Uranyl-Sensitized Photochemical Oxidation of Naphthalene by Molecular Oxygen. Role of Electron Transfer

Article abstract of DOI:10.1021/jp962961z

Naphthalene quenches the excited state *UO2(2+) in two parallel pathways: oxidation to naphthalene radical cation, C10H8(.+) (Φ = 0.3), and exciplex formation.The overall rate constant in aqueous acetonitrile containing 0.1 M H3PO4 has a value k_q = 2.20E9 M^-1 s^-1.In the presence of O2, a portion of c10H8(.+) is converted to 2-formylcinnamaldehyde and several other products, the rest reacting with UO2(1+) by back electron transfer.

Full text of DOI:10.1021/jp962961z

J. Phys. Chem. A 1997, 101, 7929-7933
7929
ARTICLES
Uranyl-Sensitized Photochemical Oxidation of Naphthalene by Molecular Oxygen. Role of
Electron Transfer
Yun Mao and Andreja Bakac*
Ames Laboratory, Iowa State UniVersity, Ames, Iowa 50011
ReceiVed: September 25, 1996; In Final Form: June 11, 1997X
2+
Naphthalene quenches the excited state *UO2 in two parallel pathways: oxidation to naphthalene radical
cation, C10H8•+ (Φ ) 0.3), and exciplex formation. The overall rate constant in aqueous acetonitrile containing
0.1 M H3PO4 has a value kq ) 2.20 × 109 M-1 s-1. In the presence of O2, a portion of C10H8•+ is converted
to 2-formylcinnamaldehyde and several other products, the rest reacting with UO2+ by back electron transfer.
Introduction
Chemicals, 99.8%) in perchloric acid. All other chemicals were
analytical grade or higher.
2+
*UO2
reacts with organic substrates by a variety of
Absorption spectra were recorded with use of a Shimadzu
3101 PC spectrometer. A Waters’ high-performance liquid
chromatograph, equipped with a C18 column (Alltech, Adsor-
bosphere) and a photodiode array detector (Waters 996), was
used to monitor the accumulation of products. The eluent was
aqueous acetonitrile. Mass spectra were measured by a GC-
MS (Magnum, Finnigan MAT) equipped with a capillary
column (DB5, 0.25 mm i.d. and 0.25 µm film), an EI/CI source,
and an ion trap assembly operated by use of the ITS40 software
package. 1H and 13C NMR spectra were recorded by use of
Varian 300 and Bruker DRX-400 spectrometers. Time-resolved
experiments used a laser photolysis system described elsewhere.5
pathways: H atom abstraction, addition to multiple bonds,
oxygen atom transfer, and exciplex formation.1-4 Electron
transfer has been observed only with easily oxidizable substrates,
such as ABTS2-,5 amino acids,6 and several others.7
+
H atom abstraction, eq 1, yields UO2 and C-centered
radicals, both of which react with molecular oxygen, eqs 2 and
3. Reaction 3 regenerates UO22+, and the photooxidation of
*UO22+ + RH f UO2+ + R• + H+
(1)
(2)
(3)
•
R• + O2 f RO2 f products
2UO2+ + O2 + 2H+ f 2UO22+ + H2O2
Because of the low solubility of naphthalene (∼0.2 mM) in
H2O, a cosolvent was required. We chose 15-50% aqueous
acetonitrile, which does not quench *UO22+ significantly.5 The
kinetic samples were prepared in 1-cm quartz cells with gastight
septa and irradiated with light from a 250-W quartz tungsten
halogen lamp (Oriel Corporation, 66181) or a solar lamp (GE,
250 W). To avoid direct excitation of naphthalene and the
photoproducts, the irradiation wavelength was adjusted with an
appropriate cutoff filter (Schott KV418) to λ > 420 nm. Most
of the reactions were conducted in 0.10 M H3PO4, where the
lifetime of the excited state2,7 (τ0 ≈ 100 µs) is much longer
than in the absence of complexing anions (τ0 ≈ 2 µs). The
samples for GC-MS and 1H NMR analyses were obtained by
extracting the reaction solutions with diethyl ether, evaporating
the ether in a hood, and dissolving the residue in CDCl3. All
experiments were carried out at room temperature (23 ( 2 °C).
