J . Chem. SOC., Faraday Trans. I , 1989, 85(8), 2481-2498

The Second Emission of the Uranyl Ion in Aqueous Solution

Minas D. Marcantonatos" and Magdalena M. Pawlowska Department of Inorganic, Analytical and Applied Chemistry, University of Geneva,

Ch-1211 Geneva 4 , Switzerland

The second lower-energy emission (LEE), weaker than the normal higher- energy luminescence (HEE) of UOi+(aq) and previously assigned to the *(OUOHOU0)4+(aq) exciplex in the exciplex-formation model (EXFM) and also recently attributed to the ntd,(X*) emission in the n3,@,(U*)e nt d,(X*) reversible-crossing model (RCM), has been investigated. Results from detailed emission studies as well as spectral data from other authors, show that, contrary to what is advanced in the RCM, the LEE at pH = 3 (attributed to the X* state in the RCM) is not identical to the LEE at pH < 2. It is unambiguously shown that the LEE at pH = 3, assigned to the

ntd,(X*) state in the RCM, is actually the emission of the well defined dinuclear species (UO,),(OHi+(aq). The exciplex-formation model is further corroborated by the influence of perchlorate and methanol on the emission of UOi+(aq).

Two-component emission from aqueous acidic solutions of the uranyl ion was observed and investigated for the first time by Marcantonatos.l(u)-(g) The normal emission with the Stokes threshold at 20500 cm-' was assigned to the 5f (6, or qbu) zt state of *UOi+(aq), while the second much weaker luminescence having its origin at 20 100 cm-' above the ground state of UOi+(aq) was attributed to the *(U0,H0,U)4+(aq) exciplex, formed between UOi+(aq) and the primary product, *(UO,H),+(aq) of H-atom abstraction from water by *UOi+(aq).

Numerous puzzling aspects of the behaviour of *UOi+ in aqueous acidic solution could be quantitatively rationalised by a formation of *(U0,H0,U)4+(aq).' The mechanism of this exciplex formation has received further support from excited-state energetics, formal state and m.0. correlations,' (g) and other studies by Marcantonatos and co-workers showed the comparable *(FU0,H0,U)3+(aq) formation between *(FUO,H)+(aq) and UOi+(aq).2(b) A qualitative description of *(U0,H0,U)4+(aq) in a v.b. formalism has also been givenl(g) and besides the 400 cm-' lower energy of its emission and the asymmetry and broadening of its vibrational components (as compared to the normal higher-energy emission), the clearest evidence for *(OUOHOU0)4+(aq) was the shortening by ca. 120 cm-l in its vibrational progression, reflecting far more axial [OUO] than equatorial [U(OH,),] modification in the *UO,(OH,)p moiety. In all the investigations by Marcantonatos and his group the pH was ,< 2 in order to avoid conspicuous and practically uncontrollable effects coming from ground-state hydrolysed species, notably (U0,),(OH)i+.3 In extreme cases' ( g )

where the maximum uranyl concentration [UO~+],,, was 0.08 mol dmb3, the pH was 1.85, corresponding to 0.36% (UO,),(OH)E+ at the maximum temperature used 7' = 65 "C, while for pH < 2.5, [UOi+],,, was 3. mol dm-3 giving < 0.035 O/O of the above bridged dimer at T = 25 "C

Recently, in a series of five publication^^-^ Formosinho and co-workers have proposed the alternative ' reversible crossing ' mechanism t U* + X* + as rationalising UO",(aq) photophysics and photochemistry, where U* and X* are the zt#, and XIS, states of

248 I

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2482 Uranyl Ion in Aqueous Solution

*UOi+(aq), exchanging equatorial water by a different mechanism. In their first three the above authors have questioned and ruled out4 the exciplex-formation

mechanism. This was done on the basis of the following essential arguments derived from their results.

(i) The bathochromic weaker emission obtained4 and assigned by Formosinho and co- workers to the niS, state (X*)44 is argued to be identical to that observed by Marcantonatos and co-workers' ("I and virtually identical4 to the higher-energy emission attributed to the n:$" state (U*).4-6

(ii) The analysis by Formosinho and co-workers4 of two photokinetic schemes involving exciplex formation and referred to by these authors as the first and second 'exciplex model ', is unable to rationalise their results.

(iii) The mechanism of Marcantonatos'(")-(g) is unable to interpret the luminescence yield versus pH dependence found by Formosinho and co-workers5 for 2.5 < pH < 4.

(iv) Contrary to the findings of Marcantonatos and co-workers2(a)-(b), ' 9 lo and in disagreement with the exciplex model, bi-exponential decay of the excited aqueous uranyl ion is found by Formosinho and co-workers in all conditions of the m e d i ~ m , ~ - ~ i.e. low and high uranyl concentrations and low and high (< 4) pH.

The object of the present work is to present emission spectroscopic and other data, as well as give an analysis of the present results and of those of Formosinho and co- w o r k e r ~ , ~ - ~ in order to clarify the origin of the contradiction between the results and assignments of Formosinho and Marcantonatos. This clarification is of necessity for the following reasons. (v) As far as point (i) is concerned, the weaker bathochromic emission obtained by Formosinho and co-workers4 [from subtraction after normalisation at 485 nm of two spectra of UOi+concentration 0.05 mol drn-,, T = 5 "C and 23 OC, pH = 3.1 and where (UO,),(OH)i+ is present at 2.5 and 6.3 %11* 12] does not appear to be the same as that obtained by Marcantonatos"") [spectral comparison of 0.01 and 0.06 mol dm-, UOi+ with normalisation at 488 nm, T = 25 "C, pH = 1.9, 0.0074 and 0.044% (UO,),(OH)i+]. (vi) The first and second exciplex model introduced by Formosinho and co-workers4 [point (ii)] cannot be compared with the photokinetic scheme proposed by Marcantonatos.'(")-(g) Fonnosinho's first exciplex model introducing infinitely negative AH and/or positive AS of exciplex formation, is extremely improbable and has never been considered by Marcantonatos. ' ( a ) - ( g ) The second has already been eliminated by Marcantonatos in his first paper"") on the two-component emission of UOi+(aq). (vii) Excited-state events are unavoidably affected by ground state hydrolysed and polymerized dioxouranium(vI) for 2.5 < pH < 4 [point (iii)]. For the 3 < pH < 4 region the luminescence yield versus pH variation found by Formosinho and co-workers5 is suggestive of complicated ground-state equilibria. (viii) The kinetic parameters published by Formosinho and c o - ~ o r k e r s ~ - ~ as resulting from bi-expo- nential decays [point (iv) above] refer either in the great majority to pH = 3, where effects from ground-state hydrolysed species may be suspected even for low uranyl concentrations [UOi+], or to 0.6 < pH < 2, where [UOi+] is always high, again giving suspicion for effects coming from the exciplex, as found by Marcantonatos and co-

In the present paper, we shall consider some of these points in more detail, the rest will workers.' (")-(g)v 2

be discussed in another work.?

