using the Shimadzu UV
Probe software installed in the Dell computer.
The Bruker EMXplus EPR spectrometer with microwave frequency 9.37 GHz (X-band)
was used for the EPR measurement throughout this work. All spectra were recorded in the frozen
state via cooling to 80 K with liquid nitrogen, with microwave power of 5 mW, a modulation
amplitude of 15 G, and a modulation frequency of 100 kHz. For each spectrum, background
subtraction from the solvent and cavity were performed.
For the variable power EPR measurements, the EPR spectra were measured at various
powers with other instrumental parameters unchanged. The peak height for a signal was taken as
the intensity of the signal.
The simulation of EPR spectra were performed using EasySpin aided by MATLAB
(http://easyspin.org) [10].
6

Uranyl(VI) nitrate hexahydrate [UO2(NO3)2∙6H2O] and uranyl(VI) acetate dihydrate
[UO2(CH3COO)2∙2H2O], both obtained from Sigma-Aldrich Chemical Company, were used as
the sources of uranyl(VI) ion (UO22+) for the reactions conducted in nearly neutral solutions and
in acidic solutions, respectively. Deionized water was used throughout this work. Methanol
(anhydrous), ethanol (anhydrous), 2-propanol (95%), and cyclohexanol were obtained from
Macron Fine Chemicals. Sulfuric acid (98%) and acetone were obtained from Fisher Chemical.
Diphenyl sulfide was obtained from Alfa Aesar. L-Ascorbic acid and 2-methyl-5-(propan-2-
yl)phenol (carvacrol) were obtained from Sigma-Aldrich.
In the experiment conducted in each medium throughout the work, the UO2(NO3)2∙6H2O
or UO2(CH3COO)2∙2H2O solid was dissolved in the corresponding medium with specific
composition at ambient conditions. A UV-visible cell (with thickness of 1 cm) was then filled
with each of the UO22+ solutions, and an electronic absorption (UV-Vis) spectrum was recorded.
2+
For EPR measurements, a UO2 solution with specific composition was made as above
mentioned and incubated in the dark (or irradiated by UV-lamp occasionally) at ambient
temperature for a specified time. Then the solution was added into a 4-mm quartz tube that was
subsequently placed in the cavity of the spectrometer. The EPR spectrum was recorded in the
frozen state at 80 K, with background subtraction as mentioned above.
3. Results and Discussion
2+
+
3.1 Thermal charge-transfer reduction of uranyl UO2 (VI) to UO2 (V) by methanol
+
(CH3OH), and evidence for spin-spin interactions between UO2 (V) and hydroxymethyl
.
radical ( CH2OH)
7

The previous investigations have shown that an excited uranyl (VI) (UO22+)* possesses
particularly strong oxidizing capabilities, allowing it to activate and cleave the relatively inert
CH bond in methanol [7, 9, 11]. However, there has been no report on the thermal oxidation of
2+
the methanol CH bond by UO2 in the ground state prior to the present work. Although
methanol is usually considered a relatively inert solvent towards oxidation-reduction reactions of
2+
common oxidants in normal ambient conditions, a thermal redox reaction between UO2 and
2+
CH3OH has been identified at normal room temperature in this work. The UO2 (as the nitrate
salt) solutions in anhydrous CH3OH and in CH3OH–H2O mixtures were incubated in the dark at
ambient temperatures for 24 h and then subjected to EPR spectroscopy in the frozen state at 80 K
+
(Figure 1). All the spectra showed a broad UO2 signal (g = 2.08) and axial and rhombic signals
[12]. By referencing to previous EPR studies of methanol radicals in frozen solution [13], the
.
axial signal is attributable to the hydroxymethyl CH2OH radical (major), and the rhombic signal
.
is attributable to the methoxy CH3O radical (minor)—the remaining signal from the methoxy
radical is overlapped by the hydroxymethyl radical. A CH bond in CH3OH can effectively
overlap with a lone pair of electrons in the OH oxygen (hyperconjugation). As a result, the
oxygen electron pair may activate the CH bond making it reductive. Comparison of the two EPR
spectra of the UO22+–CH3OH–H2O mixtures (Figure 1b and Figure 1c) also shows that the
.
+
intensities of both UO2 and CH2OH signals increased as the concentration of methanol
increased, and that as the concentration of CH3OH in the mixture was increased, the ratio of
.
+
intensity of CH2OH signal to intensity of UO2 signal increased.
EPR simulations on Figure 1 were conducted with EasySpin. Parameters used were:
Figure 1a: ratio of Axial : Rhombic = 1: 0.8. Axial: g║= 2.034 and g┴ = 2.016; broadening: 0.8.
8

