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Infrared and Raman spectroscopic study of uranyl complexes: hydroxide and acetate derivatives in aqueous solution
Vibrational Spectroscopy 18 Ž1998. 61–75
Infrared and Raman spectroscopic study of uranyl complexes: hydroxide and acetate derivatives in aqueous solution F. Quiles ` ) , A. Burneau Laboratoire de Chimie Physique pour l’EnÕironnement, U.M.R. 7564 CNRS-UniÕersite´ Henri Poincare, ´ 405 rue de VandoeuÕre, 54 600 Villers-les-Nancy, France ` Received 22 July 1998; revised 14 September 1998; accepted 16 September 1998
Abstract Infrared-attenuated total reflectance ŽIR-ATR. and Raman spectroscopies are used to identify the complexed species of uranyl with hydroxide and acetate in aqueous solutions as a function of pH and metal-to-ligand ratio. Three stoichiometries . ŽUO 2 CH 3 COOq, UO 2 ŽCH 3 COO. 2 and UO 2 ŽCH 3 COO.y 3 are observed via irregular shifts of the uranyl stretching signals. The acetate vibrational modes n ŽCO 2 . and n ŽCC., allow the identification of two different ligand structures as a function of the complex stoichiometries: UO 2 CH 3 COOq and one ligand of UO 2 ŽCH 3 COO. 2 are pseudobridging, the second acetate of UO 2 ŽCH 3 COO. 2 being bidentate. There is still uncertainty on the presence of a pseudobridging acetate in UO 2 ŽCH 3 COO.y 3. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Infrared; Raman; Vibrational spectra; Uranyl hydroxide; Uranyl acetate; Aqueous solutions; Carboxylate complexation
1. Introduction The dioxouraniumŽVI. ion, that is uranyl, is complexed by organic ligands over a wide range of pH. These complexes change the cation mobility in aqueous environment w1x, where the dominant ligand is the carboxylate group w2x. Acetic acid ŽAcOH. is chosen as one of the simplest model to study the complexation of these substances with uranyl ion, moreover that itself and some derivatives are present naturally in different media w3–5x. Uranyl is active in vibrational spectroscopy, principally through its symmetric Ž ns . and antisymmetric ) Corresponding author. Tel.: q33-83-916300; Fax: q33-83275444
Ž na . stretching modes in Raman scattering and infrared absorption, respectively. Raman spectroscopy was mainly used to study its complexation with numerous inorganic andror organic ligands in aqueous solutions w6–13x. Nguyen-Trung et al. w7x have studied by Raman spectroscopy the complexation of acetate ŽAcO. with the uranyl ion at different pH and ligand-to-cation ratios. They have limited their investigations to the uranyl signal and have found three stoichiometries with one uranyl ion and one, two or three acetates molecules. They have concluded that the addition of one acetate molecule leads to a constant red shift of 9 cmy1 from the free UO 2 symmetric stretching mode at 870 cmy1 . No structure of the corresponding complexes was given. Very few studies of aqueous solutions of uranyl used
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 4 0 - X
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F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
infrared spectroscopy. Kakihana et al. w14x discussed the coordination structures of uranyl carboxylate complexes in aqueous solution by infrared ŽIR. and 13 C nuclear magnetic resonance ŽNMR. spectroscopies. They found in their conditions only one complex type with two acetates bounded with the bidentate structure. Gal ´ et al. w12,13x have obtained the infrared spectra of a saturated solution of uranyl acetate and stated that two structures coexist in solution for the acetate ligand: the bidentate structure is majority Žwith the symmetric nsŽCO 2 . and the antisymmetric naŽCO 2 . carboxylate stretching modes at 1468 cmy1 and at 1538 cmy1 , respectively, and naŽUO 22q . at 919 cmy1 .; the monodentate structure is also present Ž nsŽCO 2 . s 1390 cmy1 , naŽCO 2 . s 1603 cmy1 and naŽUO 22q . s 930 cmy1. No mention is made about the pH and the possible presence of uranyl hydroxides that could participate to the lines near 900 cmy1 . Very few studies are made with both Raman and IR spectroscopies that, however, give complementary information. We have initiated in our previous work w15x such studies and it was found that a lot of information are available on the structure of the complexes present in aqueous solution. The aim of this work is thus to study the complexes of the uranyl ion with hydroxide and acetate by IR-attenuated total reflectance ŽATR. and Raman spectroscopies in diluted aqueous acetate solutions, down to 50 mM. As a guide for both the choice of experimental conditions and the analysis of the vibrational spectra, the concentrations of significant species in experimental solution have been calculated with the aid of the PSEQUAD program w16x, by using the complexation constants summarized in Table 1 w17x. Solutions were prepared by mixing acetic solutions Ž50 or 100 mM. with uranyl nitrate ŽRaman. or perchlorate ŽIR. in order to vary the metal-to-ligand ratio Ž1r1, 1r2 and 1r3.. The pH, determining the concentrations of free AcOH, AcO and OHy has been monitored with NaOH in conditions where no hydroxide precipitation could occur at equilibrium. No complexation of the uranyl ion with perchlorate and the formation of a weak complex with nitrate ions in aqueous solution were demonstrated with a high concentration of UO 2 ŽNO 3 . 2 Žsuperior to 1.5 M. w6x. With a formation constant of 0.66 w17x, the proportion of complexes AcONa should be weak. As a consequence,
Table 1 Decimal logarithm of the cumulative formation constants of uranyl complexes in aqueous solutions on the molar scale at 258C, for ionic strength I s 0, except as mentioned w17x Compound
OHy
AcOya
ML ML 2 ML 3 M2 L2 M3L3
8.2 – – 22.4 54.4
3.06 5.57 6.97 – –
M is used for uranyl ŽUO 22q . and L represents the ligand Žhydroxide or acetate.. a 208C, calculated for I s 0 with the Davis formulae w19x.
the spectra should essentially display the effect of complexation of uranyl with acetate and hydroxide in solution.
2. Experimental 2.1. Materials All compounds Žexcept uranyl perchlorate. were commercially available and were used without further purification. Glacial acetic acid, UO 2 ŽNO 3 . 2 , 6H 2 O4 , ŽCH 3 COO. 2UO 2 , 2H 2 O4 and molar sodium hydroxide 1 M solution were obtained from Prolabo ŽRectapure, Normapure or Titrinorme for the last one.. Deuterated acetic acid Ž99.5% D. comes from Euriso-top. Hydrated uranyl perchlorate was synthesized as follows: 20 g of hexahydrated uranyl nitrate were dissolved in water. The solution was then neutralized with NH 4 OH Ž28% NH 3 wrw in water, Lancaster. under magnetic agitation until pH was between 7 and 8. The precipitate was filtrated, washed with water and then heated under vacuum at 258C. 10 g of UO 3 , 2 H 2 O4 and 5.24 ml of HClO4 Ž70%, Prolabo, Normapur. were dissolved under magnetic agitation in 40 ml of bideionised water. After dissolution, the solution was slowly heated until total evaporation of the solvent. After cooling down in air, the sample of uranyl perchlorate was preserved in a dessicator containing silicagel. An aqueous solution of the synthesized uranyl perchlorate containing 0.171 g diluted in 50 ml of bideionised water was analyzed by ICP-AES in order to determine a mean hydration degree of the solid sample.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
The solutions were prepared with bideionised and boiled water. The pH was measured with a Tacussel glass electrode. Samples used are described in Table 2. 2.2. InductiÕe coupled plasma atomic emission spectroscopy (ICP-AES) The apparatus is a Perkin-Elmer Emission Spectrometer 2000. The analysis wavelength was 385.958 nm and the limit of detection is then 3.5 P 10y5 M. The calibration was realised with three solutions of 10y2 , 5 P 10y3 and 2 P 10y3 M of uranyl nitrate. 2.3. Infrared spectroscopy Detailed experimental settings are the same as already described in our previous work w15x. Attenuated total reflectance Fourier transform infrared ŽATR-FTIR. spectra of aqueous solutions were measured between 4000 and 800 cmy1 on a Perkin-Elmer 2000 spectrometer equipped with a KBr beam splitter and a DTGS Ždeuterated triglycine sulphate. thermal detector. The ATR accessory used is a flat horizontal ZnSe crystal prism manufactured by