Experiment a1

Uranyl solutions were prepared from UO,(NO,), (Merck p.a.), HNO, or HClO, (Merck suprapur) and tridistilled (in a quartz apparatus) water. Perchlorate solutions of UOi+ were prepared as reported previously.'(g) NaOH and NaNO, were obtained from Merck

t Detailed spectroscopic, thermodynamic and kinetic data from our work, as well as an extended analysis of mechanisms and structures, are available on request; they will appear in a review on the luminescence kinetics of the uranyl ion interactions with solvent and counter anions in aqueous solution.

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M . D. Marcantonatos and M . M . Pawlowska 2483 (suprapur) and NaClO, - H,O from B.D.H. (AnalaR). Conventional luminescence measurements were made with a Perkin-Elmer MPF-2A spectrofluorimeter by using either a Hamamatsu R955 or R106 photomultiplier (P.M.) and absorption measure- ments were performed with a Perkin-Elmer Lambda 5 spectrophotometer. All spectra were corrected against the corresponding blank solution noises and normalisation of steady-state emission spectra was always performed at the Stokes threshold of 488 nm, by changing only the apparatus sensitivity and very slightly the excitation slit. To obtain sufficient precision for the emission obtained after the normalisation and subtraction of total emissions, the mean of three independent lower-intensity spectra was subtracted from the mean of three independent higher-intensity spectra, after three independent normalisations. Figures in the present work, presenting normalised spectra, show only one of the three spectra. For luminescence measurements the emission slit was < 3.5 nm and more frequently < 2 nm, while for absorption measurements the band-width was 0.25 nm. Unless otherwise stated, steady-state emission measurements were performed with the R955 P.M. and with Aex = 337 nm, in order to have the same Aex as that mainly used in the work of Formosinho and co-~orkers.~- '

Speciation of aqueous solutions of UOz+ at various temperatures (and 0.5 mol dm-, KNO,) uia:ll- l2

UOi++ H,O + UO,(OH)+ + H+, AHll = 46 kJ mol-', ASl l = 46 J mol-' K-', K , ,

2UOi+ + 2H,O G (UO,),(OH)i+ + 2H+, AH,, = 42.7 kJ mol-l, AS,, = 29.7 J mol-' K-', K,,(298) = 1.2 x lop6 mol drn-,.

(298K) = 2.2 x mol drn-,;

We checked, by means of

that species (UO,),(OH)l is, up to pH = 2 and T = 70 "C, completely negligible [3UOi+ + 5H,O + (UO,),(OH)l + 5H+, AH,, = 105 kJ mol-', AS,, = 41.8 J mol-' K-l, K,,(298 K) = 6 x lo-'' mo12 dm-6].

Some data of Formosinho and c o - ~ o r k e r s , ~ ~ used for analysis and discussions in the present work, were taken from figures in ref. (4)-(6) by magnifying sufficiently and without distortion, in order to obtain figure values with good accuracy.

Results and Discussion As mentioned earlier, Formosinho and co-workers obtained the low-energy emission by subtracting, after normalisation at 485 nm, the emission of 0.05 mol dmP3 UO,(NO,), at pH = 3.1 and T = 5 "C from that of the same solution at T = 23 "C [fig. 4 in ref. (4)]. The resulting emission, that they attribute4 to the ni 6, state (X*), has [from fig. 4 in ref. (4)] : (a,)? vibrational components 2 s to 5s (see table 1) at 498.5 (2S), 51 7.5 (3S), 544 (4s) and 568 (5s) nm; (b,) band spacings of 744, 942 and 776cm-l, giving a stretching frequency of V,(S) = 820 f 87 cm-' for the OUO entity; (c,) rough band widths 6, (the bands deviate from Gaussian) of 481 (2S), 327 (3s) and 484 (4s) cm-l, leading to & = 431 +73 cm-'; (d,) intensity 4.23 times lower than the low-temperature (5 "C) reference emission, considered by Formosinho and co-workers as the normal uranyl emission and attributed to the higher n: $" state (U)*; (e,) its origin (see table I ) at 430 cm-l lower than the low-temperature reference emission.

Clearly, none of the above characteristics is identical to those of the low-energy luminescence, assigned to the exciplex *(U0,H0,U)4+(aq)'(u) and obtained by

t For reasons of clarity the subscript F will refer to the results or arguments of Formosinho and co-workers or to results obtained from data in their work. M will stand for previous and present results or arguments of Marcantonatos and co-workers.

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Tab

le 1

. Em

issi

on s

pect

ra o

f UO

:' in

aqu

eous

per

chlo

rate

and

nitr

ate

solu

tions

band

pos

ition

/cm

-' ba

nd p

ositi

on/c

m-'

h)

P

00

band

(4

(4

(4

(4

(4

(0

(f)

band

aS

eS

e'

S fS

P

1 ( 1

'-0)"

2

(0'4

) 3

(0'-1

) 4

(0'-2

) 5

(0'-3

) 6

(0'4

)

21 1

86

20 4

92

(294

) 19

608

(3

51)

18 7

62

(391

) 17

857

(3

87)

16 9

78

(4 14

)*

21 2

70

(638

)

(360

) (3

87)

(573

)

(570

)

2048

8 20

479

20 5

02

2049

2 20

458

2049

2

1961

6 19

616

1963

6 19

608

1957

7 19

612

1873

7 18

741

1877

4 18

744

1869

2 18

709

1785

4 17

867

1791

7 17

921

1782

5 18

008

17 0

81

17 1

88

(380

) (4

18)

(567

)

(602

)

(690

)

1s 20

790

2s

20 1

09

20 0

80

20 0

40

3s

1923

1 19

286

1928

6

4s

1834

9 18

501

1845

0

5s

1751

3 17

668

(345

)

(413

)

(369

)

(448

)

(347

)

20 0

60

(48 1

) 19

324

(3

27)

18 3

82

(484

) 17

606

$ 2 '",

2

F 35

8+41

37

0"

402

600f

46

394 f 39

43

1f73

3 3-

band

spa

cing

/cm

-' ba

nd s

paci

ng/c

m-'

b s

band

(4

(6)

(4

(4

(4

(0

(f)

band

aS

eS

e'

S fS

g 9

h

(681

) 74

4 2

1 s-2

s 75

4 1-2

(6

94)

(768

) 2-

3 88

4 87

1 86

3 86

6 88

4 88

1 88

0 2s

-3s

878

794

3-4

846

880

873

862

864

885

903

3S4S

88

2 78

5 83

6 94

2 2.