RH becomes catalytic. We have recently demonstrated this
scheme for a number or organic compounds, including alkanes,
alkenes, and side-chain aromatics.5 Benzene, on the other hand,
2+
reacts with *UO2 exclusively by exciplex formation, and no
oxidation took place when O2 was used as oxidant. A different
reaction scheme, in effect a photoinduced Fenton-like reaction,
was devised to oxidize benzene to phenol with use of H2O2 as
terminal oxidant.8
The obvious advantages of O2 over H2O2 in catalytic
oxidations have prompted us to return to O2 in an attempt to
oxidize polynuclear aromatics, many of which are potent
carcinogens. We reasoned that the lower ionization potential
of polycyclic aromatics, as compared to benzene, may open up
one-electron oxidation as a feasible pathway. In the followup
chemistry of radical cations, O2 would once again become a
competent terminal oxidant.
Results
In this work we used the noncarcinogenic naphthalene as a
model for polynuclear aromatics. This reaction provides the
Spectral measurements showed no evidence for complex
formation between UO22+ (0.25 mM) and naphthalene (1 mM)
in 0.1 M H3PO4. A new broad band did form, however, in the
2+
first example of electron transfer from a hydrocarbon to *UO2
.
2+
Experimental Section
340-550 nm range when 1 mM 1-naphthol and 0.25 mM UO2
were mixed. This result is consistent with the formation of a
Naphthalene (Aldrich) was recrystallized from methanol.
Naphthalene-d8, 1-naphthol, and 2-naphthol (all Aldrich) were
used as received. Stock solutions of uranyl perchlorate (0.10
M) were prepared by dissolving uranium trioxide (Strem
weak UO22+-naphthol complex.9
Products. Figure 1 shows the HPLC chromatogram of an
air-saturated solution of 10 mM naphthalene and 0.25 mM
UO22+ in 25% aqueous MeCN after 60 min of irradiation. The
short retention times of all the products on a C18 column are
X Abstract published in AdVance ACS Abstracts, September 15, 1997.
S1089-5639(96)02961-1 CCC: $14.00 © 1997 American Chemical Society

7930 J. Phys. Chem. A, Vol. 101, No. 43, 1997
Mao and Bakac
Figure 2. 1H NMR spectrum in CDCl3 of the reaction products
obtained by irradiating a solution of 0.25 mM UO22+, 1.5 mM
naphthalene, and 0.1 M H3PO4 for 60 min in 25% aqueous CH3CN.
TABLE 1: Effect of Experimental Conditions on the Yield
of 2-Formylcinnamaldehydea
conditions
relative yield
air
O2
Ar
(1)
0.95
∼0
b
air/H2O2
1.5
a 1.5 mM naphthalene, 1.5 mM UO22+, in 25% aqueous acetonitrile
containing 0.1 M H3PO4 . b [H2O2] ) 0.1 M.
confirms the ortho arrangement of the two substituents, -CHO
and -CHdCH-CHO.
CHO
2+
hν, UO2
(4)
O2
α
CHO
β
Figure 1. (top) HPLC chromatogram of 10 mM naphthalene and 0.25
2+
mM UO2 in 25% aqueous CH3CN (0.1 M H3PO4) after 60 min of
In the 13C NMR spectrum, two prominent peaks in the
carbonyl region (192.86 and 193.93 ppm) provide additional
evidence for the two -CHO groups.
irradiation. Eluent was 40 vol % acetonitrile in water, λdetection 220 nm.
(bottom) Growth of photochemical products with irradiation time
(dialdehyde stands for 2-formylcinnamaldehyde).