Rhombic: gx=2.034, gy=2.017, gz=2.002; broadening: 0.6. Figure 1c: ratio of Axial : Rhombic =
2:1. Axial: g║= 2.033 and g┴ = 2.016; broadening: 0.28. Rhombic: gx=2.033, gy=2.016,
gz=2.002; broadening: 0.6. The comparison of experimental and simulated EPR spectra of the
.
.
methanol radicals ( CH2OH and CH3O ) in Figure 1c is exhibited in Figure 2, and they are
essentially consistent.
.
.
The formations of two radicals CH2OH and CH3O have been further confirmed by
2+
variable power EPR measurements on the mixtures of UO2 (0.05 M)–CH3OH–H2O with
volume ratios of CH3OH : H2O = 2:1 and 1:1, respectively. For both mixtures, the intensities of
Signals A and B in the EPR spectra obtained at different microwave powers were determined.
The results are included in Figure 3. Figure 3 shows that intensities of the axial signal reached
maxima (saturation) at the power of 1.5–2.5 mW (Lines 1 and 2). As the power further increased,
the intensities decreased. However, the upper-field component (Signal B) was not saturated at
1.5–2.5 mW. Instead, its intensity kept increasing as a function of the power until 20 mW (Line
3). The results have confirmed that the lower-field peak and the derivative (marked with A in
.
Figure 1) originate from the same radical ( CH2OH), while the upper-level component (marked
.
with B in Figure 1) originates from another radical (CH3O ).
.
+
The EPR identification of both UO2 and CH2OH (the major radical) from the thermal
reaction mixtures of UO22+ and CH3OH clearly shows a charge-transfer process from a CH bond
of CH3OH to the uranium valence shell in UO22+ to lead to a redox reaction (Eq.2).
.
+
UO2 + CH2OH + H+
2+
UO2 + CH3OH
(2)
9

.
The minor CH3O radical is most likely produced by a water catalyzed isomerization of
.
initially formed CH2OH.
+
Our EPR spectroscopic studies have shown that the UO2 signal generated by reduction
2+
of UO2 with anhydrous methanol (Figure 1a) had a splitting, while in the mixed methanol-
+
water solutions the splitting on the UO2 signals was not seen (Figure 1b and 1c). Such splitting
+
on the UO2 signals was also observed and became more appreciable in the EPR spectra of the
2+
solutions of UO2 (as the acetate salt) in the CH3OH–H2SO4 mixtures (the mixtures of
+
anhydrous methanol and 98% H2SO4) (Figure 4). Conceivably, the splitting in the UO2 signals
in all the EPR spectra of Figure 1a and Figure 4 originates possibly from the spin–spin
.
+
interactions between the unpaired electrons in the two paramagnetic centers, UO2 and CH2OH.
.
+
The observation of the spin-spin interactions is indicative of the formation of a [UO2 , CH2OH]
ion-radical pair between the two paramagnetic centers in anhydrous methanol. As the content of
water in the media became appreciable (e.g. when volume ratio of CH3OH : H2O = 2:1 and 1:1),
.
+
UO2 and CH2OH were possibly separated and the spin-spin interactions did not take place.
Figure 4 also shows that sulfuric acid in the reaction media has led to a substantial
.
increase in the intensity of the upper-level component (CH3O ) in the EPR spectra, while in the
anhydrous methanol without the acid the upper-level component (Signal B) in the spectrum was
tiny (Figure 1a). The spectral data support a sulfuric-acid-catalyzed isomerization of
.
.
hydroxymethyl ( CH2OH) radical to methoxy (CH3O ) radical as shown below (Eq.3).
10

H
.
O-
O
O-
O
H
H
H
O
O
O
O
O
O
O
O
O
C
S
S
S
.
H O
-O
H
C
C
H
H
H
H
H
H
(3)
H2SO4 is ionized to HSO4 in methanol. HSO4 then relays a concerted proton transfer
-
-
.
.
.
from oxygen to carbon in CH2OH, resulting in isomerization of CH2OH to CH3O . This
mechanism is comparable with a recent theoretical study on the isomerization between different
methanol radicals [14].
We further characterized the charge-transfer reduction of UO22+ with CH3OH by UV-Vis
spectroscopy. The UV-Vis spectra of UO22+ (as the nitrate salt) in both anhydrous methanol and
in the methanol-water mixtures exhibited strong broad absorptions at 350-400 nm (Figure 5).
The absorption bands centered at ~420 nm in the spectra of samples containing lower
concentrations of methanol and higher concentrations of water (the bottom two curves in Figure
5-left) originate from the promotion of an electron from an axial oxygen 2p orbital to a uranium
(VI) 5f nonbonding orbital with vibrational fine structure resolved [15, 16]. Figure 5-left shows
that the intensity of the absorptions at 350–400 nm (represented by A375, the absorbance at 375
nm) is directly proportional to the molar concentration of methanol ([CH3OH], M) in the
CH3OH–H2O mixtures at a fixed uranyl (VI) concentration ([UO22+] = 0.050 M). Figure 5-right
shows that the intensity of the absorptions A375 is also directly proportional to the molar
2+
2+
concentration of UO2 ([UO2 ], M) in the UO22+– CH3OH–H2O solutions with a fixed volume
ratio of CH3OH : H2O = 5:1. The quantitative linear relationships between A375 and [CH3OH]
2+
and between A375 and [UO22+] show that the absorption at 350–400 nm involves both a UO2
2+
cation and a CH3OH molecule and originates from a 1:1 EDA [UO2 , CH3OH] complex
2+
(formed reversibly by interaction of UO2 and CH3OH) as a result of a single-electron transfer
11