4-5

905

876

874

857

823

866

701

4s-5

s 83

6 83

3 77

6 5

4

879

836

820

mea

nd

878f

21

876f

4 87

0f5

855f

12

857f

25

877f

8 82

6f78

m

ean

859f

17

804f

21

795

820f

87

"Tra

nsit

ion;

ha

lf-ba

nd w

idth

6 in

cm

-';

" with

out b

and

reso

lutio

n, o

nly

band

s 2

and

3 ha

ving

ca.

sam

e in

tens

ities

[see

for i

nsta

nce

the

low

-tem

pera

ture

em

issi

ons

fig. 3

(a)]

can

lead

to

appr

oxim

ate

6;

the

(1' +

0)-(0

' +

0) s

paci

ng is

not

take

n in

to a

ccou

nt; (

a) [

UO

,(ClO

,),]

= 3

x lo

-, m

ol d

m-3

at

T =

5

"C a

nd p

H =

3.1

(ba

nd r

esol

utio

n, f

ig. 1

); (6

) [U

O,(C

lO,),

= 5

x l

op2 m

ol d

m-,

at T

= 5

"C a

nd p

H =

1.5

(mea

n of

six

spe

ctra

mea

sure

d w

ith t

wo

diff

eren

t P.M

., see

expe

rimen

tal);

(c) [

UO

,(NO

,), =

5 x

lo-,

mol

dm

-3 a

t T =

5 "C

and

pH

= I

.5 (m

ean

of s

ix sp

ectra

mea

sure

d w

ith tw

o di

ffer

ent P

.M.,

see e

xper

imen

tal);

(d)

[UO

,(ClO

,),]

= 0

.218

mol

dm

-3 in

2.3

5 m

ol d

m-3

HC

lO,,

3 m

ol d

mP3

ClO

; an

d T

= 2

8 "C

[ban

d re

solu

tion,

ref.

(16)

]; (e

) and

(el)

[U

O,(C

lO,),

] an

d [U

O,(N

O,),

] 0.

3 m

ol d

m-,

in th

e pr

esen

ce o

f low

NaO

H c

once

ntra

tions

[ref

. (1 7

) and

(1 8)

] ; (f) [U

O,(N

O,),

] =

5 x

1 O-

, mol

dm

-, at

T

= 5

"C a

nd p

H =

3.1

[fro

m th

e w

ork

of F

orm

osin

ho a

nd c

o-w

orke

rs, f

ig. 4

in r

ef. (

4)]; (f

S) fr

om s

ubtr

actio

n of

spe

ctru

m (f)

from

the

spec

trum

of

[UO

,(NO

,),]

= 5

x l

o-,

mol

dm

-, at

T =

23

"C a

nd p

H =

3.1

, afte

r no

rmal

isat

ion

at 4

85 n

m [

from

the

wor

k of

For

mos

inho

and

co-

wor

kers

, fig

. 4 in

re

f. (4

)].

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M . D. Marcantonatos and M . M . Pawlowska 2485

Marcantonatos and co-workers by subtracting after normalisation at 488 nm, the emission of 0.01 mol dm-3 UO,(NO,), at pH = I .9 and T = 25 "C from that of 0.06 mol dm-3 UO,(NO,), at the same pH and temperature. In fact, this low-energy luminescence has {see fig. 4 in ref. [l (a)]} (a,) bands at 497.7 (2S), 514 (3S), 534 (4s) and 560 (5s) nm; (6,) band spacings of 637, 728 and 870 cm-', giving V,(S) = 745 k96 cm-'; (c , ) highly asymmetric and broad bands, excluding any 6, estimation ; (d,) intensity 7.7 times lower than the 0.06 mol dm-3 U0,(N03), emission; (e,) its origin 394 cm-' lower than the well defined 20486 & 13 cm-' (see band 2 in table 1) origin of the aqueous UOi+ luminescence.

Spectral data from emissions at pH < 2 and different temperatures will be presented below. For the moment, we may ask whether the low-temperature ( T = 5 "C) reference emission in fig. 4 of ref. (4) [assigned to the n: #u state (U*)14 is actually identical to the bathochromic weaker emission [n;J, state (X*)], as argued by Formosinho and co- workers and as expected for U* and x* assumed to have the same equatorial-water geometry.6

As can be readily seen from fig. 4 in ref. (4), the low-temperature ( T = 5 "C) reference spectrum is not a normal uranyl luminescence, as supposed by Formosinho and co- w o r k e r ~ . ~ In fact, it is a two-component emission spectrum, as can be judged from the considerable emission at the minima of all bands, which in turn deviate seriously from Gaussian bands. More clear evidence for this comes from fig. 1 below, showing a UOt+(aq) emission spectrum taken at the same pH and temperature as the low- temperature spectrum in fig. 4 of ref. (4) and with an even lower uranyl concentration than in ref. (4). It is therefore not surprising that the +78 cm-' scattering in the band spacing of the spectrum of Formosinho and co-workers [table 1, column (f)] is 680 YO higher than the average scattering of band spacings of spectra (a) , (6) and (c) , and 420 YO higher than that of spectra ( d ) , (e) and (e') published by other Also, the average band spacing of 826 cm-l (table 1) of the low-temperature ( 5 "C) spectrum of Formosinho and co-workers is 49 cm-' lower than that of spectra (a) , (6) and (c) (875 f 3 cm-' at T = 5 "C) and 37 cm-' lower than that of spectra ( d ) , (e), (el) (863+ 10 cm-I), despite the fact that these latter were measured under conditions favouring slight lowering of the v, stretching frequency of the ground-state OUO entity. In fact, though the low [857cm-', table 1, column (e)] average spacing of the spectrum of Pant and Khandelwal in ClO, s01ution'~~ l8 is open to some experimental uncertainty owing to the (0' + 2)-(0' + 3) spacing, the lower value (855 2 12 cm-') of v, and the largest 6 of Bell and Biggers16 compared to those of spectrum (a) (table 1, second column) are quite informative. In fact, much higher ClO, and H,O' concentrations may result in some inner-sphere ClO, penetration or in mixtures of D,, D,, equatorial water geometr ie~ '~ or even in some H,O+ perturbation of the axial oxygens, all these effects acting most p r ~ b a b l y ' ~ , ~ ~ in the same direction of slightly lowering v, and increasing 6.