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Specac Žsix internal reflections or bounces on the upper surface, angle of incidence: 458.. ATR spectra are shown with an absorbance scale corresponding to logŽ R referencerR sample ., where R is the internal reflectance of the device, and the resolution of the spectra were 4 cmy1 . In order to eliminate as much as possible the strong absorbance of liquid water, a reference spectrum of pure water was simply used. It was additionally necessary to eliminate the contribution of water vapour, obtained separately from the difference between two spectra of ‘wet’ and dry air. Despite the absorbance scale used, the ATR spectra are not strictly proportional to the absorption coefficients at every wavenumber since no further correction has been applied w18x. 2.4. Raman spectroscopy Raman spectra were obtained with a Jobin-Yvon ISA T64000 spectrometer as already described elsewhere w15x. An usual macroscopic sample chamber, with a scattering collection at 908 to both the direction of incidence and the polarization vector of the exciting laser beam, was used to record the total radiant power of Stokes Raman spectra. A spectral
Table 2 Total initial concentrations ŽmM. used for the preparation of the solutions in this study Solution
UO 2 ŽAcO. 2
UO 2 ŽNO 3 . 2
UO 2 ŽClO4 . 2
AcOH
NaOH
Final pH a
Final pH b
A B I Ia Ib Ic II IIa IIb IIc III IIIa IIIb IIIc IV V Vb Vc
– – – – – – – – – – – – – – 200 – – –
100 100 0 17 25 50 0 17 25 50 0 17 25 50 – 0 50 100
– – 0 15 23 46 0 15 23 46 0 15 23 46 – – – –
0 0 50 50 50 50 50 50 50 50 50 50 50 50 – 100 c 100 c 100 c
0 91 0 0 0 0 33 33 33 33 48 ar47 b 48 ar47 b 48 ar47 b 48 ar47 b – 96 96 96
1.0 4.0 3.1 2.7 2.6 2.5 3.9 3.3 3.1 2.7 6.2 4.6 4.3 3.6 3.9 6.1 4.2 3.7
– – 3.1 3.0 3.0 2.9 3.9 3.4 3.2 3.2 5.9 4.7 4.4 3.9 – – – –
a
Nitrate or acetate salt. Perchlorate salt. c CD 3 COOH. b
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Table 3 Concentrations Žin mM except footnote b . calculated by PSEQUAD for the solutions used to study the complexation of acetate by the uranyl cation
A B I Ia Ib Ic II IIa IIb IIc III IIIa IIIb IIIc IV V Vb Vc a b
1.0 4.0 3.1 2.7 2.6 2.5 3.9 3.3 3.1 2.7 6.2 4.6 4.3 3.6 3.9 6.1 4.2 3.7
1.0 3.8 3.15 2.54 2.43 2.30 3.87 2.90 2.78 2.58 6.10 4.83 4.41 3.71 4.19 5.97 4.31 3.54
– – 48 46 46 45 44 42 42 41 1 9 12 18 37 3 19 30
– – 2 -1 -1 -1 6 1 1 1 49 17 9 3 16 97 12 3
– – – 3 3 5 – 5 6 7 – 2 5 14 30 – 9 29
– – – -1 -1 -1 – 1 1 1 – 7 9 7 99 – 21 17
– – – 0 0 0 – 0 0 0 – 3 2 -1 40 – 6 1
– – – 3 3 6 – 7 8 9 – 25 29 30 348 – 69 66
100 22 – 13 21 45 0 11 18 41 – -1 1 13 4 – 2 27
0 30 – 0 0 -1 – -1 -1 -1 – -1 2 7 7 – 3 12
0 6 – 0 0 0 – 0 0 0 – 1 2 -1 5 – 2 -1
300 200 1 46 70 145 6 43 68 141 49 54 64 123 66 97 124 245
Only the data corresponding to the solutions prepared with the nitrate salt ŽTable 2. are presented. No significant differences were observed with the perchlorate salt data. Calculated in the molar scale.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
x Total wUO 22q x wŽUO 2 . 2 ŽOH. 22q x wŽUO 2 . 3 ŽOH.q x I Solution Experimental PSEQUAD: wAcOHx wAcOy x wUO 2 AcOq x wUO 2 ŽAcO. 2 x wUO 2 ŽAcO.y 3 5 pH of the ylogwHq x b complexed solutiona AcO
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
resolution of about 2.5 cmy1 has been obtained. The 514.53 nm line of a Spectra Physics 2017 Stabilite´ model, argon ion laser was used for excitation at low pH Ž- 3.5.. In cases of luminescence ŽpH ) 3.5., the Raman effect was excited with a HerNe laser at 632.8 nm with a total power of 35 mW. Wavenumbers in vacuum were calibrated by using the mercury line at 18 312.7 cmy1 . The wavenumber accuracy depends much on the band profile and intensity. It is estimated between 1 and 4 cmy1 . 2.5. Peak decomposition of Raman and infrared spectra Spectra were converted into ASCII files and read by the Microcal Origin Peak Fitting Module for Windows using the Leven–Marquardt non-linear least-square curves fitter for the determination of each band parameters. Mixtures of Lorentzian and Gaussian lines were used since they gave better results than pure Lorentzians. 2.6. PSEQUAD PSEQUAD w16x is a FORTRAN program used to study equilibrium systems in solutions. Experiments are carried out as titrations: the concentration of one of the components is changing step by step. PSEQUAD allows the evaluation of titration curves and all the concentrations can be calculated for a given sample knowing the total concentrations of components in solution and the formation quotients w15x. For a given solution of acetic acid and NaOH, the ‘titration’ at constant volume was made by addition of aliquots of a virtual solution of uranyl at the concentration 1000 M. The calculation of equilibrium concentrations have been made in the hypothesis of homogeneous solutions by using the formation constants summarized in Table 1. For every solution, a starting ionic strength I was assumed in order to calculate starting values of the concentration quotients b j . The model of activity coefficients selected for this study was the Davies one Žlimited to ionic strength - 0.5. w19x. The more precise ionic strength deduced from the first program run allowed a refinement of the quotients b j . Convergence was quickly attained on equilibrium concentrations through this iterative process. Results are presented in Table 3.
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3. Results and discussion 3.1. The uranyl cation and its hydroxides For the study of acetate complexed with uranyl, it was first necessary to assign the uranyl hydroxides bands in the Raman and IR-ATR spectra of the examined aqueous solutions. The symmetric Ž nsŽUO 2 .. and antisymmetric Ž naŽUO 2 .. stretching wavenumbers of the hydrated uranyl ion Žnoted Ž1,0.. in aqueous solution at very low pH are 870 and 962 cmy1 , respectively Žspectra not shown. w6,11,13,20x. Among many papers dealing with the Raman spectra of uranyl cation in aqueous solutions w6–13x, only few of them take into account the hydroxides derivatives. Toth and Begun w11x have studied solutions of 0.1 M uranyl nitrate at different pH values and assigned two new bands at 851 and 836 cmy1 , appearing with the pH increase, to ŽUO 2 . 2 ŽOH. 2q 2 Žnoted Ž2,2.. and ŽUO 2 . 3 ŽOH.q Žcalled Ž3,5.., re5 spectively. We have also prepared solutions of 0.1 M uranyl nitrate at pH 1.0 and 4.0 and both the Raman and IR-ATR spectra of these solutions were obtained ŽFig. 1.. The Raman spectra obtained by Toth and Begun were reproduced ŽFig. 1A.. Free hydrated uranyl and the two hydroxides previously cited are present Žsolution B, Table 3.. The comparison of the measured areas of the uranyl scattering in the range 780–880 cmy1 and the nitrate Raman signal in the range 975–1100 cmy1 for 0.1 M nitrate uranyl aqueous solutions at different pH shows that the molar scattering coefficient of nsŽUO 2 . does not change with hydroxide complexation. The value 2q x x w J s I w nsŽUO 2 .xrI w nsŽNO 3 .x = wNOy s 2.65 3 r UO 2 0.15 is found. So, it is possible to compare directly the intensity of the bands with the concentration of hydroxides in solution. Three unresolved bands can be seen on both infrared and Raman spectra. As a first approach, we have decomposed these bands and found three couples: 870; 9624 , 853; 9434 and 835; 9234 cmy1 , for nsŽUO 2 .; naŽUO 2 .4 of Ž1,0., Ž2,2. and Ž3,5. species, respectively ŽTable 4.. These assignments will be used as a first approximation. A more precise study of uranyl hydroxides is in progress to better understand the spectra of such multinuclear complexes.
66
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
Fig. 1. Raman ŽA. and IR-ATR ŽB. spectra of a 0.1 M solution of uranyl nitrate at pH 4.0 Žsolution B.. Calculated individual lines are also indicated.
3.2. The complexation of uranyl with acetate It is generally observed that the UO force constants of uranyl, and consequently both the nsŽUO 2 . and naŽUO 2 . wavenumbers, decrease while the ligand number in the equatorial plane of uranyl increase. Thus the uranyl spectra in the ranges 900–980 cmy1 Ž naŽUO 2 . observed in infrared. and 800–880 cmy1 Ž nsŽUO 2 . in Raman. are mainly used to define the complex stoichiometries, as previously shown by Nguyen-Trung et al. w7x. Information on the ligand structures are rather available in the ligand spectra,
mainly through the COOy stretching modes Žbetween about 1300–1600 cmy1 . and the n ŽCC. vibration Ž880–960 cmy1 . w15x. Although the n ŽCC. and naŽUO 2 . wavenumbers overlap around 930 cmy1 , there is no significant interference between their infrared absorptions because the molar absorption coefficient is about 30 times larger for naŽUO 2 . at 962 cmy1 than for n ŽCC. at 928 cmy1 , and the highest concentration ratio acetateruranyl used is about 3 ŽTable 2.. Thus the infrared spectrum between 880 and 980 cmy1 is essentially related to the naŽUO 2 . mode.
n ŽC5O.
naŽCOOy .
nsŽCOOy .
d ŽCH 3 .
d ip ŽOH.
n ŽC–O. with d ip ŽCOH.
ATR-IR Raman ATR-IR Raman ATR-IR
1712 1710 – – –
– – 1553 1551 1595
– – 1415 1416 1389 Žcoupled.
1369 1368 1348 1347 1339 Žcoupled.