2+
from a CH bond in CH3OH (electron-donor) to the uranium valence shell of UO2 (electron
acceptor) within the [UO22+, CH3OH] complex. Its absorbance is directly proportional to
concentrations of both uranyl (VI) and methanol, being consistent with the equilibrium for the
formation of the EDA complex. The features of high intensity and a band being very broad for
the absorption are also characteristic of intermolecular charge-transfer absorptions. The single-
electron transfer, which can take place photochemically as well as thermally in the ground state,
.
+
gives rise to formations of both UO2 and CH2OH (identified by EPR) in the UO22+–CH3OH–
H2O solutions. The charge-transfer absorptions of the UO22+–CH3OH–H2O solutions are similar
to those for the UO22+–X-–H2O (X- = Br- and I-) solutions we observed before [2]. Very recently,
charge-transfer absorptions for the UO22+–Cl-–H2O solutions have been observed at elevated
o
temperatures up to 250 C, showing their intensities were enhanced by increasing temperature
[17].
Previous investigations have shown that the OH group in an alcohol (e.g. ethanol and
2+
PVA) can coordinate to the uranium center of UO2 in the equatorial positions forming a
uranyl–alcohol adduct [3,18]. By its nature, the EDA complex [UO22+, CH3OH] that we have
observed from the above UV-Vis study could be such a uranyl–alcohol adduct. We believe that
within this EDA complex, a single-electron transfer between a methanol CH bond and the
2+
uranium center of UO2 (an inner-sphere charge-transfer) possibly occurs leading to the
.
+
formation of an ion-radical [UO2 , HOCH2 ] pair (adduct) via the same oxygenuranium
.
+
coordination bond. Formation of a [UO2 , HOCH2 ] adduct (ion-radical pair) is comparable to
.
.
+
the connection of UO2 to the OH groups in PVA radical (–CH2–C (OH)–CH2–) produced from
2+
the -ray induced photochemical reduction of UO2 by PVA [3]. The overall possible
12

2+
mechanism for the thermal charge-transfer reduction of UO2 by CH3OH is illustrated in the
following scheme (Eq.4).
2+
+
CT
- H+
.
UO2+ + CH2OH
UO22+ + CH3OH
O=U=O
O=U=O
e
O
O
H
H
H
.
H2C
CH2
.
+
[UO22+, CH3OH]
[UO2 , CH2OH]
(4)
2+
First, the hydroxyl –OH in methanol interacts with uranium in UO2 to form an EDA
2+
complex [UO2 , CH3OH] via a weak oxygenuranium coordination bond. The reversible
formation of [UO22+, CH3OH] is followed by an irreversible charge-transfer within the EDA
.
+
complex to lead to an ion-radical pair [UO2 , CH2OH] (adduct). We believe that the spin-spin
.
+
interactions between UO2 and CH2OH evidenced by EPR spectroscopy (Figures 1 and 4) in
anhydrous methanol take place within the ion-radical pair. In anhydrous methanol, the ion-
radical pair is stable and its dissociation is unfavorable. As the content of water in the solution
.
+
increases, the CH2OH ligand coordinating to the uranium center in UO2 can be effectively
.
+
+
displaced by the H2O molecules, and the [UO2 , HOCH2 ] adduct dissociates to UO2
.
(subsequently hydrated) and the discrete CH2OH radical (Eq.5).
.
.
+
+
[UO2 , H2O] + HOCH2
[UO2 , HOCH2 ] + H2O
(5)
+
As a result, the splitting in the UO2 signal (spin-spin interaction) disappears.
2+
We performed a photochemical reaction of UO2 with CH3OH by irradiating a mixture
2+
of UO2 (0.05 M, as the nitrate salt) and anhydrous methanol with a mercury lamp (350 nm).
Then the reaction mixture was characterized by EPR spectroscopy in the frozen state at 80 K
.
.
(Figure 6), showing signals of both CH2OH (axial signal) and CH3O (rhombic signal), but no
13

+
+
UO2 signal was observed. The lack of UO2 is consistent with the previous investigations on
.
bleaching of the uranyl (VI) excited state (UO22+)* by CH3OH, for which CH2OH was identified
+
2+
experimentally, but UO2 was not [7]. By comparison with the thermal reduction of UO2 with
+
CH3OH in the dark, we have reinforced that the lack of UO2 in photochemical reactions is due
+
2+
to a facile disproportionation of the initially formed UO2 to UO2 and U4+ in the light [7,9].
+
Alternatively, the more recent research has suggested that UO2 may also be oxidized back to
2+
UO2 photochemically by the O2 dissolved in the solvent in the presence of the UV light [19,
2+
20]. By comparison with the thermal reaction of UO2 with CH3OH which essentially gave
.
.
CH2OH (Figure 1a), we believe that the initially formed CH2OH underwent subsequent
.
photochemical isomerization to CH3O likely catalyzed by the methanol solvent molecule (Eq.6).
.
.
.
[
]
CH2OH
CH O H
+
CH3O
H
CH2OH
CH3O + CH3OH
3
(6)
.
The formation of CH3O on flash photolysis of the UO22+–CH3OH mixtures was not
found previously [7,9]. We are here reporting the substantial photochemical isomerization of
.
.
CH2OH to CH3O for the first time.
2+
+
3.2 Thermal charge-transfer reduction of uranyl UO2 (VI) to UO2 (V) by ethanol, 2-
propanol, and cyclohexanol, and the variable power EPR studies of -hydroxyalkyl and
alkoxy radicals
2+
We characterized the reaction mixtures of UO2 (0.05 M, as the nitrate salt) with
anhydrous ethanol, and with ethanol and water (volume ratio EtOH : H2O = 1:1) by EPR (Figure
7a and Figure 7b) after the mixtures were incubated at room temperature in the dark for 24 h. We
14