Since, to the best of our knowledge, a band spacing of ca. 830 cm-l [spectrum (f), table I] for a normal emission (pH < 2 and low [UOi']) of aqueous UOi+ has never been reported and is most unlikely, the very similar v, obtained by Formosinho and co- workers from the higher- (826 cm-') and lower-energy (820 cm-l) emissions is more than questionable. Therefore, in order to obtain a consistent comparison? between v, (obtained from the higher energy normal emission) and v,(S) (from the second lower- energy one), we compare the average V, from spectra (a), (6) and ( c ) [together with v, = 873 & 3 cm-' and 6 = 385 ( 5 "C) and 418 cm-' (23 "C) from fig. 3 (a)] with the average v,(S) from spectra as , f S in table 1 and BS in table 2 (fig. 5 (b) Spectrum BS and fig. 2 (B). The result is:

(1) v,-v,(S) = 874-841 = 33 cm-'; pH = 3.1; T = 23-5 "C

t For reasons mentioned above spectrum d16 is not taken into account. The unresolved two-component emission spectra e-e' and e(S) -e'(S)" are also disregarded for this comparison.

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2486 Uranyl Ion in Aqueous Solution

h

3

1

- d 6 0 480 500 520 540 560 580 600

aim Fig. 1. Emission of 0.03 mol dm-3 UO,(ClO,), excited at 337 nm at pH = 3.0 and T = 5 "C

(0.21 mol dm-3 ClO;), Gauss band model resolution; (-), (a); (. * .), (as) , see table 1.

B

A

480 500 520 540 560 580 600 Alnm

Fig. 2. Emissions of UO,(NO,), in aqueous nitrate solution (concentrations are in mol drn-,) at pH = 3.1 ; narrow or dashed line spectra T = 5 "C, broad line spectra T = 23 "C; A,,, = 337 nm for all spectra except for (A,,,) (A,,, = 406 nm). (A) upper spectra [UOi+]/[NO;] = 0.05/0.1; lower spectra [UO2,+]/lr\rOJ = 0.02/0.04 (Ak); 0.05/0.1 (A,) and (A,,,). (B) [UOi+]/[NO;] = 0.02/0.04.

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M . D. Marcantonatos and M . M . Pawlowska 2487

Fig. 3. narrow

I - I . I ' I . I . I r ,

480 500 520 540 560 580 6 00 A/nm

Normalised emissions of 0.05 mol dm-3 UO,(NO,), at pH = line 5 "C. (a) Spectrum C, [NO;] = 0.125 mol dm-3 and broad line

F, [NO;] = 0.13 mol dmP3 and broad line 70 "C.

1.5. A,,, = 337 nm; 23 "C. (b) Spectrum

and for the half-band widths:

(2) 6-6(S) = 358-394 = -36 cm-l; pH = 3.1; T = 5 "C

8-&(S) = 387-465 = -78 cm-l; pH = 3.1; T = 23-5 "C

The E.N.S.? BS in fig. 5(b) refers to UO,(NO,), = 0.02 mol dm-3 at pH = 3.1 and T = 5 "C and 23 "C [(fig. 2B)]. With this pH, T = 5 "C and UO,(NO,), = 0.05 mol dm-3, we were unable, even with the highest resolution of our apparatus, to obtain a spectrum similar to the low-temperature spectrum obtained by Formosinho and co- workers under the same conditions [compare fig. 2A, dashed-line spectra with fig. 4 in ref. (4)) The striking and not so unexpected result is that, apart from an approximately common origin, the E.N.S. (BS) in fig. 5 (b) has essentially different characteristics from those of (CS) in the same figure. In fact (see table 2) the CS spectrum (pH 1.5) has relative to BS (pH 3.1): (fM) its vibrational components (maxima of the asymmetric bands 3S, 4s and 5s) hypochromically shifted by 11 1, 136 and 124 cm-l; (gM) an average band spacing AV 27 cm-' lower ; (h,) highly asymmetric bands, each of them consisting of two major components; (i,) an average very rough half-band width 6, 12 cm-I higher.

t E.N.S. and E.I.N.S. stand, respectively, for the emission and the emission intensity obtained after normalisation and subtraction.

82-2

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Tab

le 2

. C

hara

cter

istic

s of

the

em

issi

on o

btai

ned

afte

r no

rmal

isat

ion

and

subt

ract

ion

(E.N

.S)

of s

pect

ra a

t fr

om t

hose

at

(T, >

q)

band

spa

cing

ba

nd p

ositi

on/n

m

G,(S

)/cm

-'"

Av(

S)/c

m-'

med

ium

T-

T,

[W'I

sp

ectru

m

fig.

pH

/mol

dmd3

/"

C

/10-

2mol

dm

-3

2s

3s

4s

5s

2s

3s

4s

5s

&(S)

2s

-3s

3W

S 4

s-5s

A

v(S)

BS

cs

DS

ES

FS

0s"

Ps"

QS

5; 2

B 3.

1 0.

04N

O;

5; 3

(a)

1.5

0.12

5 N

O;

6; 4

(a)

3.1b

0.2

C10

; 6;

4(b

) 1.

5 0.

2 C

lO;

6; 3

(6)

1.5

0.13

NO

; 9

1.5

0.2

ClO

; 9

1.5

2.2

ClO

; 10

1.

0 0.

18 C

lO;

23-5

2

23-5

5

70-5

5

70-5

5

70-5

5

70-5

5

70-5

5

25

1+

mol

dm

-3 M

eOH

498

521

545

506

529

553

498

518

542

498

519

542

499

519

541

499

519

542

-

518

541

-

516

537

570

562

571

573 -

569+

5 56

6 -

605

585

552

581 f

22

58

0 39

0 37

5 45

6 44

6 41

7535

56

6 58

4 55

3 55

4 52

1 55

3+22

56

7 60

2 60

7 62

1 58

2 60

3f 1

4 -

602

552 - -

577

-

601

551 - -

576

56

2-

--

-

-

886

845

805

845k

33

%

-

821

816

818

2

859

820

842

840f

16

775

855

782

804k

36 S

812

818

813

814k

3 gb

772

818

-

795

Q

-

758

828

793

5 3

772

783

-

777

g 3 - "H

alf-

band

wid

ths

6, a

re v

ery

appr

oxim

ate,

esp

ecia

lly f

or t

he h

ighl

y as

ymm

etric

ban

ds o

f th

e E.

N.S

at

pH =

1.5

; ba

djus

ted

with

dilu

ted

NaO

H;

cmea

sure

d w

ith t

he R106

Ham

amat

su P

.M.

and

the

othe

r sp

ectra

with

the

R95

5.