With an electron ionization source (EI), the main peak in the
GC-MS chromatogram has a mass number 131, which we
assign to cinnamaldehyde (C6H5C2H2CHO - H ) 131), a
fragment of 2-formylcinnamaldehyde. The main peak in the
chemical ionization (CI) GC-MS chromatogram has a mass
number 161, which matches that of 2-formylcinnamaldehyde
[C6H4(CHO)(C2H2CHO + H]. The two minor peaks with mass
numbers 159 and 175/177 were not fully identified, but are
consistent with naphthoquinone and 2-formylcinnamic acid,
respectively.
The precise determination of the quantum yield of 2-formyl-
cinnamaldehyde could not be carried out without an independent
source of the compound for HPLC calibration. The yield was
estimated from the absorbance at 220 nm, which was ap-
proximately 5 times greater than the absorbance of the 1-naph-
thol band at the same wavelength. Under the reasonable
assumption that the molar absorptivities of the two compounds
do not differ at this wavelength by more than a factor of 10,
consistent with the products being more polar than naphthalene.
Only the peak at 4.1 min increased consistently with irradiation
time.
The species with the retention times of 11.8 and 12.8 min
were identified as 1-naphthol and 2-naphthol (C10H7OH),
respectively, by comparison to the chromatograms of the
commercial compounds. The greater yield of 1-naphthol is
reasonable in view of the greater electron density at the C1
position of naphthalene.10 The quantum yield of 1-naphthol
was determined by comparison with the yield of benzaldehyde
produced by oxidation of toluene in a similar system.11 The
concentrations of naphthalene (1 mM) and toluene (3.8 mM)
in two separate experiments were chosen such that 98% of
2+
*UO2 was quenched by the substrates, the rest decaying
spontaneously. The yield ratio of the respective products,
1-naphthol and benzaldehyde, was determined by HPLC, and
led to Φ ≈ 1 × 10-4 for 1-naphthol.
and taking Φ ) 10-4 for 1-naphthol, we obtain 5 × 10-5
<
The 4.1-min species was identified as 2-formylcinnamalde-
hyde on the basis of GC-MS, 1H NMR, and 13C NMR spectra.
The 1H NMR spectrum, Figure 2, exhibits resonances at 10.16
ppm (singlet, -CHO bound to benzene), 9.74 ppm (doublet, J
) 8 Hz, -CHO adjacent to R-CH), 6.61 ppm (doublet of
doublets, R-CH), 8.55 ppm (doublet, J ) 15.6 Hz, â-CH), and
7-8 ppm (aromatic resonances). A weak signal at ∼7 ppm
was not assigned. The areas under the two aldehydic hydrogens
are comparable, suggesting that both -CHO groups belong to
the same compound derived from naphthalene by ring cleavage
and oxidation.12-17 The large downfield shift of the â-hydrogen
Φ(2-formylcinnamaldehyde) < 5 × 10-3
.
The appearance of HPLC chromatograms and the yield of
dialdehyde were unchanged when naphthalene was replaced by
naphthalene-d8. As shown in Table 1, no products were detected
when the reaction was carried out under argon, but air-saturated
and O2-saturated samples had similar product yields.
The presence of H2O2 in the reaction system increases the
dialdehyde yield, Table 1. The reaction scheme under these
conditions may be similar to that in the benzene/UO22+/H2O2
system, where H2O2 reacts with UO2+ and produces an oxidizing

Photochemical Oxidation of Naphthalene by O2
J. Phys. Chem. A, Vol. 101, No. 43, 1997 7931
2+
TABLE 2: Kinetic and Product Data on Photochemical Reactions between UO2 and Aromatic Compounds
quenchera
kq/M-1 s-1 a,b
products
naphthalene
naphthalene-d8
1-naphthol
2.20(3) × 109
2.14(3) × 109
2.90(4) × 109
2.38(4) × 109
3.81(7) × 109
2.30(10) × 109
1.33(2) × 109
1-naphthol, 2-naphthol, 2-formylcinnamaldehyde
1-naphthol-d7, 2-naphthol-d7, 2-formylcinnamaldehyde-d6
2-naphthol
1,5-dihydroxynaphthalene
1,4-dimethoxybenzenec
ABTS2- d
•+
(CH3O)2C6H4
ABTS•+
a Rate constants for the quenching of *UO22+. Conditions: 1.0 mM UO22+, 0.1 M H3PO4, 23 ( 2 °C. The reaction with naphthalene was carried
out in 50% aqueous acetonitrile. All other reactions used H2O as solvent. b Numbers in parantheses represent one standard deviation of the last
significant figure. c Dimethoxybenzene radical cation was observed at 460 nm. d Reference 5.