1390 1390 – – –
1280
Raman ATR-IR
– –
1527
1467
– –
– –
Raman
– – – – – – –
– – – – – –
– – – – – –
– – – – – – –
– – – – – – –
Assignment AcOH AcOy Pseudobridging uranyl acetate Bidentate uranyl acetate ŽAcO.UOq 2 ŽAcO. 2UO 2 ŽAcO. 3UO 2 UO 22q ŽUO 2 . 2 ŽOH. 2q 2 ŽUO 2 . 3 ŽOH.q 5
– – – – – –
naŽUO 22q .
nsŽUO 22q .
– – – – –
– – – – –
943
– –
– –
952 – – – – – –
– 954 928
– 861 841 823 870 853 835
n ŽC–C.
892 – – –
928
962 943 923
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
Table 4 Principal wavenumbers Žcmy1 . and assignments modes of acetic acid, sodium acetate, uranyl acetates and uranyl hydroxides in aqueous solutions between 1800 and 400 cmy1 . The notations n , d and r are used for the stretching, the bending and rocking modes
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68
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
Nitrate is not a good counter anion for the study of infrared spectra of the complexed uranyl acetate species since large and intense bands occur between 1200 and 1500 cmy1 overlapping with acetate modes. That is why we have synthesized uranyl perchlorate that has only one stretching band at 1106 cmy1 . The Raman spectrum of a solution of 0.163 g uranyl perchlorate in 5 ml water ŽpH s 3.1. shows the same features between 800 and 900 cmy1 than those observed for the nitrate salt. We have also verified that no residual nitrate stay in the solution by the absence of a band at 1047 cmy1 Ž nsŽNO 3 ... The only observable band was the symmetric stretching of perchlorate at 934 cmy1 . The results of the titration of the synthesized salt by ICP-AES allow the calculation of a mean molecular weight of 535.4 grmol that corresponds to a hydrated state of 3.7 water molecule per UO 2 ŽClO4 . 2 molecule. The hydration of the salt was not controlled strictly and the crystallised solid shows two mixed colors Žyellow and orange.. This mean molecular weight was used to determine all the concentrations of uranyl in the solutions studied by IR-ATR spectroscopy ŽTable 2.. 3.2.1. Uranyl signals Figs. 2–4 show the symmetric and antisymmetric stretching vibrations of the uranyl group in the ATR-
IR and Raman spectra. In the case of the complexation of acetate with uranyl at low pH, the complexes are in too small concentrations Žsolutions Ia–c, Table 3. to see modifications of the n ŽUO 2 . profiles, except a broadening of the line in the lower frequencies in each spectrum ŽFig. 2A.. The case of the complexation at initial pH 3.9 Žsolutions IIa–c, Table 3. is more interesting since there is no contribution of Ž3,5. and UO 2 ŽAcO.y 3 compounds and very little of Ž2,2. and UO 2 ŽAcO. 2 . The subtraction of the Ž1,0. and Ž2,2. contributions leads to the apparition of an asymmetric peak at 861 cmy1 in the Raman spectra with an intensity increasing with the total uranyl concentration Žfrom IIa to IIc., when compared to n ŽCC. of acetic acid that is approximately constant in concentration ŽTable 3, Fig. 3.. This line is assigned to UO 2 AcOq which is the main acetate complex in the medium. With the decrease of the ratio wUO 2 AcOqx rwUO 2 ŽAcO. 2 x from IIc to IIa ŽTable 3., the asymmetry of the band at 861 cmy1 toward low wavenumber is bigger Žsolution IIa.. A similar procedure of elimination of the signals of Ž1,0. and Ž2,2. species from the IR-ATR spectra of Fig. 2A, leads to the apparition of two bands at 954 cmy1 and the other at 928 cmy1 ŽFig. 2B.. By analogy with the observations made on the equivalent Raman spectra, the first line is assigned to
Fig. 2. IR-ATR spectra of uranyl perchlorate solutions defined in Table 2. ŽA. total signal; ŽB. spectra obtained after subtraction of hydrated UO 22q and ŽUO 2 . 2 ŽOH. 2q from solutions IIa–c. 2
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
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Fig. 3. Raman spectra Žnormalized on the AcOH band at 892 cmy1 . of uranyl nitrate solutions IIa–c defined in Table 2. Dashed spectra are obtained by subtraction of the contribution of Ž1,0. and Ž2,2. uranyl species.
the UO 2 AcOq complex and the second one to UO 2 ŽAcO. 2 , all the more as the first one increases from IIa to IIc as the metal-to-ligand ratio increases. At still higher pH values Žsolutions IIIa–c, Fig. 4., the above defined mean Raman scattering efficiency J of the nsŽUO 2 . mode increases 2.78 for solution IIIc to 3.56 for solution IIIa, while the fraction of uranyl complexed to acetate increases ŽTable 3.. The molar scattering of uranyl acetates are then higher than for hydroxides. In solution IIIc, the concentrations of uranyl in Ž1,0., UO 2 AcOq and Ž2,2. species are about the same and equal to approximately twice that of UO 2 ŽAcO. 2 , while compounds Ž3,5. and UO 2 ŽAcO.y Ž . 3 are negligible Table 3 . The decomposition of the spectrum shows three strong bands at 870, 861 and 853 cmy1 , assigned respec-
tively to Ž1,0., UO 2 AcOq and Ž2,2., and an additional line at 841 cmy1 assigned to UO 2 ŽAcO. 2 . This peak increases when wUO 2 ŽAcO. 2 x increases in IIIb and stays important in IIIa. This assignment is not in accordance with the one proposed by Nguyen-Trung et al. w7x at 861 cmy1 for UO 2 ŽAcO. 2 , but better agrees with the shift already observed in IR-ATR spectra: y8 cmy1 for UO 2 AcOq and then an additional y26 shift cmy1 for UO 2 ŽAcO. 2 ŽFig. 2B, Table 5.. Moreover the relative intensities well agree with the concentrations calculated with PSEQUAD ŽFig. 4.. Big differences in the structures of UO 2 AcOq and UO 2 ŽAcO. 2 are anticipated from these different shifts for the uranyl signals. ŽCH 3 COO. 3UOy 2 is generally low in concentration, even in solutions IIIb and IIIa Ž2 and 3 mM.. Fig. 4
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F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
tainty in the IR-ATR spectra to assign an additional component to this last complex. Nguyen-Trung et al. w7x found that the Raman shifts of the acetate complexes follow the relation: n s 870 y 9n where n is the average ligand number. This supposes that each added ligand leads to the same perturbation on the uranyl group and then it could follow that all the ligands have the same configuration. As we will discuss next paragraph, two structures coexist in the solutions analysed. This fact is in line with the result previously found for copperŽII. acetate w15x, i.e., that different structures can occur as a function of ligand number.
Fig. 4. Raman spectra of uranyl nitrate solutions IIIa–c defined in Table 2, with different intensity scales. Decomposed lines are indicated. For comparison, the intensities of stick spectra Ždashed lines are for uranyl hydroxides. represent the corresponding relative concentrations calculated by PSEQUAD, without taking into account the increase of the nsŽUO 2 . molar scattering ŽTable 3..
shows a shoulder at 823 cmy1 for spectra of solutions IIIa and IIIb. Even though the concentration of the product is weak, this line is assigned to UO 2 ŽAcO.y 3 complex. This makes a new red shift of 18 cmy1 consequently to the addition of a third ligand. Unfortunately, there was too much uncer-
3.2.2. Acetate signals Fig. 5 compares the ATR-IR spectra of the free acetate anion Žsodium acetate in solution III. to those of complexed species in the region of the carboxylate stretchings. These last spectra have been obtained after subtraction of the acetic acid and ‘free’ acetate contributions. As expected by the examination of PSEQUAD calculations, the higher the initial pH is, from solutions of type I to III, the more concentrated the complexes are. As it was already observed about copper acetate w15x, the complexation constant is not strong enough to make directly the reaction of uranyl with acetic acid with the release of one proton. Four bands appear in the region 1580–1200 cmy1 , at 1527, 1467, 1389 and 1339 cmy1 . The region above 1580 cmy1 is not well resolved because of the non perfect elimination of the water bending absorption near 1640 cmy1 and because solute bands have less than 0.01 ‘absorbance’ unit. The couples 1527, 14674 and 1389, 13394 cmy1 vary parallely as a function
Table 5 Comparison of the area ratios observed for pseudobridging and bidentate n ŽCC. Ž A 943 r A 951 . with possible structure concentration ratios calculated with PSEQUAD Ž y1 r y 2 . Žfrom solutions IIIa–c and IV.. UO 2 AcOq is supposed pseudobridging Solution
A 943 r A 951
y1 r y 2a
y 1 r y 2b
y1 r y 2c
IIIa IIIb IIIc IV
0.7 1.1 3.1 0.9
0.9 1.2 2.9 0.9
0.6 1.0 2.5 0.6
0.1 0.2 0.9 0.1
a,b,c
See Section 3.2.3.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
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Fig. 5. IR-ATR spectra of the stretching region of acetate complexed forms with uranyl from solutions defined in Table 2 Žall these spectra, except III, are obtained after subtraction of free acetic acid and free acetate spectra..
of the initial quantity of uranyl perchlorate added: the higher this amount is, from solutions of type a to c, the more the second couple grows in intensity.