2+
also measured the EPR spectra (Figure 7c and Figure 7d) of mixtures of UO2 (0.05 M, as the
acetate salt) with anhydrous ethanol and 98% sulfuric acid, with overall molar concentrations of
H2SO4 in the media being 1 M and 2 M respectively, after the mixtures were incubated at room
temperature in the dark for 24 h. Both Figure 7a and Figure 7b showed strong axial signals
2+
(Signals A) analogous to that of the UO2 –CH3OH mixture (Figure 1a). The signals are
.
+
attributable to the -hydroxyethyl CH3 CHOH radical. The UO2 signal is either not observed
(Figure 7a) or very weak (Figure 7b). Figure 7c shows that when [H2SO4] = 1 M in the medium,
+
a broad UO2 signal and a stronger rhombic signal (Signal B) were observed in addition to an
.
+
axial signal (Signal A, CH3 CHOH). As the concentration of H2SO4 increased to 2 M, the UO2
+
2+
signal disappeared presumably due to disproportionation of the initially formed UO2 to UO2
2+
and U4+. Strong rhombic signals were also observed (Figure 7d). By analogy to the UO2
–
CH3OH system (Figure 4), the higher-field portion of the rhombic signal (B) observed in Figure
.
7c and Figure 7d is attributable to the ethoxy CH3CH2O radical.
The experimental EPR spectrum in Figure 7d has been simulated (Figure 8). The
.
simulation shows that the g-values for the axial signal (Signal A, CH3 CHOH) are g║= 2.033 and
.
g┴ = 2.014 with broadening 0.52, and the g-values for the rhombic signal (Signal B, CH3CH2O )
are gx = 2.033, gy = 2.017, and gz = 2.001 with broadening 0.52. The ratio of axial to rhombic
signal is 2:9. Figure 8 shows that the simulated and experimental spectra are essentially
consistent.
The EPR data has indicated that there was a thermal charge-transfer redox reaction
.
2+
+
between UO2 and CH3CH2OH to give UO2 and CH3 CHOH (Eq.7), and in the presence of
15

.
.
H2SO4 some -hydroxyethyl CH3 CHOH radical isomerizes to the ethoxy CH3CH2O radical
.
.
partially (Eq.8), following the same mechanism as that for isomerization of CH2OH to CH3O
(Eq.3).
.
+
UO22+ + CH3CH2OH
UO2 + CH3 CHOH + H+
(7)
(8)
-
HSO4
.
.
CH3 CHOH
CH3CH2O
2+
The thermal (in the dark) and photochemical (with Hg UV lamp) reactions of UO2 (as
the nitrate salt) with 2-propanol (CH3)2CHOH were conducted at room temperature and the
2+
products were identified by EPR spectroscopy. Since both solubility of UO2 in (CH3)2CHOH
and solubility of (CH3)2CHOH in water are low, the reactions were performed in the liquid
acetone media, with volume ratio of 2-propanol : acetone = 1:1. An EPR spectrum (Figure 9a)
was recorded on a frozen mixture of UO22+(0.05 M)–2-propanol–acetone at 80 K after the
mixture was incubated in the dark at room temperature for 24 h. The spectrum exhibited a weak
+
broad UO2 signal and a strong axial signal. By analogy to the spectra of UO22+–CH3OH and
UO22+–CH3CH2OH systems, the axial signal is attributable to the -hydroxyl-2-propyl
.
(CH3)2C OH radical. Figure 9b shows the EPR spectrum of a frozen mixture of UO22+(0.05 M)–
2-propanol–acetone at 80 K after the mixture was irradiated by a Hg UV-lamp (350 nm) for 0.5 h
+
at room temperature. A broad UO2 signal was observed clearly. A moderately strong upper-field
portion of the rhombic signal is apparent. The rhombic signal is attributable to the 2-propy
.
(CH3)2CH–O radical by comparison to the equivalent signal in Figures 1, 6, and 7.
Figure 9b has been simulated, with ratio of axial to rhombic signal being 1:5. The g-
values of axial signal are g║ = 2.033 and g┴ = 2.017. The g-values of rhombic signal are gx =
16

2.033, gy = 2.017, and gz = 2.002. The broadening is 0.4 for both axial and rhombic signals.
Similar to the simulations of Figure 1c and Figure 7d, the simulated spectrum for Figure 9b is
essentially consistent with the experimental spectrum.
2+
The EPR data indicate that UO2 and (CH3)2CHOH underwent both thermal and
.
+
photochemical charge-transfer redox reactions to give UO2 and (CH3)2C OH (Eq.9). In the light,
.
.
some (CH3)2C OH isomerized to (CH3)2CH–O partially (Eq.10).
.
+
UO22+ + (CH3)2CHOH
(CH3)2CHOH
UO2 + (CH3)2 COH + H+
(9)
.
.
(CH3)2CH_O
(CH3)2 COH
(10)
The photochemical isomerization (Eq.10) is catalyzed by (CH3)2CHOH, and it reinforces
.
.
the analogous methanol-catalyzed photochemical isomerization of CH2OH to CH3O (Eq.6).
.
Most likely, the 2-propanol-catalyzed photochemical isomerization of (CH3)2 COH to
.
(CH3)2CH–O follows the same mechanism as that of the methanol-catalyzed photochemical
.
.
isomerization of CH2OH to CH3O .
Variable power EPR measurements were conducted on a thermal reaction mixture of
2+
UO22+–CH3CH2OH–H2SO4 (1 M) and a photochemical reaction mixture of UO2 –
(CH3)2CHOH–acetone. For both systems, the EPR spectra exhibited several signals (Figure 7c
and Figure 9b). The intensity of each signal was determined at different microwave powers. The
results are included in Figure 10. Figure 10a (the UO22+–CH3CH2OH–H2SO4 mixture) shows
that intensities of both the lower-field peak and middle spectral component reached a maxima
(saturation) at the power of 1.5 mW (Lines 1 and 3). As the power further increased, the
intensities decreased. However, the upper-field signal was not saturated at 1.5 mW. Instead, its
17