TABLE 3: Oxidation Products of Naphthalene
reaction
reaction system
type
products
2+ a
C10H8/air/UO2
photolysis
γ-radiolysis
thermal
thermal
thermal
1-naphthol, 2-naphthol, 2-formylcinnamaldehyde, others
1-naphthol (68%), 2-naphthol (32%)
1-naphthol, 2-naphthol, salicylic acid, carboxyhydroxycinnamic acid, CO2, others
1-naphthol, 1,4-naphthoquinone, 2-formylcinnamaldehyde
1-naphthol, 2-naphthol, dihydroxydihydronaphthalene, 2-formylcinnamaldehyde, others
phthalic acid (49%)
C10H8/Fe(CN)63-/N2Ob
c
C10H8/air/Fe2+/H2O2
d
C10H8/PdCl2/HAc/SiO2
e
C10H8/Fe/mercaptobenzoate/O2
C10H8/air/SiO2(Al2O3)f
photolysis
a This work. Conditions: air-saturated 0.1 M H3PO4 in 20% aqueous CH3CN, 0.2 mM naphthalene, 0.5 mM UO22+, λirr > 420 nm. b γ-Radiolysis
in N2O-saturated aqueous solution, 0.2 mM naphthalene, 1 mM K3Fe(CN)6, ref 10. c Aqueous solution, ref 27 . d In acetic acid, ref 16. e Udenfriend/
Ullrich system, ref 28. f Solid state, naphthalene loading 2 × 10-5 mol/g, λirr > 300 nm, ref 36.
intermediate, presumably HO•, which initiates further oxidation
of substrate.8
The Kinetics of the quenching of *UO22+ with naphthalene
and several other compounds were measured by monitoring the
2+
luminescence of *UO2
. The data for all the compounds
obeyed the rate law of eq 5, where k0 represents the rate constant
for the spontaneous decay of *UO22+, kq is the quenching
constant, and [Q] is the concentration of the quencher.
2+
-d[*UO22+]/dt ) kobs[*UO22+] ) (k0 + kq[Q])[*UO2
]
(5)
From a linear plot of kobs Vs [naphthalene], we obtain kq ) 2.20
× 109 M-1 s-1. The phenolic derivatives have similar rate
constants. For example, kq ) 2.9 × 109 M-1 s-1 for 1-naphthol.
The adherence of all the data to eq 5 shows that either the
UO22+-naphthol complex does not form in appreciable amounts
at the low concentrations of 1-naphthol (<0.1 mM) used in the
kinetics experiments or that the complex is kinetically insig-
nificant.
Figure 3. Transient absorption spectrum obtained 3.5 µs after the laser
flash. Conditions: 4.0 mM UO22+, 1.5 mM naphthalene, 0.1 M H3-
PO4, 25% aqueous CH3CN, λexc 423 nm.
(C10H8•+) ) [C10H8•+]/[*UO22+] ) 0.3 ( 0.1. As expected,
this yield was the same irrespective of whether the reaction was
conducted under O2 or Ar.
The value for naphthalene-d8, kq ) 2.14 × 109 M-1 s-1, is
comparable to that for naphthalene. The negligible kinetic
isotope effect, combined with the lack of deuterium isotope
effect on product yields, is consistent with the reaction taking
place by electron transfer and/or exciplex formation, i.e. a
reaction that does not involve a C-H bond breaking in a
kinetically significant step. The kinetic data for all the reactions
studied are summarized in Table 2.