2. x

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M . D. Marcantonatos and M . M . Pawlowska 2489

n , . , . , . , . , . , ,

, . l - , , l . l ' , - ,

480 500 520 540 5 6 0 seo 600 Alnm

Fig. 4. Normalised emissions of 0.05 mol dm-3 UO,(CIO,),; [CIO;] = 0.2 mol dm-3; A,,, = 337 nm; narrow line 5 O C , broad line 70 "C. (a) Spectrum D, pH = 3.1 (b) Spectrum E, pH = 1.5.

480 500 520 5 4 0 560 580 600 Llnm

Fig. 5. Emission intensity obtained after normalisation and subtraction (E.I.N.S.) versus wavelength. The intensity is given with reference to the I,, measured at 5 "C (a) Spectrum CS, see

fig. 3(a), and (b) Spectrum BS, see fig. 2(b).

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2490 Uranyl Ion in Aqueous Solution

A i m

Fig. 6. As fig. 5, (a) Spectrum DS, see fig. 4(a), (b) Spectrum ES, see fig. 4(b) and (c) Spectrum FS, see fig. 3(b).

Despite the fact that the pH decreases slightly when the temperature increases, the differences (fM) to (iM) become even more sharp, when the high-temperature emission spectrum is run at 70 "C and compared to the low-temperature spectrum at 5 "C. The E.N.S. DS [pH = 3.1, fig. 6(a)] now has no common origin with either the BS [pH = 3.1, fig. 5(b)] or the CS [PH = 1.5, fig. 5(a)] spectra and comparison of the origin WS (pH = 3.1), the band position VS (pH = 3.1), the half-band width 6,s (pH = 3.1) and the band spacing AvS (pH 3.1) or spectrum DS with the mean values of those of the spectra ES to PS (fig. 6 and 9) gives (pH in parenthesis):

WS (1.5)-WS (3.1) = 300 cm-l v3S (lS)-v3S (3.1) = 373 cm-I

v4S (1.5)-v4S (3.1) = 375 cm-'

v5S (1.5)-v5S (3.1) = 41 1 cm-I 5,s (1.5)-8$(3.1) = 160 cm-'

AvSS (1.5)-AijsS (3.1) = 43 cm-'; T = 70- 5 "C. (3) The differences u,) to (i,) and those in eqn (lt(3) clearly show that the second low-

energy weaker emission of the aqueous uranyl ion, obtained at pH = 3.1 by Formosinho and co-workers4 and by us in the present work, under either identical or different temperature conditions, has no relation to that previously reported1(") and presently obtained by Marcantonatos and co-workers at pH < 2. (jM): For pH 1.5, the band positions and the half-band widths 6, of the E.N.S.

(second emission) do not perceptibly change on going from 23 "C to 70 "C (see spectra CS, ES, FS, OS, PS and spectral characteristics in table 2). However, for pH = 3.1, the E.N.S. undergoes an average bathochromic shift of 293 f 19 cm-' in the band positions and a 152 an-' lowering in 6,, when the temperature is raised from 23 "C to 70 "C (see fig 5 (a), 6 ( d ) and table 2). This can, by no means, be expected for an assumed4 well defined

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M . D. Marcantonatos and M . M . Pawlowska 249 1

R A

4

3

2

1

0

R A

1.5

1.3

1.1

0.9

4

Rn

3

2

1

1 2 3

PH 7 (4

0

J 1 PH

1 1 3

R I;*

2 .o

1.75

1.5

1.25

1.0

Fig. 7. Ratio of absorbances RA (ApH/ApH& for ,labs = 406nm (a) and A,,/A,,-,,, for Aabs = 337 nm (b), of emission yields Ra((DpH/(DpH-o,5 from 460 to 620 nm (integrated intensities) and of total absorbed intensities (I~pHJI~pH-, ,5J as a function of pH ; [UO;'] = 0.02 mol dm-3,

[ClO;],,, = 0.5 mol dm-3.

electronic state such as the n:S, state of UO,(OH,)V. On the contrary, it strongly suggests that X* is some other uranyl species, suffering pronounced changes in structure or composition as temperature increases.

These, as well as points (aM)-( jM) , definitely show that the second lower-energy emission, appearing for 2.5 < pH < 4 at low and ambient temperatures, is that of the well defined3* 11* 12 ,15 [U02(OH),U0,]2+(aq) complex transforming to other polymeric species for higher temperatures, and that it has nothing to do either with the second emission obtained at pH < 2 or with the ntS, state of UOi+(aq), as claimed by Formosinho and c o - w ~ r k e r s . ~

Fig. 7 shows the emission yield ((9) variation with pH. A decrease in pH from 0.5 to ca. 1.2, a plateau for 1.2 < pH < 2.5 and then a strong increase up to pH 3.5 are observed. Previously, this type of variation, for 0 < pH < 2.5, was extensively investigated by Marcantonatos and co-workers and an excellent fit was obtained with equations derived from the EXFM.l(e).(g)

[fig. 1 in ref. (5) ] , but in the poorly explored (seven 0 value^)^ 0.5 < pH < 3.5 domain, the plateau appears as a flat minimum. Fig. 1 in ref. 5 shows a 54% decrease in <D on moving from pH 3.5 to 4. We also found a similar decrease (four points, not represented in fig. 7), but for our purposes this pH region is of little interest, as at pH values around 4

Formosinho and co-workers5 report an analogous pH dependence of

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2492 Uranyl Ion in Aqueous Solution

triangular-trinuclear and possibly quadrangular-tetranuclear3 complexes are expected to be coexcited with UOi+(aq) and (UO,),(OH)i+(aq); we found indeed that the magnitude of the Q, drop, from pH 3.5 to 4 for the [UOi+] = 0.02 mol dmP3, is A,,, dependent.

In regard to point (vii) of the present work, it is of interest to analyse and discuss the statement of Formosinho and co-workers [p. 1737 in ref. ( 5 ) ] : (a) 'although the Marcantonatos expression :

Q, = ( A + B[H,O+])/(C+ D[H,O+]) (4)

can be used to fit 0 data upsto pH 2.5.' (b) 'it is clear that it is unable to interpret the acidity dependence for pH > 2.5.' As far as the statement (a) is concerned, we may first compare eqn (4) with the expression:5

( 5 ) @ = [k: + (ki k",k, + k,)] [k, + ki - (ki k , /k , + k,)]-'

of Formosinho and co-workers and see that, contrary to eqn (4) (the expressions of the pH-independent A , B, C, D are given in ref [ 1 (e), (g) ] } Q, in eqn (5) is not explicitly related to [H,O+].