The first couple of frequencies with a difference D s 60 cmy1 is assigned to a bidentate complex w21x, notably when the proportion of complexes
Fig. 6. IR-ATR spectra of solution IV ŽTable 2.. ŽA. After subtraction of water; ŽB. after additional subtraction of free acetate and acetic acid spectra.
72
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
UO 2 ŽAcO. 2 and UO 2 ŽAcO.y 3 are preponderant versus the monoliganded one ŽTable 3.. Kakihana et al. w14x have already shown the possibility of a bidentate structure for the acetate–uranyl complex in aqueous medium with naŽCO 2 . at 1538 cmy1 and nsŽCO 2 . at 1466 cmy1 . The two other lines are assigned to a monodentate or, more probably in aqueous solution, a pseudobridging form w22x. The nsŽCO 2 . splitting into two strong components at 1389 cmy1 and 1339 cmy1 is assigned to a coupling with dsŽCH 3 ., as previously demonstrated through the substitution of CH 3 to CD 3 in mercuryŽII. acetate complexes w23x. The phenomenon is observed each time the nsŽCO 2 . mode undergoes a large decrease by acetate complexation. The possibility of the existence of two structures for the uranyl acetate was also observed by Gal ´ et al. w13x in the spectra of a saturated aqueous solution of UO 2 ŽAcO. 2 . They assigned the lines at 1538 and 1468 cmy1 to naŽCO 2 . and nsŽCO 2 ., respectively, for the bidentate acetate form and two bands, less intense, at 1603 and 1390 cmy1 to the monodentate acetate structure. Fig. 6 displays spectra from a 0.2 M aqueous solution of UO 2 ŽAcO. 2 after subtraction of the acetic acid and ‘free’ acetate spectra. The four lines previously observed are present. An other weak one also appears at 1595 cmy1 that can be assigned to the antisymmetric stretching of the hydrated unidentate complex structure ŽTable 4.. Figs. 5 and 6 display a striking decrease in the intensity ratio I w naŽCO 2 .xrw nsŽCO 2 .x from the free acetate ion to the bidentate structure. This fact is probably related to a decrease of OCO angle, and as a result a decrease of the angle F between the transitions moments corresponding to the stretching of the two bonds, since: I w naŽCO 2 .xrI w nsŽCO 2 .x s tan2 ŽFr2.. The n ŽCC. band, intense in the Raman spectra, has been shown to be very sensitive to the complexation of acetate in aqueous solution w5,15,24,25x. Notably, we have suggested the following assignment in the case of the complexation of copperŽII.: the line at 928 cmy1 is related to ‘free’ sodium acetate, another one at 938 cmy1 was assigned to the stretching vibration of the pseudobridging form and a third one at 948 cmy1 to the bidentate form. Here, we have to be careful because the antisymmetric stretching of the free hydrated uranyl and its complex derivatives have their wavenumbers close to the one of acetate
C–C stretching. It was then important to be sure that the antisymmetric stretching, forbidden in Raman scattering, was not ‘activated’ by complexation, even though no activation was observed for the hydroxide derivatives ŽFig. 1A.. Fig. 7 shows the Raman spectra of the n ŽCC. stretching mode of solutions V and Vb–c ŽTable 3. made with trideuterated acetate at initial pH 6.1 complexed by uranyl ŽTable 2.. The ‘free’ trideuterated acetate n ŽCC. band is shifted from 928 for acetate to 884 cmy1 Žsolution V.. With addition of the uranyl cation, two new lines appear at 901 and 910 cmy1 corresponding by analogy with acetate to the n ŽCC. of the pseudobridging and the bidentate structures of the complexes, respectively. No scattering band is observed in the region 920–980 cmy1 ŽFig. 7.. This makes evidence that naŽUO 2 . signal is not enhanced by complexation and does not interfere with n ŽCC.. If we except the line at 928 cmy1 , traducing the presence of ‘free’ acetate, the two lines at 943 and 952 cmy1 , observed in Fig. 8, are assigned to the pseudobridging and the bidentate forms, respectively. This is confirmed by the evolution of the intensities with the quantity of uranyl added: the line at 952 cmy1 is the most intense at the same time that both bands at 1527 cmy1 and 1467 cmy1 in the ATR-IR spectrum, that is when the metal-to-ligand ratio is 1r3 ŽFigs. 5 and 8.. On the contrary, the line at 943 cmy1 is the biggest when this ratio is 1r1, parallely to the lines at 1389 and 1339 cmy1 . Similar conclusions can be drawn for the n ŽCC. of trideuterated acetate since the evolution of the different peaks are the same as a function of pH and cation-to-ligand ratio. The principal assignments are summarized in Table 4. 3.2.3. Structurer stoichiometry relation We now try to establish a relation between the structure and the stoichiometry of the complexes. We note:
° x s UO AcO ~ x s UO Ž AcO. ¢x s UO Ž AcO.
q
1
2
2
2
2
3
2
3
and
½
y 1 s w pseudobridgingx y 2 s w bidentatex
we have: x 1 q 2 x 2 q 3 x 3 s wtotal complexed acetatex s y 1 q y 2 ŽTable 3.. The n ŽCC. region of the Raman spectra of solutions were decomposed in
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
73
Fig. 7. Raman spectra in the n ŽCC. range of deuterated acetate complexed with uranyl Žsolutions V and Vb–c, Table 2..
individual components ŽFig. 8. and the areas were measured. The scattering molar coefficients of every component were difficult to calculate because there is some overlap between the n ŽCC. component of AcOH at 892 cmy1 and the strong uranyl signal. By assuming a constant relative differential Raman scattering cross-section for the n ŽCC. modes of all complexed species, the peak areas are proportional to the concentration of the related structure. The calculated concentrations by PSEQUAD for the stoichio-
metric complexes were used and combined in order to calculate the concentrations of the bidentate and the pseudobridging forms. UO 2 AcOq is assumed pseudobridging because the line at 943 cmy1 is intense when the complex UO 2 AcOq is in high quantity. Three further hypotheses are proposed: Ža. one ligand of UO 2 ŽAcO. 2 and of UO 2 ŽAcO.y 3 is pseudobridging, the others being bidentate; Žb. UO 2 ŽAcO. 2 is like in Ža. and UO 2 ŽAcO.y 3 is entirely bidentate; Žc. all acetates in UO 2 ŽAcO. 2 and
Fig. 8. Quantitative analysis of the n ŽCC. range in the Raman spectra of species AcO, either free or complexed with uranyl.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
74
UO 2 ŽAcO.y 3 are bidentate. We can then calculate y 1 and y 2 as:
Ž a. Ž b. Ž c.
½ ½ ½
y1 s x 1 q x 2 q x 3 y2 s x 2 q 2 x 3 y1 s x 1 q x 2 y2 s x 2 q 3 x 3 y1 s x 1 y2 s 2 x 2 q 3 x 3
and compare the ratio y 1ry 2 to the one obtained by areas measurements A 943rA 951 , where A 943 and A 951 are the measured areas of the pseudobridging acetate at 943 cmy1 and of the bidentate form at 952 cmy1 , respectively. The results for the three proposed configurations are given in Table 5. It is concluded that both hypotheses Ža. and Žb. are possible. Anyway, there is little doubt that one acetate ligand of UO 2 ŽAcO. 2 has a pseudobridging structure, the other one being in the bidentate structure. This conclusion does not agree with the one of Kakihana et al. w14x who state a unique bidentate structure for UO 2 ŽAcO. 2 . But if we have a precise look to their spectra, they have neglected Ž1. two weak bands near 1390 and 1330 cmy1 on their IR spectra Ž2. the asymmetry of the COO peak near 190 ppm in their 13 C NMR spectra. The short repetition time makes this last peak analysis difficult. For UO 2 ŽAcO.y 3, there is still some uncertainty. But it is quite possible that the solvated complex does not display three bidentate acetates as it was shown for solid NaUO 2 ŽCH 3 COO. 3 by X-Ray w26x and EXAFS w27x. The hypothesis of different structures in the same complex was already perceptible regarding the uranyl signals. The pseudobridging form leads to the release of only one water molecule Žand an 8 cmy1 red shift. which could be thermodynamically favourable for the formation of the complex from free hydrated uranyl cation. After that, by steric considerations, the bidentate structure could be better, leads to the release of two water molecules and it is traduced by a mean red shift of 22 cmy1 . 4. Conclusions In this work, we have studied the interactions of uranyl with the hydroxide and acetate molecules
their probable structures in diluted aqueous solutions. We have shown that IR and Raman spectroscopies are sensitive methods to characterize different structures of dioxouraniumŽVI. complexes in aqueous diluted solutions. For example they allow us to show that for the same complex the ligation could be realised with different structural modes. It appears that for UO 22q as for Cu2q w15x, the first complexation step in water involves a pseudobridging structure of acetate, rather than a bidentate complexation. This fact suggests that this step keeps a hydration number of the cation as high as possible. Further research is needed to generalise this trend which is governed by both energy and entropy contributions.
Acknowledgements The authors are grateful to the Laboratoire de Geologie et de Gestion des Ressources Minerales et ´ ´ Energetiques, Nancy, France for the gracious furni´ ture of the synthesized UO 3 , 2 H 2 O4 .