•+
The lifetime of C10H8 was ∼10 µs in 50% aqueous
•+
acetonitrile at 0.1 M H3PO4. As discussed later, most of C10H8
+
disappears in back electron transfer with UO2
.
2+
The quenching of *UO2 by 1,4-dimethoxybenzene takes
place with a rate constant k ) 2.3 × 109 M-1 s-1. The reaction
produces a radical cation, 1,4-C6H4(OCH3)2•+, which was
identified by its UV-visible spectrum.21
A point-by-point UV-visible spectrum of the intermediate
Discussion
2+
produced by quenching of *UO2 by naphthalene, Figure 3,
shows the characteristic broad absorption of C10H8•+ 18,19 in the
600-750-nm range. Transient absorption also was observed
in the 300-500-nm range, as expected for C10H8•+, although
these measurements were complicated by high background
absorbance. Thus at least part of the quenching reaction takes
place by electron transfer. The concentration of naphthalene
radical cations (ꢀ690 ) 1350 M-1 cm-1),18 produced from
*UO22+ and naphthalene (1.5 mM) was determined in laser flash
A significant fraction (∼30%) of the *UO22+-naphthalene
reaction produces naphthalene radical cations, C10H8•+, clearly
demonstrating the electron-transfer nature of the process, eq 6a.
2+
The 70% of *UO2 that does not yield any observable
intermediates presumably takes place by exciplex formation,
eq 6b, as suggested previously for other unsaturated com-
pounds.3,22 The experiments were carried out in 0.1 M H3PO4,
2+
2+
a medium where phosphate complexes of UO2 and *UO2
2+
experiments. The concentration of *UO2 (ꢀ570 ) 4500 M-1
predominate2 and almost certainly take part in reaction 6 and
beyond. For the sake of simplicity, the coordinated phosphate
ions are not shown.
cm-1 5,20
was determined under identical conditions in the
)
absence of naphthalene. The data were combined to give Φ-

7932 J. Phys. Chem. A, Vol. 101, No. 43, 1997
Mao and Bakac
+
•
UO2+ + C10H8
(6a)
(6b)
SCHEME 1
*UO22+ + C10H8
2+
(UO2 • • •C10H8)*
UO22+ + C10H8
2+
Even though the oxidation of organic substrates by *UO2
is fairly common,1,2,5,7,23-25 such reactions typically take place
by hydrogen atom abstraction or initial addition to multiple
bonds. To our knowledge, an outer-sphere electron transfer
2+
between *UO2 and a hydrocarbon has not been reported
previously. As expected, the rate constant k6a (∼0.3 × k6 ∼ 7
× 108 M-1 s-1) is somewhat smaller than those for the more
easily oxidizable ABTS2- (1.3 × 109 M-1 s-1) and 1,4-
dimethoxybenzene (2.3 × 109 M-1 s-1). A more quantitative
comparison would be difficult to make, because all three
reactions approach the diffusion-controlled limit. The data do
suggest, however, that the proportion of electron transfer to
exciplex formation would become even greater for the larger,
more strongly reducing26 polynuclear aromatics.
2+
(C16H10•+) with k ) 1.0 × 1010 M-1 s-1 34
,
and Euaq reacts
with C10H8•+ with k ) 3.9 × 109 M-1 s-1 35
.
The driving force26
for reaction 10 is smaller than that for the reaction of C10H8
with Eu2+, and the rate constant k10 on the order of 108-109
M-1 s-1 appears reasonable.
•+
Naphthols are typical products of oxidation of naphthalene
in aqueous solvents.10,16,27,28 In the commonly accepted mech-
Table 3 compares the products of oxidation of naphthalene
in several systems. Naphthols are produced in most of the
reactions, but the yield and distribution of all the products vary
widely, presumably because of the high reactivity of intermedi-
ates involved. The Fenton reaction,27 which involves HO•
radicals, and Udenfriend’s system,28 which is believed to utilize
iron-oxo and -peroxo species, yield a large number of products.