Expressions such as eqn (4)1(e)9(g) were found to fit very well 18 values at two A,,, for pH 0.2-11(e) (HNO,), and 84 values at two A,,, for pH 0.5-2.5 (HNO, and HC104).1(B) These results may be compared with those obtained by eqn ( 9 , fitting poorly [see fig. 1 in ref. (5)] five Q, values for pH 0.5-2.5.

With regard to statement (b) above,5 Marcantonatos was initially interested in the influence of H30+ on the behaviour of *UOi+(aq) and not on the coexcited dinuclear (UO,),(OH)~+(aq) which is present in appreciable concentrations above pH 2.5. The quantitative rationalisation of the emission intensity dependence on pH > 2.5 becomes straightforward (see table 3 and fig. 8) if the exciplex-formation mechanism is considered.

In fact, according to the EXFM,l(")-(B) the formation of species *UOg (Y*) is favoured while that of *U0,H02U4+ (E*) is unfavoured by increasing the pH. Therefore, for low or moderate uranyl concentrations and below a certain [H,O+], the total emission intensity I = I , + IE {see eqn (21) and (22) in ref. [l (g)] and eqn (9) in ref. [l (e ) ] } becomes

(6) {see eqn (24)-(26) in ref. [l (g)]} corresponding to Z us. pH {fig. 1 in ref. [l (g ) ] } or to the Q, us. pH (fig. 7) plateau.

Above pH ca. 2.5 where, for [UOi+] = 0.02 mol drn-,, coexcitation of (UO,),(OH)i+(aq) (Z,) becomes significant and below a certain pH, at which the formation and coexcitation of higher polynuclear hydro-complexes is no longer negligible, the total emission intensity is :

(7)

As the excitation of UO,(OH)+(aq) is insignificant and as equilibrium between 2U*

I = I , = kg) k& I,"

Iem = I , + Iz, = kg) [U*] + kg: [Z,*].

and Z,* is not attained within the U* lifetime (see footnote on p. 2482):

Iem = 0, I," + Q Z p If2

and Iern/C' = Q,zz ( C 2 / I : ) (8)

which is perfectly obeyed (fig. 8; r = 0.998, slope = 131, intercept = 12.5). Note that @z2/Q,u = 10.5, and since (see footnote on p. 2482) k , = k,, = (5.2 f 0.3) x l F s-l, k,, = k, = (6.1 & 1.1) l@ s-l and kg) = 930 s-l, we obtain (D,, (295 K) = 1.9 x lop2; k!&) = 1 (60 s-l and k& (295 K) = 6 x lo4 s-l.

The above results 'clearly show that Formosinho and co-workers have made the statement (b) without any foundation. The interesting feature is that the radiative

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M . D . Marcantonatos and M . M . Pawlowska 2493

Table 3. Ground-state aqua-uranyl species,= absorbed (I.$ and integrated (Iem)e emission intensities for various pH at 295 K

[U]/10-2 [z,]/lo-4 r,u/io-2 10-4 Ie rn

pH /mol dm-3 /mol dm-3 Nhv dm-3 min-' Nhv dm-3 min-' (arb. units)

0.50 0.57 0.70 1.14 1.85 2.10 2.35 2.52 2.64 2.72 2.82 2.93 3.01 3.13 3.24 3.32 3.43 3.56

0.50 1.13 1.25 1.45 1.69 2.04 2.37 2.49 2.58 2.66 2.76 2.87 2.99 3.08 3.17 3.25 3.36 3.59

-~

Aexc = 337 nm, 1; = (1.48 f 0.02) x 1 0-4 Nhv min-'(d) 2.000 0.000 2 . 0 4 0.000 2.000 0.000 2.004 0.000 2.000 0.000 2.004 0.000 2.000 0.000 2.004 0.000 1.999 0.02 1 2.003 0.378 1.998 0.066 2.000 1.192 1.995 0.209 1.992 3.748 1.990 0.455 1.978 8.123 1.983 0.784 1.958 13.91 1.976 1.126 1.939 19.85 1.962 1.760 1.903 30.66 1.940 2.855 1.843 48.74 1.916 4.025 1.782 67.24 1.863 6.614 1.653 105.4 1.791 10.15 1.492 151.9 1.722 13.56 1.353 191.3 1.602 19.48 1.140 249.0 1.427 28.13 0.886 313.9

A,,, = 406 nm, I: = (3.26f.0.04) x Nhv min-l(d) 0.000 0.000 0.000 0.030 0.098 0.495 2.253 3.894 5.867

2.000--- 2.000 2.000 2.000 2.000 1.999 1.995 1.991 1.987

0.000 0.000 0.000 0.000 0.0 10 0.050 0.229 0.396 0.597

2.954 2.954 2.954 2.954 2.954 2.952 2.944 2.937 2.929

.98 1

.97 1

.953

.922 3 8 8 340 .783 .682

0.859 1.347 2.196 3.697 5.395 7.755

10.53 15.54

2.9 18 2.898 2.863 2.803 2.737 2.646 2.540 2.355

8.434 13.20 2 1.46 35.95 52.15 74.35 00.00 45.00

1.383 30.33 1 342 269.20

0.210 0.186 0.166 0.125 0.1 16 0.1 17 0.132 0.161 0.206 0.242 0.31 1 0.405 0.53 1 0.737 1.034 1.181 1.406 1.618

0.593 0.356 0.338 0.330 0.3 16 0.299 0.310 0.33 1 0.362 0.401 0.490 0.600 0.769 0.986 1.278 1.572 2.121 3.194

a [UO,ClO,] = 2 x mol dm-3, [ClO;],,, = 0.5 mol dm-3 for all pH, U = UOi+(aq), Z, = (UO,)(OH);+(aq); I , (in Nhv dmP3 min-') calculated with ~,(337), ~,*(337), ~ ~ ( 4 0 6 ) and E, (406) are 1 1.3,203, 6.9,46 mol-' dm-3 cm-', respectively (from absorbance measurements with [dOi+]tot = 2 x lo-, mol dm-3, see footnote on p. 4). I,,,, (in arb. units) from Aem = 460-620 nm and with excitation and emission slits identical for all measurements ; incident light intensities from five independent determinations.

probability of the excited dinuclear species differs only slightly from that of the excited UO;+(aq), a trend generally shown by anhydrous uranyl compounds having lifetimes varying from 0.95 to 1.6 ms at 80 K. Another interesting aspect is linked to the rate constant of deactivation of Z; at 278 K. This can be obtained from the spectra shown in fig. 1 at T = 278 K, pH = 3, [UO;'] = 0.03 rnol dm-3 and A,,, = 337 nm, corres-

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2494 Uranyl Ion in Aqueous Solution

200 -

160-

I

3.36

0'

12 11: 0 0.4 0.8 1.2

Fig. 8. The linear dependence of the ratio of integrated emission to the uranyl absorbed intensities (with reference to 4 = 3.26 x Nhv min-') upon I?/[: according to eqn (8); (O), for A,,, =

406 nm; (e), A,,, = 337 nm; [UO;'] = 0.02 mol dm-3, [ClO;], = 0.5 mol dm-3.

ponding to [Ul = 2.94 x lo-, and [Z,] = 2.95 x mol dmP3, I," = 2.49 x lo-, and 1:' = 4.49 x lop3 Nhv dm3 min-'.