References w1x D. Porcelli, P.S. Andersson, G.J. Wasserburg, J. Ingri, M. Baskaran, Geochim. Cosmochim. Acta 61 Ž19. Ž1997. 4095. w2x M. Filella, N. Parthasarathy, J. Buffle, Humic and fulvic compounds, Encyclopedia of Analytical Science, 1995, p. 2017. w3x J. Berthelin, Pedologie, Gand 32 Ž1982. 313. ´ w4x A. McCulloch, J.C. Boutonnet, J. Franklin, T. Kawano, J.M. Libre, I. Sobolev, oral communication, SETAC-Europe 8th Annual Meeting, Interfaces in Environmental Chemistry and Toxicology: From the Global to the Molecular Level, 14–18 April 1998, Bordeaux, France. w5x J. Semmler, D.E. Irish, T. Oseki, Geochim. Cosmochim. Acta 54 Ž1990. 947. w6x M.H. Brooker, C.H. Huang, J. Sylwestrowicz, J. Inorg. Nucl. Chem. 42 Ž1980. 1431. w7x C. Nguyen-Trung, G.M. Begun, D.A. Palmer, Inorg. Chem. 31 Ž1992. 5280. w8x A. Burneau, M. Tazi, G. Bouzat, Talanta 39 Ž7. Ž1992. 743. w9x L. Maya, G.M. Begun, J. Inorg. Nucl. Chem. 43 Ž11. Ž1981. 2827. w10x E. Faulques, R.E. Russo, D.L. Perry, Spectrochimica Acta A 50 Ž4. Ž1994. 757.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75 w11x L.M. Toth, G.M. Begun, J. Phys. Chem. 85 Ž1981. 547. w12x M. Gal, ´ P.L. Goggin, J. Mink, J. Mol. Struct. 114 Ž1984. 459. w13x M. Gal, ´ P.L. Goggin, J. Mink, Spectrochimica Acta A 48 Ž1. Ž1992. 121. w14x M. Kakihana, T. Nagumo, M. Okamoto, H. Kakihana, J. Phys. Chem. 91 Ž1987. 6128. w15x F. Quiles, ` A. Burneau, Vib. Spectrosc. 16 Ž2. Ž1998. 105. w16x L. Zekany, I. Nagyral, ´ ´ in: D.J. Leggett ŽEd.., Computational Methods for the Determination of Formation Constants, Chap. 8, Plenum, New York, 1985. w17x R.M. Smith, A.E. Martell, Critical Stability Constants, Vol. 3, Other Organic ligands, Plenum, New York, 1977. w18x F.M. Mirabella Jr., N.J. Harrick, Internal Reflection Spectroscopy: Review and Supplement, Harrick Scientific, New York, 1985, p. 84. w19x C.W. Davies, J. Chem. Soc. Ž1938. 2093.
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w20x L.H. Jones, R.A. Penneman, J. Chem. Phys. 21 Ž3. Ž1953. 542. w21x K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd edn., Wiley, 1978, p. 232. w22x M. Nara, H. Torii, M. Tasumi, J. Phys. Chem. 100 Ž1996. 19812. w23x N. Gross, Diplome d’Etudes Approfondies, Universite´ H. ˆ Poincare´ Nancy I, 1998. w24x R.I. Bickley, H.G.M. Edwards, S.J. Rose, R. Gustar, J. Mol. Struct. 238 Ž1990. 15. w25x Z. Nickolov, I. Ivanov, G. Georgiev, D. Stoilova, J. Mol. Struct. 377 Ž1996. 13. w26x D.H. Templeton, A. Zalkin, H. Ruben, L.K. Templeton, Acta Cryst. C 41 Ž1985. 1439. w27x M.A. Denecke, T. Reich, S. Pompe, M. Bubner, K.H. Heise, N. Nitsche, P.G. Allen, J.J. Bucher, N.M. Edelstein, D.K. Shuh, J. Phys. IV France 7 ŽC2. Ž1997. 637.
Infrared and Raman spectroscopic study of uranyl complexes: hydroxide and acetate derivatives in aqueous solution F. Quiles ` ) , A. Burneau Laboratoire de Chimie Physique pour l’EnÕironnement, U.M.R. 7564 CNRS-UniÕersite´ Henri Poincare, ´ 405 rue de VandoeuÕre, 54 600 Villers-les-Nancy, France ` Received 22 July 1998; revised 14 September 1998; accepted 16 September 1998
Abstract Infrared-attenuated total reflectance ŽIR-ATR. and Raman spectroscopies are used to identify the complexed species of uranyl with hydroxide and acetate in aqueous solutions as a function of pH and metal-to-ligand ratio. Three stoichiometries . ŽUO 2 CH 3 COOq, UO 2 ŽCH 3 COO. 2 and UO 2 ŽCH 3 COO.y 3 are observed via irregular shifts of the uranyl stretching signals. The acetate vibrational modes n ŽCO 2 . and n ŽCC., allow the identification of two different ligand structures as a function of the complex stoichiometries: UO 2 CH 3 COOq and one ligand of UO 2 ŽCH 3 COO. 2 are pseudobridging, the second acetate of UO 2 ŽCH 3 COO. 2 being bidentate. There is still uncertainty on the presence of a pseudobridging acetate in UO 2 ŽCH 3 COO.y 3. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Infrared; Raman; Vibrational spectra; Uranyl hydroxide; Uranyl acetate; Aqueous solutions; Carboxylate complexation
1. Introduction The dioxouraniumŽVI. ion, that is uranyl, is complexed by organic ligands over a wide range of pH. These complexes change the cation mobility in aqueous environment w1x, where the dominant ligand is the carboxylate group w2x. Acetic acid ŽAcOH. is chosen as one of the simplest model to study the complexation of these substances with uranyl ion, moreover that itself and some derivatives are present naturally in different media w3–5x. Uranyl is active in vibrational spectroscopy, principally through its symmetric Ž ns . and antisymmetric ) Corresponding author. Tel.: q33-83-916300; Fax: q33-83275444
Ž na . stretching modes in Raman scattering and infrared absorption, respectively. Raman spectroscopy was mainly used to study its complexation with numerous inorganic andror organic ligands in aqueous solutions w6–13x. Nguyen-Trung et al. w7x have studied by Raman spectroscopy the complexation of acetate ŽAcO. with the uranyl ion at different pH and ligand-to-cation ratios. They have limited their investigations to the uranyl signal and have found three stoichiometries with one uranyl ion and one, two or three acetates molecules. They have concluded that the addition of one acetate molecule leads to a constant red shift of 9 cmy1 from the free UO 2 symmetric stretching mode at 870 cmy1 . No structure of the corresponding complexes was given. Very few studies of aqueous solutions of uranyl used
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 4 0 - X
62
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
infrared spectroscopy. Kakihana et al. w14x discussed the coordination structures of uranyl carboxylate complexes in aqueous solution by infrared ŽIR. and 13 C nuclear magnetic resonance ŽNMR. spectroscopies. They found in their conditions only one complex type with two acetates bounded with the bidentate structure. Gal ´ et al. w12,13x have obtained the infrared spectra of a saturated solution of uranyl acetate and stated that two structures coexist in solution for the acetate ligand: the bidentate structure is majority Žwith the symmetric nsŽCO 2 . and the antisymmetric naŽCO 2 . carboxylate stretching modes at 1468 cmy1 and at 1538 cmy1 , respectively, and naŽUO 22q . at 919 cmy1 .; the monodentate structure is also present Ž nsŽCO 2 . s 1390 cmy1 , naŽCO 2 . s 1603 cmy1 and naŽUO 22q . s 930 cmy1. No mention is made about the pH and the possible presence of uranyl hydroxides that could participate to the lines near 900 cmy1 . Very few studies are made with both Raman and IR spectroscopies that, however, give complementary information. We have initiated in our previous work w15x such studies and it was found that a lot of information are available on the structure of the complexes present in aqueous solution. The aim of this work is thus to study the complexes of the uranyl ion with hydroxide and acetate by IR-attenuated total reflectance ŽATR. and Raman spectroscopies in diluted aqueous acetate solutions, down to 50 mM. As a guide for both the choice of experimental conditions and the analysis of the vibrational spectra, the concentrations of significant species in experimental solution have been calculated with the aid of the PSEQUAD program w16x, by using the complexation constants summarized in Table 1 w17x. Solutions were prepared by mixing acetic solutions Ž50 or 100 mM. with uranyl nitrate ŽRaman. or perchlorate ŽIR. in order to vary the metal-to-ligand ratio Ž1r1, 1r2 and 1r3.. The pH, determining the concentrations of free AcOH, AcO and OHy has been monitored with NaOH in conditions where no hydroxide precipitation could occur at equilibrium. No complexation of the uranyl ion with perchlorate and the formation of a weak complex with nitrate ions in aqueous solution were demonstrated with a high concentration of UO 2 ŽNO 3 . 2 Žsuperior to 1.5 M. w6x. With a formation constant of 0.66 w17x, the proportion of complexes AcONa should be weak. As a consequence,
Table 1 Decimal logarithm of the cumulative formation constants of uranyl complexes in aqueous solutions on the molar scale at 258C, for ionic strength I s 0, except as mentioned w17x Compound
OHy
AcOya
ML ML 2 ML 3 M2 L2 M3L3
8.2 – – 22.4 54.4
3.06 5.57 6.97 – –
M is used for uranyl ŽUO 22q . and L represents the ligand Žhydroxide or acetate.. a 208C, calculated for I s 0 with the Davis formulae w19x.
the spectra should essentially display the effect of complexation of uranyl with acetate and hydroxide in solution.