γ-Radiolysis in the presence of Fe(CN)63- produces naphthols
anism of eqs 7-9, the OH adduct is produced either directly
•+
by addition of HO• to naphthalene or by hydrolysis of C10H8
The intermediates produced by one-electron oxidation (C10H8
.
•+
)
and in Fenton-type reactions (C10H8(OH•)) are thus related by
acid-base chemistry of eq 8. Further oxidation of C10H8(OH)•
then yields a mixture of naphthols.29,30 In the *UO22+-initiated
2+
oxidation, the oxidant in eq 9 is either O2 or UO2
. No
in a scheme6 that involves rapid oxidation of C10H8(HO•)
products were observed in the absence of O2, which may mean
that it plays a role in eq 9. Another possibility is that O2 helps
the reaction only indirectly by keeping the concentration of
3-
adducts by Fe(CN)6
.
A PdCl2-based system16 produces
2-formylcinnamaldehyde and 1-naphthol. The latter is oxidized
further to 1,4-naphthoquinone. In the photochemical UO2
2+
UO2+ low,31 eq 3, thus slowing down the back electron transfer
/
2+
O2 system described here, 2-formylcinnamaldehyde is the major
product, Scheme 1. The yields are low, but the results are
encouraging in that electron transfer is a significant pathway in
the quenching process. We expect the yields of products,
especially naphthols, to increase in the presence of sacrificial
of eq 10. In this scenario, the oxidant in eq 9 is UO2
.
C10H8 + HO• f C10H8(OH)•
(7)
(8)
C10H8•+ + H2O a C10H8(OH)• + H+
+
oxidants, which would rapidly remove UO2 and thus prevent
C10H8(OH)• 9Ox8 C10H7(OH)
(9)
the back electron transfer of eq 10.
UO2+ + C10H8•+ f UO22+ + C10H8
(10)
Conclusions
The photochemical oxidation of naphthalene by UO22+/O2
yields 2-formylcinnamaldehyde as a major product. Minor
amounts of 1- and 2-naphthols were also found. The reaction
takes place by one-electron oxidation of naphthalene, followed
by further reactions of C10H8•+. The quantum yield for the
The formation of 2-formylcinnamaldehyde as a major product
shows that the ring cleavage is an important process. As shown
in Scheme 1, this pathway probably starts with deprotonation
of the strongly acidic32 radical cation followed by the reaction
with O2 and formation of the peroxyl radicals. The radical
center may couple with the neighboring ring carbon to produce
a transient cyclic peroxide,13,15 although there is no direct
evidence for such intermediates.28 Irrespective of whether the
cyclic peroxide is involved or not, further reaction requires a
hydrogen source (possibly solvent) or one-electron reduction
(possibly by UO2+) followed by protonation to yield the
dialdehyde. A similar mechanism has been proposed in some
other oxidations of naphthalene.28,33 The UO2+, produced in
reaction 6a, reacts either with O2 to regenerate UO22+ (eq 3) or
in a back electron transfer of eq 10.
Despite the fact that ∼30% of quenching in eq 6 takes place
by electron transfer, the total quantum yield of all the products
is less than 1%. The most apparent reason is the back electron
transfer of eq 10, which restores the reactants in their ground
states. On the basis of the known precedents, the rate constant
k10 is expected to be large and probably close to diffusion
control. For example, ferrocene reduces pyrene radical cations
•+
formation of C10H8 in the quenching process is 0.3, but the
overall quantum yield for the formation of products is <0.01,
•+
suggesting that most of C10H8 is re-reduced to naphthalene
+
by back electron transfer with UO2
.
Acknowledgment. We are grateful to Dr. J. H. Espenson
for helpful discussions and to Mr. J. Jacob for his assistance
with some experiments. This work was supported by the U.S.
Department of Energy, Office of Basic Energy Science, Division
of Chemical Sciences, under Contract W-7405-Eng-82. An
Ames Laboratory Directed Research and Development Grant
is gratefully acknowledged.
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