From the ratio I zZ / Iu = 0.532 of the integrated emission intensities, ((Dz2/(Du) = 2.95 at 278 K and from the 75.8% decrease in k, , on moving from 295 to 278 K,1° k , (kg) = 1160 s-',kg) = 930 s-')canbecalculatedandfoundtobekZz(278 K) = 5.31 x lo4 s-l;

?he decrease in k, due to change in T from 295 to 278 K is 13.1 % (to compare with the 75.8 YO fall in k,,j, which strongly suggests, as previously assumed, an inductive axial perturbation by the bridging O H groups. In fact, this perturbation most probably causes an increase in the electron density of the partially occupied n: (or 0:) m.0.s of the dioxouranium entities, thus reducing their H-from-H,O abstracting strength.

The reversible crossing model is contradicted by the effect of ClO, on the intensity of the LEE (fig. 9). In fact, it has been showng that the enhancement of the *UOi+(aq) emission results from an axially L . oT~o(oH,)~,+ or an equatorially O~(L)O(OH,): orientated L- (= ClO;), thus directly or inductively increasing the electron density of the partially occupied 7~: orbitals and reducing their H-abstracting strength for the

According to the reversible-crossing mechanism the nt 6, OU(L)O(OH,)Z complex formation should be more favoured (the half-filled 6, being at 45" to the equatorial ligand plane) than the nt$,OU(L)O(OH,)~ one ($" lying in the plane). Therefore, the expected result ought to be the enhancement of the LEE, while both the HEE and the LEE should have their intensities increased by the axially perturbed *OU02+ in

15* 21 process of H-abstraction from water. *

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M . D . Marcantonatos and M . M . Pawlowska 2495

;.. .... *.. PS *,

, _.*..

....... .. I . . ., I . . . .. . , I , , .

\ : . . : ; 480 ' 500 ' 520 ' 540 ' 560

Fig. 9. (a) Normalised emissions of 0.05 mol dmP3 UO,(ClO,), at pH = 1.5 (A,,, = 337 nm). Broad line at 5 "C ([ClO;] = 0.2 mol dm-3), dashed line at 70 "C ([ClO;] = 2.2 mol dm-3) (P), narrow line at 70 "C ([ClO;] = 0.2 mol dmP3) (0). (b) Emissions obtained after normalisation and subtraction

for 0 and P above (the intensity is given with reference to the I,, measured at 5 "C).

L. OUO(OH,):. Nonetheless, the experimental results (fig. 9) are contrary to these expectations.

A slight increase in the UOi+(aq) luminescence by MeOH has previously been shownl(g) by normalising at 533 nm the emissions of 0.02 mol dm-3 aqueous UO,(NO,), in the absence and in the presence of 0.24.4mol dmP3 MeOH. Obtaining the emission spectrum after normalisation and substraction (E.N.S.) at T = 25 "C (fig. 10) is difficult, owing to the only very slight effect produced by 1 mol dm-3 MeOH, which cannot be amplified by using higher MeOH concentrations without serious loss of precision, as MeOH is a powerful H-donor quencher of the UOi+(aq) e r n i ~ s i o n . ~ ~ - ~ ~ Nonetheless, fig. 10 shows that the E.N.S. is not significantly different from the LEE obtained under different conditions [fig. 5(a) CS; fig. 6(b) ES, (c) FS; fig. 9) and the question which further arises is why H-abstraction from H-CH,OH by *UOi+(aq) should favour the n;Ju(X*) emission. Indeed it is difficult, if not impossible, to find a rationalisation for this effect required by the reversible-crossing model, the more so, since studies by Formosinho and co-workers of the quenching of *UOi+(aq) by electron-donor quenchers Q (Q = T1+, Ag+, Fe2+, Pb2+, Mn2+, Ce3+) give:'

(k: + kF))/(k,& + k:) = 1.07 t- 0.19.

According to the exciplex-formation mechanism,l-, *(U0,H0,U)4+(aq) results from the reaction :

*U0,H2+(aq) + UOi+(aq) *(U0,H0,U)4+(aq) ; EX (9)

where *U0,H2+(aq) is formed as follows:

*UOi+(aq) + H,O *U0,H2+(aq) + HO'. (10)

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2496 Uranyl Ion in Aqueous Solution

Q

. * .. l ' . " ' 1 . 1 ' I ' 1 - '

480 500 520 540 5 6 0 580 600 Alnm

Fig. 10. (a) Normalised emissions of 0.01 mol dmP3 UO,(ClO,), at pH = 1.0 and T = 25 "C (A,,,, = 337 nm); broad line [ClO,] = 0.18 mol dm-3; dashed line [ClO,] and = 0.18 [CH30H] =

1 mol dm-3 (Q). (b) Emission obtained after normalisation and subtraction (QS).

The 400cm-' bathochromic emission, as compared to *UOi+(aq) is fairly well explained by the direct axial 0-U-0 perturbation in the EX {see fig. 11 below and scheme (59) in ref. [l (g) ] ) and the nearly coincident bathochromic shift for the dinuclear species *(UO,),(OH)i+(aq) (pH x 3) is certainly due to an inductive axial perturbation by the bridging OH groups.