2. Experimental 2.1. Materials All compounds Žexcept uranyl perchlorate. were commercially available and were used without further purification. Glacial acetic acid, UO 2 ŽNO 3 . 2 , 6H 2 O4 , ŽCH 3 COO. 2UO 2 , 2H 2 O4 and molar sodium hydroxide 1 M solution were obtained from Prolabo ŽRectapure, Normapure or Titrinorme for the last one.. Deuterated acetic acid Ž99.5% D. comes from Euriso-top. Hydrated uranyl perchlorate was synthesized as follows: 20 g of hexahydrated uranyl nitrate were dissolved in water. The solution was then neutralized with NH 4 OH Ž28% NH 3 wrw in water, Lancaster. under magnetic agitation until pH was between 7 and 8. The precipitate was filtrated, washed with water and then heated under vacuum at 258C. 10 g of UO 3 , 2 H 2 O4 and 5.24 ml of HClO4 Ž70%, Prolabo, Normapur. were dissolved under magnetic agitation in 40 ml of bideionised water. After dissolution, the solution was slowly heated until total evaporation of the solvent. After cooling down in air, the sample of uranyl perchlorate was preserved in a dessicator containing silicagel. An aqueous solution of the synthesized uranyl perchlorate containing 0.171 g diluted in 50 ml of bideionised water was analyzed by ICP-AES in order to determine a mean hydration degree of the solid sample.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
The solutions were prepared with bideionised and boiled water. The pH was measured with a Tacussel glass electrode. Samples used are described in Table 2. 2.2. InductiÕe coupled plasma atomic emission spectroscopy (ICP-AES) The apparatus is a Perkin-Elmer Emission Spectrometer 2000. The analysis wavelength was 385.958 nm and the limit of detection is then 3.5 P 10y5 M. The calibration was realised with three solutions of 10y2 , 5 P 10y3 and 2 P 10y3 M of uranyl nitrate. 2.3. Infrared spectroscopy Detailed experimental settings are the same as already described in our previous work w15x. Attenuated total reflectance Fourier transform infrared ŽATR-FTIR. spectra of aqueous solutions were measured between 4000 and 800 cmy1 on a Perkin-Elmer 2000 spectrometer equipped with a KBr beam splitter and a DTGS Ždeuterated triglycine sulphate. thermal detector. The ATR accessory used is a flat horizontal ZnSe crystal prism manufactured by
63
Specac Žsix internal reflections or bounces on the upper surface, angle of incidence: 458.. ATR spectra are shown with an absorbance scale corresponding to logŽ R referencerR sample ., where R is the internal reflectance of the device, and the resolution of the spectra were 4 cmy1 . In order to eliminate as much as possible the strong absorbance of liquid water, a reference spectrum of pure water was simply used. It was additionally necessary to eliminate the contribution of water vapour, obtained separately from the difference between two spectra of ‘wet’ and dry air. Despite the absorbance scale used, the ATR spectra are not strictly proportional to the absorption coefficients at every wavenumber since no further correction has been applied w18x. 2.4. Raman spectroscopy Raman spectra were obtained with a Jobin-Yvon ISA T64000 spectrometer as already described elsewhere w15x. An usual macroscopic sample chamber, with a scattering collection at 908 to both the direction of incidence and the polarization vector of the exciting laser beam, was used to record the total radiant power of Stokes Raman spectra. A spectral
Table 2 Total initial concentrations ŽmM. used for the preparation of the solutions in this study Solution
UO 2 ŽAcO. 2
UO 2 ŽNO 3 . 2
UO 2 ŽClO4 . 2
AcOH
NaOH
Final pH a
Final pH b
A B I Ia Ib Ic II IIa IIb IIc III IIIa IIIb IIIc IV V Vb Vc
– – – – – – – – – – – – – – 200 – – –
100 100 0 17 25 50 0 17 25 50 0 17 25 50 – 0 50 100
– – 0 15 23 46 0 15 23 46 0 15 23 46 – – – –
0 0 50 50 50 50 50 50 50 50 50 50 50 50 – 100 c 100 c 100 c
0 91 0 0 0 0 33 33 33 33 48 ar47 b 48 ar47 b 48 ar47 b 48 ar47 b – 96 96 96
1.0 4.0 3.1 2.7 2.6 2.5 3.9 3.3 3.1 2.7 6.2 4.6 4.3 3.6 3.9 6.1 4.2 3.7
– – 3.1 3.0 3.0 2.9 3.9 3.4 3.2 3.2 5.9 4.7 4.4 3.9 – – – –
a
Nitrate or acetate salt. Perchlorate salt. c CD 3 COOH. b
64
Table 3 Concentrations Žin mM except footnote b . calculated by PSEQUAD for the solutions used to study the complexation of acetate by the uranyl cation
A B I Ia Ib Ic II IIa IIb IIc III IIIa IIIb IIIc IV V Vb Vc a b
1.0 4.0 3.1 2.7 2.6 2.5 3.9 3.3 3.1 2.7 6.2 4.6 4.3 3.6 3.9 6.1 4.2 3.7
1.0 3.8 3.15 2.54 2.43 2.30 3.87 2.90 2.78 2.58 6.10 4.83 4.41 3.71 4.19 5.97 4.31 3.54
– – 48 46 46 45 44 42 42 41 1 9 12 18 37 3 19 30
– – 2 -1 -1 -1 6 1 1 1 49 17 9 3 16 97 12 3
– – – 3 3 5 – 5 6 7 – 2 5 14 30 – 9 29
– – – -1 -1 -1 – 1 1 1 – 7 9 7 99 – 21 17
– – – 0 0 0 – 0 0 0 – 3 2 -1 40 – 6 1
– – – 3 3 6 – 7 8 9 – 25 29 30 348 – 69 66
100 22 – 13 21 45 0 11 18 41 – -1 1 13 4 – 2 27
0 30 – 0 0 -1 – -1 -1 -1 – -1 2 7 7 – 3 12
0 6 – 0 0 0 – 0 0 0 – 1 2 -1 5 – 2 -1
300 200 1 46 70 145 6 43 68 141 49 54 64 123 66 97 124 245
Only the data corresponding to the solutions prepared with the nitrate salt ŽTable 2. are presented. No significant differences were observed with the perchlorate salt data. Calculated in the molar scale.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
x Total wUO 22q x wŽUO 2 . 2 ŽOH. 22q x wŽUO 2 . 3 ŽOH.q x I Solution Experimental PSEQUAD: wAcOHx wAcOy x wUO 2 AcOq x wUO 2 ŽAcO. 2 x wUO 2 ŽAcO.y 3 5 pH of the ylogwHq x b complexed solutiona AcO
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
resolution of about 2.5 cmy1 has been obtained. The 514.53 nm line of a Spectra Physics 2017 Stabilite´ model, argon ion laser was used for excitation at low pH Ž- 3.5.. In cases of luminescence ŽpH ) 3.5., the Raman effect was excited with a HerNe laser at 632.8 nm with a total power of 35 mW. Wavenumbers in vacuum were calibrated by using the mercury line at 18 312.7 cmy1 . The wavenumber accuracy depends much on the band profile and intensity. It is estimated between 1 and 4 cmy1 . 2.5. Peak decomposition of Raman and infrared spectra Spectra were converted into ASCII files and read by the Microcal Origin Peak Fitting Module for Windows using the Leven–Marquardt non-linear least-square curves fitter for the determination of each band parameters. Mixtures of Lorentzian and Gaussian lines were used since they gave better results than pure Lorentzians. 2.6. PSEQUAD PSEQUAD w16x is a FORTRAN program used to study equilibrium systems in solutions. Experiments are carried out as titrations: the concentration of one of the components is changing step by step. PSEQUAD allows the evaluation of titration curves and all the concentrations can be calculated for a given sample knowing the total concentrations of components in solution and the formation quotients w15x. For a given solution of acetic acid and NaOH, the ‘titration’ at constant volume was made by addition of aliquots of a virtual solution of uranyl at the concentration 1000 M. The calculation of equilibrium concentrations have been made in the hypothesis of homogeneous solutions by using the formation constants summarized in Table 1. For every solution, a starting ionic strength I was assumed in order to calculate starting values of the concentration quotients b j . The model of activity coefficients selected for this study was the Davies one Žlimited to ionic strength - 0.5. w19x. The more precise ionic strength deduced from the first program run allowed a refinement of the quotients b j . Convergence was quickly attained on equilibrium concentrations through this iterative process. Results are presented in Table 3.