2+
intensity kept increasing as a function of the power until 8 mW (Line 2). Figure 10b (the UO2
–
(CH3)2CHOH–acetone mixture) shows that intensities of the axial system reached its maximum
(saturation) at the power of 1.3 mW (Lines 1 and 3). As the power further increased, the
intensities decreased. However, the upper-field portion of the rhombic signal was not saturated
(maximum intensity) until the power reached 4-5 mW (Line 2). The results have confirmed that
the axial signal A (the lower-level peak and middle component) in each spectrum originates from
.
.
the same -hydroxyalkyl (CH3 CHOH or (CH3)2C OH) radical, while the upper-level portion of
.
.
rhombic signal (B) originates from another alkoxy (CH3CH2O or (CH3)2CH–O ) radical. The
variable power EPR measurements for all the UO22+–alcohol (methanol, ethanol, and 2-
propanol) mixtures are consistent, supporting the assignment of the axial signals to an –
hydroxyalkyl radical and the assignment of the rhombic signal to an alkoxy radical.
The thermal reaction of UO22+ with cyclohexanol was performed by incubating a mixture
2+
of UO2 (0.05M, as the nitrate salt) and cyclohexanol (0.3 M) in liquid acetone in the dark at
room temperature for 24 h. Then an EPR spectrum was recorded on the frozen reaction mixture
+
at 80 K (Figure 11). A weak UO2 signal and an axial signal were observed, which were
attributed to the -hydroxycyclohexyl radical. The spectral data shows that a thermal charge-
2+
+
transfer reduction of UO2 by cyclohexanol has taken place to give UO2 and -
hydroxycyclohexyl radical (Eq.11).
OH
.
2+
+
+ UO2 + H+
+ UO2
OH
H
(11)
+
Although we have got EPR evidence for the spin-spin interactions between UO2 and
.
+
hydroxymethyl CH2OH radical, no evidence for the spin-spin interactions between UO2 and
18

other -hydroxyalkyl (-hydroxyethyl, -hydroxyl-2-propyl, and -hydroxycyclohexyl) radicals
was found in this research.
We further characterized the charge-transfer reduction of UO22+ with ethanol, 2-propanol,
and cyclohexanol by UV-Vis spectroscopy. Similar to the UO22+–CH3OH–H2O mixtures, the
2+
UV-Vis spectra of UO2 (0.05 M, as the nitrate salt) in the mixtures containing ethanol, 2-
propanol, and cyclohexanol exhibited strong broad absorptions at 350–400 nm (Figure 12). The
absorption bands centered at ~420 nm in the spectra of samples containing lower concentrations
of the alcohols originate from the promotion of an electron from a UO22+ axial oxygen 2p orbital
to a uranium (VI) 5f nonbonding orbital [15, 16]. As the concentration of the alcohol increased in
each sample, a strong absorption evolved at 350–400 nm, with A375 (the absorbance at 375 nm)
being directly proportional to the molar concentration of the alcohol. By comparison to the
spectra of the UO22+–CH3OH–H2O mixtures, the absorption at 350–400 nm is due to a 1:1 EDA
2+
[UO2 , ROH] complex (ROH = ethanol, 2-propanol, or cyclohexanol) as a result of a single-
2+
electron transfer from an –CH bond in ROH to the uranium valence shell of UO2 within the
2+
[UO2 , ROH] complex. The single-electron transfer, which can take place photochemically as
+
well as thermally in the ground state, gives rise to the formations of both UO2 and an –
hydroxyalkyl radical (identified by EPR) in the UO22+–alcohol mixtures.
2+
+
3.3 Thermal charge-transfer reduction of uranyl UO2 (VI) to UO2 (V) by diphenyl sulfide
+
(Ph2S), and evidence for spin-spin interactions between UO2 (V) and diphenyl sulfide
.
radical cation (Ph2S+ )
Photolytic studies of the reaction of UO22+ and diaryl sulfides (R2S) have shown that R2S
.
can be oxidized by the excited (UO22+)* in the UV light to a cationic diaryl sulfide radical R2S+ ,
19

but no reduced uranium species was observed experimentally [8, 21]. In the present work, we
2+
have carried out the redox reaction of UO2 and Ph2S thermally in the dark at normal room
temperature and then characterized the reaction products by EPR spectroscopy. Figure 13 shows
the EPR spectrum of a reaction mixture of UO22+ (as the nitrate salt) and Ph2S in liquid acetone
+
that was incubated in the dark at ambient temperature for 24 h. Both UO2 (g = 2.075) and the
.
Ph2S+ radical cation (g = 1.999) were observed, showing that a single-electron transfer took
place from Ph2S (electron donor) to UO22+ (electron acceptor) to lead to an oxidation-reduction
reaction (Eq.12).
.
2+
+
UO2 + Ph2S
UO2 + Ph2S+
(12)
2+
Similar to the reduction of UO2 by anhydrous methanol, the EPR spectrum of the
+
UO22+–Ph2S–acetone mixture (Figure 13) exhibited splitting on the UO2 signal, suggesting the
.
+
spin–spin interaction between UO2 and Ph2S+ . Most likely, such an interaction occurs between
.
.
+
+
the unpaired electrons of UO2 and Ph2S+ within a [UO2 , Ph2S+ ] ion-radical pair.
The thermal charge-transfer redox between UO22+ and Ph2S has been further studied in
this work by UV-Vis spectroscopy. Figure 14-left shows the UV-Vis spectra of the initial
2+
mixtures of UO2 (0.05 M, as the nitrate salt) and various concentrations of Ph2S in liquid
acetone. All the spectra exhibited a broad band with the maximum absorption being at ~420 nm.
It is due to the transfer of an electron from a UO22+ axial oxygen 2p orbital to a uranium (VI) 5f
nonbonding orbital [15, 16]. In addition, a strong absorption at 350–400 nm evolved as the
concentration of Ph2S increased gradually. The intensity of the absorption at 350–400 nm
(represented by the absorbance A375 at 375 nm) was shown to be directly proportional to the
molar concentration of Ph2S at a fixed concentration of UO22+ (0.05 M). We also recorded the
20