The finding (see fig. 3(a) and spectrum CS) that the integrated intensity of the lower- energy emission I(LEE) is 18 times lower than the I(HEE) of the higher-energy emission can be readily explained by a low photostationary concentration of EX and/or by the fact that in YEx = C, Y, + C, Y , + C, Y,, the wave functions Yl, Y , and Y, stand for charge-transfer (c.t.), locally excited (1.e.) and back-charge-transfer (b.c.t.) structures, respectively, all of which (fig. 11) have one uranyl entity with at least partial character of uranyl(v), which is well known to be non-radiative. In parallel, the structures B or B' and C or C' have the other uranyl entity with a configuration which is partially of the excited UOi+, thus conferring to EX its radiative pr0perties.l (g)*

From the valence bond description in fig. 11, the EX should have, according to Jarrgensen's nt(g5, or 8,) hypothesis (upper part of fig. l l ) , a structure where both the axial and equatorial (probably pentagonal or tetragonal) moieties are displaced and parallel, while the Denning's o,, d,, hypothesis would suggest a co-axial structure with parallel equatorial planes (lower part of fig. 11). For such structures all vibrational modes (symmetric stretching, bending and rocking) of the uranyl groups should be perturbed compared to those of the aqua-uranyl monomer. Further on, though the relative contributions of yl, y, and y, to yEx are unknown, the EX is seen (fig. 11) to be composed of two distinct uranyl entities (cf. OUOH and OUO), whatever the values of CJC,, C1/C3 and C,/C,.provided, they are not zero. Therefore, the EX transition to its unrelaxed (before dissociation) Franck-Condon ground-state is expected to present

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M . D. Marcantonatos and M . M . Pawlowska 2497

B A

L 2 n x , 1 r y ; 2u ; 'fly, 2flX,l(@,'

C

16.16 ;2a: la 1 c'

Fig. 11. Valence-bond description of the uranyl exciplex *(OUOHOUO)". (I) H' transfer (1) to (2) or electron transfer (2) to (1). (11) H transfer (1) to (2) or electron transfer (1) to (2).1(g) (O), (0) and ( x ) are a $, or d,, a nu and a flu electron, respectively. The z axis is the axial uranyl one and Y 2, are the wavefunctions whose linear combination gives Y EX of the exciplex. Upper and

lower parts stand according to Jerrgensen's 7c: ($, or 8,) and Denning's C J , , ~ , , hypothesis.

a two-component vibrational progression, instead of a single-component symmetric stretching one, which is typical for UOi+ in aqueous or other solutions.

The above accounts fairly well for the LEE consisting of two major components and although Marcatonatosl (a) originally reported an exaggerated 120 cm-l shortening of the vibrational progression (owing to less detailed measurements of the LEE than in the present work), the better value of 75 cm-l still reflects clearly a direct axial uranyl perturbation in the exciplex EX.

The influence of ClO, on the intensity of the LEE further supports the exciplex- formation model, since disfavouring the formation of *U02H2+(aq) results in a lowering of the exciplex concentration [EX].

The effect of MeOH (which has already been investigated under various experimental conditions and which has been shown1(')' (g) to rationalise quantitatively the exciplex- formation mechanism) clearly shows that the increase of [EX] results from the increase of the *U02H2+(aq) concentration in the presence of MeOH, this being a far more powerful H-donor quencher than H,O. ( ~ 3 (9)~ 22-25

Finally, while the EXFM is not at all contradicted by the ou 6, hypothesis of Denning and co-workers, the RCM is ruled out by the energy difference 5000 cm-l > W0,$- Wo, > 1000 cm-1.26

References 1 M. D. Marcantonatos, (a) Znorg. Chim. Acta, 1978, 26, 41 ; (b) 1977, 24, L37; (c) 1977, 25, L87; ( d )

1977, 25, LlOl ; (e) J . Chern. SOC., Furaduy Trans. 1, 1979, 75, 2273; (J) 1979, 75, 2252; (g) 1980, 76, 1093.

2 M. Deschaux and M. D. Marcantonatos, (a) Chem. Phys. Lett . , 1979, 63, 283; (b) J . Znorg. Nucl. Chey., 1981, 43, 361.

3 M. Aberg, Acta Chem. Scand., 1969, 23, 791; 1970, 24, 2901.

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2498 Uranyl Ion in Aqueous Solution 4 S. J. Formosinho and M. G. Miguel, J. Chem. Soc., Faraday Trans. I , 1984, 80, 1717. 5 M. G. Miguel, S. J. Formosinho, A. C. Cardoso and H. D. Burrows, J. Chem. SOC., Faraday Trans. 1 ,

6 S . J. Formosinho and M. G. Miguel, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 1745. 7 H. D. Burrows, A. C. Cardoso, S. J. Formosinho and M. G. Miguel, J. Chem. Soc., Faraday Trans. I ,

8 S. J. Formosinho and M. G. Miguel, J. Chem. SOC., Faraday Trans. 1 , 1985, 81, 1891. 9 M. D. Marcantonatos, M. Deschaux, F. Celardin and M. Levental, Chem. Phys. Lett., 1979, 65, 316.

10 M. D. Marcantonatos, M. Deschaux and F. Celardin, Chem. Phys. Let t . , 1980, 69, 144. I1 L. G. Sillen and A. E. Martell, Stability Constants (The Chemical Society, Special Publication no. 17,

12 C. Fd Baes Jr and R. E. Mesmer, The Hydrolysis of Cations (Wiley, New York, 1976), pp. 176-179. 13 M. Aberg, D. Ferri, J. Glaser and J. Grenthe, Znorg. Chem., 1983, 22, 3986. 14 C. K. Joergensen, Rev. Chim. Miner., 1977, 14, 127. 15 C. K, Joergensen and R. Reisfeld, Struct. Bonding (Berlin), 1982, 50, 121. 16 J. T. Bell and R. E. Biggers, J. Mol. Spectrosc., 1965, 18, 247. 17 D. D. Pant and D. P. Khandelwal, Proc. Indian Acad. Sci., Sect. A , 1959, 50, 323. 18 E. Rabinowitch and R. L. Belford, Spectroscopy and Photochemistry of Uranyl Compounds (Pergamon,

19 G. Gorller-Walrand and S. De Jaegere, Spectrochim. Acta, Part A , 1972, 28, 257. 20 S. F. Lincoln, A. Ekstrom and G. J. Honan, Aust. J. Chem., 1982, 35, 2385. 21 M. Moriyasu, Y. Yokoyama and S. Ikeda, J. Znorg. Nucl. Chem., 1977, 39, 2211. 22 R. J. Hill, T. J. Kemp, D. M. Allen and A. Cox, J. Chem. SOC., Faraday Trans. I , 1974, 70, 847. 23 V. Balzani, F. Bolletta, M. T. Candolfi and M. Maestri, Top. Cum. Chem., 1978, 75, 1. 24 S. R. Allsopp, A. Cox, T. J. Kemp, W. J. Reed, V. Carassiti and 0. Traverso, J. Chem. SOC., Faraday

25 A. Cox, T. J. Kemp, W. J. Reed and 0. Traverso, J. Chem. SOC., Faraday Trans. I , 1980, 76, 804. 26 R. G. Denning, T. R. Snellgrove and D. R. Woodwark, Mol. Phys., 1979, 37, 1109.

1984, 80, 1735.

1985, 81, 49.

London, 1964), pp. 50 apd 51.

Oxford, 1964) p. 105-109.

Trans. 1, 1979, 75, 342.

Paper 81039421 ; Received 3rd October, 1988

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