65
3. Results and discussion 3.1. The uranyl cation and its hydroxides For the study of acetate complexed with uranyl, it was first necessary to assign the uranyl hydroxides bands in the Raman and IR-ATR spectra of the examined aqueous solutions. The symmetric Ž nsŽUO 2 .. and antisymmetric Ž naŽUO 2 .. stretching wavenumbers of the hydrated uranyl ion Žnoted Ž1,0.. in aqueous solution at very low pH are 870 and 962 cmy1 , respectively Žspectra not shown. w6,11,13,20x. Among many papers dealing with the Raman spectra of uranyl cation in aqueous solutions w6–13x, only few of them take into account the hydroxides derivatives. Toth and Begun w11x have studied solutions of 0.1 M uranyl nitrate at different pH values and assigned two new bands at 851 and 836 cmy1 , appearing with the pH increase, to ŽUO 2 . 2 ŽOH. 2q 2 Žnoted Ž2,2.. and ŽUO 2 . 3 ŽOH.q Žcalled Ž3,5.., re5 spectively. We have also prepared solutions of 0.1 M uranyl nitrate at pH 1.0 and 4.0 and both the Raman and IR-ATR spectra of these solutions were obtained ŽFig. 1.. The Raman spectra obtained by Toth and Begun were reproduced ŽFig. 1A.. Free hydrated uranyl and the two hydroxides previously cited are present Žsolution B, Table 3.. The comparison of the measured areas of the uranyl scattering in the range 780–880 cmy1 and the nitrate Raman signal in the range 975–1100 cmy1 for 0.1 M nitrate uranyl aqueous solutions at different pH shows that the molar scattering coefficient of nsŽUO 2 . does not change with hydroxide complexation. The value 2q x x w J s I w nsŽUO 2 .xrI w nsŽNO 3 .x = wNOy s 2.65 3 r UO 2 0.15 is found. So, it is possible to compare directly the intensity of the bands with the concentration of hydroxides in solution. Three unresolved bands can be seen on both infrared and Raman spectra. As a first approach, we have decomposed these bands and found three couples: 870; 9624 , 853; 9434 and 835; 9234 cmy1 , for nsŽUO 2 .; naŽUO 2 .4 of Ž1,0., Ž2,2. and Ž3,5. species, respectively ŽTable 4.. These assignments will be used as a first approximation. A more precise study of uranyl hydroxides is in progress to better understand the spectra of such multinuclear complexes.
66
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
Fig. 1. Raman ŽA. and IR-ATR ŽB. spectra of a 0.1 M solution of uranyl nitrate at pH 4.0 Žsolution B.. Calculated individual lines are also indicated.
3.2. The complexation of uranyl with acetate It is generally observed that the UO force constants of uranyl, and consequently both the nsŽUO 2 . and naŽUO 2 . wavenumbers, decrease while the ligand number in the equatorial plane of uranyl increase. Thus the uranyl spectra in the ranges 900–980 cmy1 Ž naŽUO 2 . observed in infrared. and 800–880 cmy1 Ž nsŽUO 2 . in Raman. are mainly used to define the complex stoichiometries, as previously shown by Nguyen-Trung et al. w7x. Information on the ligand structures are rather available in the ligand spectra,
mainly through the COOy stretching modes Žbetween about 1300–1600 cmy1 . and the n ŽCC. vibration Ž880–960 cmy1 . w15x. Although the n ŽCC. and naŽUO 2 . wavenumbers overlap around 930 cmy1 , there is no significant interference between their infrared absorptions because the molar absorption coefficient is about 30 times larger for naŽUO 2 . at 962 cmy1 than for n ŽCC. at 928 cmy1 , and the highest concentration ratio acetateruranyl used is about 3 ŽTable 2.. Thus the infrared spectrum between 880 and 980 cmy1 is essentially related to the naŽUO 2 . mode.
n ŽC5O.
naŽCOOy .
nsŽCOOy .
d ŽCH 3 .
d ip ŽOH.
n ŽC–O. with d ip ŽCOH.
ATR-IR Raman ATR-IR Raman ATR-IR
1712 1710 – – –
– – 1553 1551 1595
– – 1415 1416 1389 Žcoupled.
1369 1368 1348 1347 1339 Žcoupled.
1390 1390 – – –
1280
Raman ATR-IR
– –
1527
1467
– –
– –
Raman
– – – – – – –
– – – – – –
– – – – – –
– – – – – – –
– – – – – – –
Assignment AcOH AcOy Pseudobridging uranyl acetate Bidentate uranyl acetate ŽAcO.UOq 2 ŽAcO. 2UO 2 ŽAcO. 3UO 2 UO 22q ŽUO 2 . 2 ŽOH. 2q 2 ŽUO 2 . 3 ŽOH.q 5
– – – – – –
naŽUO 22q .
nsŽUO 22q .
– – – – –
– – – – –
943
– –
– –
952 – – – – – –
– 954 928
– 861 841 823 870 853 835
n ŽC–C.
892 – – –
928
962 943 923
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
Table 4 Principal wavenumbers Žcmy1 . and assignments modes of acetic acid, sodium acetate, uranyl acetates and uranyl hydroxides in aqueous solutions between 1800 and 400 cmy1 . The notations n , d and r are used for the stretching, the bending and rocking modes
67
68
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
Nitrate is not a good counter anion for the study of infrared spectra of the complexed uranyl acetate species since large and intense bands occur between 1200 and 1500 cmy1 overlapping with acetate modes. That is why we have synthesized uranyl perchlorate that has only one stretching band at 1106 cmy1 . The Raman spectrum of a solution of 0.163 g uranyl perchlorate in 5 ml water ŽpH s 3.1. shows the same features between 800 and 900 cmy1 than those observed for the nitrate salt. We have also verified that no residual nitrate stay in the solution by the absence of a band at 1047 cmy1 Ž nsŽNO 3 ... The only observable band was the symmetric stretching of perchlorate at 934 cmy1 . The results of the titration of the synthesized salt by ICP-AES allow the calculation of a mean molecular weight of 535.4 grmol that corresponds to a hydrated state of 3.7 water molecule per UO 2 ŽClO4 . 2 molecule. The hydration of the salt was not controlled strictly and the crystallised solid shows two mixed colors Žyellow and orange.. This mean molecular weight was used to determine all the concentrations of uranyl in the solutions studied by IR-ATR spectroscopy ŽTable 2.. 3.2.1. Uranyl signals Figs. 2–4 show the symmetric and antisymmetric stretching vibrations of the uranyl group in the ATR-
IR and Raman spectra. In the case of the complexation of acetate with uranyl at low pH, the complexes are in too small concentrations Žsolutions Ia–c, Table 3. to see modifications of the n ŽUO 2 . profiles, except a broadening of the line in the lower frequencies in each spectrum ŽFig. 2A.. The case of the complexation at initial pH 3.9 Žsolutions IIa–c, Table 3. is more interesting since there is no contribution of Ž3,5. and UO 2 ŽAcO.y 3 compounds and very little of Ž2,2. and UO 2 ŽAcO. 2 . The subtraction of the Ž1,0. and Ž2,2. contributions leads to the apparition of an asymmetric peak at 861 cmy1 in the Raman spectra with an intensity increasing with the total uranyl concentration Žfrom IIa to IIc., when compared to n ŽCC. of acetic acid that is approximately constant in concentration ŽTable 3, Fig. 3.. This line is assigned to UO 2 AcOq which is the main acetate complex in the medium. With the decrease of the ratio wUO 2 AcOqx rwUO 2 ŽAcO. 2 x from IIc to IIa ŽTable 3., the asymmetry of the band at 861 cmy1 toward low wavenumber is bigger Žsolution IIa.. A similar procedure of elimination of the signals of Ž1,0. and Ž2,2. species from the IR-ATR spectra of Fig. 2A, leads to the apparition of two bands at 954 cmy1 and the other at 928 cmy1 ŽFig. 2B.. By analogy with the observations made on the equivalent Raman spectra, the first line is assigned to
Fig. 2. IR-ATR spectra of uranyl perchlorate solutions defined in Table 2. ŽA. total signal; ŽB. spectra obtained after subtraction of hydrated UO 22q and ŽUO 2 . 2 ŽOH. 2q from solutions IIa–c. 2
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
69
Fig. 3. Raman spectra Žnormalized on the AcOH band at 892 cmy1 . of uranyl nitrate solutions IIa–c defined in Table 2. Dashed spectra are obtained by subtraction of the contribution of Ž1,0. and Ž2,2. uranyl species.
the UO 2 AcOq complex and the second one to UO 2 ŽAcO. 2 , all the more as the first one increases from IIa to IIc as the metal-to-ligand ratio increases. At still higher pH values Žsolutions IIIa–c, Fig. 4., the above defined mean Raman scattering efficiency J of the nsŽUO 2 . mode increases 2.78 for solution IIIc to 3.56 for solution IIIa, while the fraction of uranyl complexed to acetate increases ŽTable 3.. The molar scattering of uranyl acetates are then higher than for hydroxides. In solution IIIc, the concentrations of uranyl in Ž1,0., UO 2 AcOq and Ž2,2. species are about the same and equal to approximately twice that of UO 2 ŽAcO. 2 , while compounds Ž3,5. and UO 2 ŽAcO.y Ž . 3 are negligible Table 3 . The decomposition of the spectrum shows three strong bands at 870, 861 and 853 cmy1 , assigned respec-
tively to Ž1,0., UO 2 AcOq and Ž2,2., and an additional line at 841 cmy1 assigned to UO 2 ŽAcO. 2 . This peak increases when wUO 2 ŽAcO. 2 x increases in IIIb and stays important in IIIa. This assignment is not in accordance with the one proposed by Nguyen-Trung et al. w7x at 861 cmy1 for UO 2 ŽAcO. 2 , but better agrees with the shift already observed in IR-ATR spectra: y8 cmy1 for UO 2 AcOq and then an additional y26 shift cmy1 for UO 2 ŽAcO. 2 ŽFig. 2B, Table 5.. Moreover the relative intensities well agree with the concentrations calculated with PSEQUAD ŽFig. 4.. Big differences in the structures of UO 2 AcOq and UO 2 ŽAcO. 2 are anticipated from these different shifts for the uranyl signals. ŽCH 3 COO. 3UOy 2 is generally low in concentration, even in solutions IIIb and IIIa Ž2 and 3 mM.. Fig. 4
70
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
tainty in the IR-ATR spectra to assign an additional component to this last complex. Nguyen-Trung et al. w7x found that the Raman shifts of the acetate complexes follow the relation: n s 870 y 9n where n is the average ligand number. This supposes that each added ligand leads to the same perturbation on the uranyl group and then it could follow that all the ligands have the same configuration. As we will discuss next paragraph, two structures coexist in the solutions analysed. This fact is in line with the result previously found for copperŽII. acetate w15x, i.e., that different structures can occur as a function of ligand number.