2+
UV-Vis spectra of the initial UO2 –Ph2S(1.4 M)–acetone mixtures with variable concentrations
of UO22+ (Figure 14-right) and found that A375 is directly proportional to molar concentration of
2+
UO2 as well at a fixed concentration of Ph2S (1.4 M). The linear relationships between A375
2+
and [Ph2S] (molarity) and between A375 and [UO2 ] (molarity) indicate the reversible formation
2+
of a 1:1 charge-transfer complex [UO2 , Ph2S] and the absorption at 350-400 nm originates
from a single-electron transfer from sulfur in Ph2S (electron donor) to the uranium center in
.
2+
+
UO2 (electron acceptor) within the EDA [UO22+, Ph2S] complex to give UO2 and Ph2S+
(Eq.13).
O
+U
O
O
+2U
O
.
+
..
:SPh2
.
CT
+
UO2 + Ph2S+
:SPh2
2+
UO2 + Ph2S
.
[UO2 , Ph2S+ ]
+
[UO22+, Ph2S]
(13)
The absorbance for this absorption is directly proportional to concentrations of both
2+
2+
UO2 and Ph2S, being consistent with the equilibrium for the formation of the EDA [UO2
,
2+
Ph2S] complex. Analogous to the [UO22+, CH3OH] complex, the EDA complex [UO2 , Ph2S] is
reasonably formed reversibly via an SU coordination bond. Then an irreversible single-
2+
2+
electron transfer from sulfur of Ph2S to the uranium center of UO2 occurs within the [UO2 ,
.
+
Ph2S] complex to lead to an ion-radical pair [UO2 , Ph2S+ ] which has been identified by EPR.
.
+
The ion-radical pair can undergo a reversible dissociation to separate UO2 and Ph2S+ . However,
in our experimental conditions, the dissociation seems unfavorable. Direct identification of both
.
+
UO2 and Ph2S+ by EPR from a mixture of UO22+ and Ph2S, together with the UV-Vis study, has
confirmed the above overall charge-transfer mechanism in the ground state (Eq.13).
21

2+
+
3.4 Thermal charge-transfer reduction of uranyl UO2 (VI) to UO2 (V) by L-ascorbic acid
L-Ascorbic acid (AA) is a biologically relevant antioxidant (reductant). We carried out a
2+
thermal reduction of UO2 (0.05 M, as the acetate salt) by AA (0.5 M) in aqueous H2SO4 (0.5
M) solution. The reaction mixture was incubated in the dark at ambient temperature for 24 h. The
+
resulting EPR spectrum (Figure 15a) showed a broad signal of UO2 (g = 2.07) and a strong
.
sharp singlet signal (g = 2.00) attributable to the L-ascorbic acid radical (AA ). The result
revealed a charge-transfer redox reaction between UO22+ and AA effected by transfer of a single-
.
+
electron from an OH group of AA to uranium in UO22+ to give UO2 and AA (Eq.14).
HO
e
HO
HO
OH
OH
CT
- H+
.
2+
+
UO2
UO2
+
O
+
O
O
U(VI)
U(V)
O
HO
O
HO
L-Ascorbic Acid
(AA)
L-Ascorbic Acid Radical
.
(AA )
(14)
.
The unpaired electron in AA can be possibly strongly attracted to the uranium valence
.
+
shell of UO2 . This may be comparable to the CH2OH radical formed photochemically in the
AgNa-A zeolite, the unpaired electron of which is attracted to the valence shell of silver to form
an Ag.CH2OH one-electron "half-bond" [22]. The possible interaction between the unpaired
.
+
electron in AA and the uranium center in UO2 can prevent the electron from delocalizing to the
ring, giving rise to a singlet EPR signal.
Similar to the UO22+/alcohol and UO22+/Ph2S mixtures, the UV-Vis spectra of the initial
2+
mixtures of UO2 (0.05 M, as the acetate salt) and L-ascorbic acid (AA) in the aqueous media
22

containing H2SO4 (0.5 M) exhibited a strong absorption at 350–400 nm (Figure 15b). This
absorption became particularly strong when the AA concentration was greater than 0.4 M. The
intensity of the absorption (represented by the absorbance A375 at 375 nm) was shown to be
directly proportional to the molar concentrations of AA and UO22+, respectively, showing
2+
formation of an EDA complex between UO2 and AA in the initial reaction mixtures. The
single-electron transfer process in Eq.14 takes place within the EDA complex [UO22+, AA] as
illustrated in Eq.15.
.
2+
+
UO22+ + AA
[UO2 , AA]
[UO2 , AA ]
(15)
2+
+
3.5 Thermal charge-transfer reduction of uranyl UO2 (VI) to UO2 (V) by 2-methyl-5-
(propan-2-yl)phenol (carvacrol)
2-Methyl-5-(propan-2-yl)phenol (carvacrol) is a biological antioxidant [23, 24]. We
2+
carried out a thermal reduction of UO2 (0.05 M, as the nitrate salt) by carvacrol at different
concentrations (0.04 M, 0.09 M, and 0.22 M respectively) in acetone. The reaction mixtures were
incubated in the dark at ambient temperature for 24 h and then characterized by EPR
+
spectroscopy (Figure 16). All the spectra in Figure 16 exhibited a broad signal of UO2 (g =
2.06) and a sharp singlet of 2-methyl-5-(propan-2-yl)phenoxyl (carvacrol radical, g = 1.99). The
EPR results have shown that a single-electron transfer from the OH oxygen in carvacrol to the
2+
uranium valence shell in UO2 took place to bring about the oxidation-reduction reaction
(Eq.16).
.
OH
O
+
2+
+ UO2
+ H+
+ UO2
Carvacrol
Carvacrol
radical
(16)
23

Figure 16 also exhibited that the ratio of the intensity of carvacrol radical signal to
+
intensity of the UO2 signal increases as the initial concentration of carvacrol in the mixtures
.
increases. The spin density (unpaired electron) in various phenoxyl radicals (ArO ) has been
demonstrated to undergo substantial delocalization to the aromatic ring [25]. In the presence of
+
UO2 , the unpaired electron in carvacrol radical can be possibly strongly attracted to the uranium
.
+
+
valence shell of UO2 , analogous to the above AA /UO2 interactions. This may have prevented
the delocalization of the unpaired electron to the aromatic ring and gives rise to a sharp singlet
EPR signal for carvacrol radical as seen in Figure 16.
2+
Analogous to the UO2 /alcohol (methanol, ethanol, 2-propanol, and cyclohexanol)
mixtures, UO22+/Ph2S mixtures, and UO22+/ascorbic acid mixtures, the UV-Vis spectra of the
initial mixtures of UO22+ (0.05 M, as the nitrate salt) and carvacrol (ArOH) in acetone exhibited
a strong absorption at 350–400 nm (Figure 17). The intensity of the absorption (represented by
the absorbance A375 at 375 nm) has been shown to be directly proportional to the molar
concentrations of ArOH and UO22+, respectively, indicating the reversible formation of an EDA
2+
2+
complex [ArOH, UO2 ] between UO2 and ArOH in the initial reaction mixtures. Comparable
2+
2+
to the [UO2 , Ph2S] complex, [ArOH, UO2 ] is formed most likely via a weak
oxygenuranium coordination bond. Then an irreversible single-electron transfer takes place
.
2+
+
within the EDA complex [ArOH, UO2 ] to give ArO and UO2 (Eq.17) as observed by EPR.
e
CT
- H+
.
2+
+
2+
[ArOH, UO2 ]
ArO + UO2
ArOH + UO2
EDA complex
(17)
It has been shown that the antioxidant activity of carvacrol is directly related to the
stability of the carvacrol radical [23, 24]. The observation of the stable carvacrol radical in this
24

work by EPR at ambient conditions has provided further supporting information for the studies
of biological antioxidant functions of carvacrol.
Similar to carvacrol, salicylic acid (SA) also belongs to phenols (ArOH), but contains an
electron withdrawing carboxyl (–CO2H) group in the aromatic ring. The UV-Vis spectra of the
UO22+/SA mixtures exhibited analogous strong absorption at 350–400 nm (Figure 18, Line 1).
By comparison with a spectrum of UO22+/benzoic acid (PhCO2H) mixture which lacks the
absorption at 375 nm (Figure 18, Line 2), the absorption due to the carboxyl group in salicylic
acid can be excluded. The absorption at 350–400 nm for salicylic acid is attributable to the
charge-transfer transition from the hydroxyl (–OH) oxygen attaching to the aromatic ring to
uranium in a [UO22+, SA] EDA complex reasonably formed via the interaction of the –OH in the
ring with the uranium center of UO22+ (Eq.18).
OH
hCT
.
+
[UO2 , SA ] + H+
[UO22+, SA]
2+
+ UO2
CO2H
EDA complex
Salicylic Acid (SA)
(18)
Due to the electron withdrawing –CO2H, SA does not undergo thermal charge-transfer
oxidation by UO22+. After irradiated by Hg lamp (350 nm), a UO22+–SA mixture in acetone
.
+
exhibited weak EPR signals attributable to UO2 and SA (a phenoxyl radical), supporting the
photochemical charge-transfer redox between UO22+ and SA.
4. Conclusions
Thermal charge-transfer reductions of UO22+(VI) by alcohols (methanol, ethanol, 2-
propanol, and cyclohexanol), diphenyl sulfide (Ph2S), ascorbic acid (AA), and 2-methyl-5-
(propan-2-yl)phenol (carvacrol, ArOH) have been conducted. For each thermal reduction, both
25

.
.
.
.
.
+
UO2 (V) and a radical ( CH2OH, CH3 CHOH, (CH3)2 COH, –hydroxycyclohexyl, Ph2S+ , AA ,
.
+
or ArO ) have been identified by EPR spectroscopy, while only a radical, but not UO2 (V), was
observable in most of the photochemical reactions due to facile disproportionation of the initially
+
formed UO2 to UO22+ and U4+ in the light.
.
.
+
Spin-spin interactions between UO2 and a radical ( CH2OH or Ph2S+ ) have been
evidenced from EPR studies. This and the UV-Vis studies together have supported the
.
.
+
+
formations of the [UO2 , CH2OH] and [UO2 , Ph2S+ ] ion-radical pairs in the course of the
thermal redox reactions of UO22+ with CH3OH and Ph2S, respectively.
A ground-state charge-transfer mechanism has been developed for thermal reductions of
UO22+(VI) by methanol and diphenyl sulfide on the basis of EPR and UV-Vis studies (Eq.4 and
Eq.13).
.
.
.
The sulfuric-acid-catalyzed isomerizations of CH2OH to CH3O and CH3 CHOH to
.
CH3CH2O have been found by EPR studies. The identifications of the two alcohol radicals for
methanol and ethanol have been confirmed by variable power EPR measurements.
Reduction of uranyl UO22+(VI) to the lower oxidation states of uranium [such as U(V)
and U(IV)] forms an interesting aspect of its chemistry. Among other things, it facilitates
separation of uranium from aqueous media (the water system) [2]. The present research not only
possesses academic significance, but may also have societal impacts in terms of radioactive
waste (uranium) disposal.
5. Acknowledgement
26

This work was funded by a National Science Foundation (NSF) grant (Award #
CHE1229498). It has also been supported financially by the University of Charleston through a
faculty sabbatical leave (X.S.) and aided by the University Chemistry Program operating fund.
We would like to thank Mr. Trevor Duncan, Mr. Roger Deal, and Ms. Joy Wu for helping obtain
some UV-vis spectroscopic data.
6. References
[1] J. J. Katz, G. T. Seaborg, L. R. Morss, The Chemistry of the Actinide Elements, Chapman and
Hall: London (1986), pp. 340-347.
[2] X. Sun, D. R. J. Kolling, H. Mazagri, B. Karawan, C. Pierron, Inorg. Chim. Acta 435 (2015)
117-124.
[3] M. Kumar, R. M. Kadam, A. R. Dhobale, M. D. Sastry, J. Nucl. Radiochem. Sci. 1 (2000),
77-80.
[4] C. Miyake, T. Nakase, Y. Sano, J. Nucl. Sci. Technol. 30 (1993), 1256-1260.
[5] M. Sarakha, M. Bolte, J. Phys. Chem. A. 104 (2000) 3142-3149.
[6] H. D. Burrows, Inorg. Chem. 29 (1990) 1549-1554.
[7] M. E. D. G. Azenha, H. D. Burrows, S. J. Formosinho, M. de G. M. Miguel, J. Chem. Soc.,
Faraday Trans. 1, 85 (1989) 2625-2634.
[8] T. J. Kemp, M. A. Shand, Inorg. Chem. 25 (1986) 3840-3843.
[9] S. Kannan, A. E. Vaughn, E. M. Weis, C. L. Barnes, P. B. Duval, J. Am. Chem. Soc., 128
(2006) 14024-14025.
[10] S. Stoll, A. Schweiger, J. Magn. Reson. 178 (2006) 42-55.
[11] S. Tsushima, Inorg. Chem. 48 (2009) 4856-4862.
27

[12] J. A. Well, J. R. Bolton, Electron Paramagnetic Resonance: Elementary Theory and
Practical Applications, 2nd ed., John Wiley & Sons: Hoboken, New Jersey (2007).
[13] Sr. P. J. Sullivan, W. S. Koski, J. Am. Chem. Soc. 85 (1963) 384-387.
[14] R. J. Buszek, A. Sinha, J. S. Francisco, J. Am. Chem. Soc. 133 (2011) 2013-2015.
[15] R. G. Denning, J. Phys. Chem. A, 111 (2007), 4125-4143.
[16] L. S. Natrajan, Coordination Chem. Rev. 256 (2012), 1583-1603.
[17] A. A. Migdisov, H. Boukhalfa, A. Timofeev, W. Runde, R. Roback, A. E. Williams-Jones,
Geochimica et Cosmochimica Acta, 222 (2018) 130-145.
[18] T. Harazono, S. Sato, H. Fukutomi, Bull. Chem. Soc. Jpn., 57 (1984) 768-770.
[19] K.-X. Wang, J.-S. Chen, Acc. Chem. Res., 44 (2011), 531-540.
[20] S. G. Thangavelu, C. L. Cahill, Inorg. Chem., 54 (2015), 4208-4221.
[21] C. P. Baird, T. J. Kemp, Prog. Reaction Kinetics, 22 (1997) 87-139.
[22] J. Michalik, J. Sadlo, A. van der Pol, E. Reijerse, Acta Chim. Scand., 51 (1997) 330-333.
[23] J. Mastelic, I. Jerkovic, I. Blazevic, M. Poljac-Blazi, S. Borovic, I. Ivancic-Bace, V.
Smrecki, N. Zarkovic, K. Brcic-Kostic, D. Vikic-Topic, N. Muller, J. Agric. Food Chem., 56
(2008) 3989-3996.
[24] N. V. Yanishlieva, E. M. Marinova, M. H. Gordon, V. G. Raneva, Food Chemistry, 64
(1999) 59-66.
[25] C. Xie, P. M. Lahti, C. George, Org. Lett. 2 (2000) 3417-3420.
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Figure 1
The EPR spectra of UO22+ (0.050 M, as the nitrate salt) in (a) anhydrous CH3OH, (b) the medium
with CH3OH : H2O = 2:1 (volume ratio), and (c) the medium with CH3OH : H2O = 1:1 (volume
ratio). UO22+ was incubated in each medium for 24 h in the dark at room temperature, and then a
+
spectrum recorded in the frozen state at 80 K. The splitting of the UO2 signal in (a) is circled. A:
.
The axial signal, assigned to hydroxymethyl CH2OH radical (major). B: The rhombic signal,
.
assigned to methoxy CH3O radical (minor).
29