Fig. 4. Raman spectra of uranyl nitrate solutions IIIa–c defined in Table 2, with different intensity scales. Decomposed lines are indicated. For comparison, the intensities of stick spectra Ždashed lines are for uranyl hydroxides. represent the corresponding relative concentrations calculated by PSEQUAD, without taking into account the increase of the nsŽUO 2 . molar scattering ŽTable 3..
shows a shoulder at 823 cmy1 for spectra of solutions IIIa and IIIb. Even though the concentration of the product is weak, this line is assigned to UO 2 ŽAcO.y 3 complex. This makes a new red shift of 18 cmy1 consequently to the addition of a third ligand. Unfortunately, there was too much uncer-
3.2.2. Acetate signals Fig. 5 compares the ATR-IR spectra of the free acetate anion Žsodium acetate in solution III. to those of complexed species in the region of the carboxylate stretchings. These last spectra have been obtained after subtraction of the acetic acid and ‘free’ acetate contributions. As expected by the examination of PSEQUAD calculations, the higher the initial pH is, from solutions of type I to III, the more concentrated the complexes are. As it was already observed about copper acetate w15x, the complexation constant is not strong enough to make directly the reaction of uranyl with acetic acid with the release of one proton. Four bands appear in the region 1580–1200 cmy1 , at 1527, 1467, 1389 and 1339 cmy1 . The region above 1580 cmy1 is not well resolved because of the non perfect elimination of the water bending absorption near 1640 cmy1 and because solute bands have less than 0.01 ‘absorbance’ unit. The couples 1527, 14674 and 1389, 13394 cmy1 vary parallely as a function
Table 5 Comparison of the area ratios observed for pseudobridging and bidentate n ŽCC. Ž A 943 r A 951 . with possible structure concentration ratios calculated with PSEQUAD Ž y1 r y 2 . Žfrom solutions IIIa–c and IV.. UO 2 AcOq is supposed pseudobridging Solution
A 943 r A 951
y1 r y 2a
y 1 r y 2b
y1 r y 2c
IIIa IIIb IIIc IV
0.7 1.1 3.1 0.9
0.9 1.2 2.9 0.9
0.6 1.0 2.5 0.6
0.1 0.2 0.9 0.1
a,b,c
See Section 3.2.3.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
71
Fig. 5. IR-ATR spectra of the stretching region of acetate complexed forms with uranyl from solutions defined in Table 2 Žall these spectra, except III, are obtained after subtraction of free acetic acid and free acetate spectra..
of the initial quantity of uranyl perchlorate added: the higher this amount is, from solutions of type a to c, the more the second couple grows in intensity.
The first couple of frequencies with a difference D s 60 cmy1 is assigned to a bidentate complex w21x, notably when the proportion of complexes
Fig. 6. IR-ATR spectra of solution IV ŽTable 2.. ŽA. After subtraction of water; ŽB. after additional subtraction of free acetate and acetic acid spectra.
72
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
UO 2 ŽAcO. 2 and UO 2 ŽAcO.y 3 are preponderant versus the monoliganded one ŽTable 3.. Kakihana et al. w14x have already shown the possibility of a bidentate structure for the acetate–uranyl complex in aqueous medium with naŽCO 2 . at 1538 cmy1 and nsŽCO 2 . at 1466 cmy1 . The two other lines are assigned to a monodentate or, more probably in aqueous solution, a pseudobridging form w22x. The nsŽCO 2 . splitting into two strong components at 1389 cmy1 and 1339 cmy1 is assigned to a coupling with dsŽCH 3 ., as previously demonstrated through the substitution of CH 3 to CD 3 in mercuryŽII. acetate complexes w23x. The phenomenon is observed each time the nsŽCO 2 . mode undergoes a large decrease by acetate complexation. The possibility of the existence of two structures for the uranyl acetate was also observed by Gal ´ et al. w13x in the spectra of a saturated aqueous solution of UO 2 ŽAcO. 2 . They assigned the lines at 1538 and 1468 cmy1 to naŽCO 2 . and nsŽCO 2 ., respectively, for the bidentate acetate form and two bands, less intense, at 1603 and 1390 cmy1 to the monodentate acetate structure. Fig. 6 displays spectra from a 0.2 M aqueous solution of UO 2 ŽAcO. 2 after subtraction of the acetic acid and ‘free’ acetate spectra. The four lines previously observed are present. An other weak one also appears at 1595 cmy1 that can be assigned to the antisymmetric stretching of the hydrated unidentate complex structure ŽTable 4.. Figs. 5 and 6 display a striking decrease in the intensity ratio I w naŽCO 2 .xrw nsŽCO 2 .x from the free acetate ion to the bidentate structure. This fact is probably related to a decrease of OCO angle, and as a result a decrease of the angle F between the transitions moments corresponding to the stretching of the two bonds, since: I w naŽCO 2 .xrI w nsŽCO 2 .x s tan2 ŽFr2.. The n ŽCC. band, intense in the Raman spectra, has been shown to be very sensitive to the complexation of acetate in aqueous solution w5,15,24,25x. Notably, we have suggested the following assignment in the case of the complexation of copperŽII.: the line at 928 cmy1 is related to ‘free’ sodium acetate, another one at 938 cmy1 was assigned to the stretching vibration of the pseudobridging form and a third one at 948 cmy1 to the bidentate form. Here, we have to be careful because the antisymmetric stretching of the free hydrated uranyl and its complex derivatives have their wavenumbers close to the one of acetate
C–C stretching. It was then important to be sure that the antisymmetric stretching, forbidden in Raman scattering, was not ‘activated’ by complexation, even though no activation was observed for the hydroxide derivatives ŽFig. 1A.. Fig. 7 shows the Raman spectra of the n ŽCC. stretching mode of solutions V and Vb–c ŽTable 3. made with trideuterated acetate at initial pH 6.1 complexed by uranyl ŽTable 2.. The ‘free’ trideuterated acetate n ŽCC. band is shifted from 928 for acetate to 884 cmy1 Žsolution V.. With addition of the uranyl cation, two new lines appear at 901 and 910 cmy1 corresponding by analogy with acetate to the n ŽCC. of the pseudobridging and the bidentate structures of the complexes, respectively. No scattering band is observed in the region 920–980 cmy1 ŽFig. 7.. This makes evidence that naŽUO 2 . signal is not enhanced by complexation and does not interfere with n ŽCC.. If we except the line at 928 cmy1 , traducing the presence of ‘free’ acetate, the two lines at 943 and 952 cmy1 , observed in Fig. 8, are assigned to the pseudobridging and the bidentate forms, respectively. This is confirmed by the evolution of the intensities with the quantity of uranyl added: the line at 952 cmy1 is the most intense at the same time that both bands at 1527 cmy1 and 1467 cmy1 in the ATR-IR spectrum, that is when the metal-to-ligand ratio is 1r3 ŽFigs. 5 and 8.. On the contrary, the line at 943 cmy1 is the biggest when this ratio is 1r1, parallely to the lines at 1389 and 1339 cmy1 . Similar conclusions can be drawn for the n ŽCC. of trideuterated acetate since the evolution of the different peaks are the same as a function of pH and cation-to-ligand ratio. The principal assignments are summarized in Table 4. 3.2.3. Structurer stoichiometry relation We now try to establish a relation between the structure and the stoichiometry of the complexes. We note:
° x s UO AcO ~ x s UO Ž AcO. ¢x s UO Ž AcO.
q
1
2
2
2
2
3
2
3
and
½
y 1 s w pseudobridgingx y 2 s w bidentatex
we have: x 1 q 2 x 2 q 3 x 3 s wtotal complexed acetatex s y 1 q y 2 ŽTable 3.. The n ŽCC. region of the Raman spectra of solutions were decomposed in
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
73
Fig. 7. Raman spectra in the n ŽCC. range of deuterated acetate complexed with uranyl Žsolutions V and Vb–c, Table 2..
individual components ŽFig. 8. and the areas were measured. The scattering molar coefficients of every component were difficult to calculate because there is some overlap between the n ŽCC. component of AcOH at 892 cmy1 and the strong uranyl signal. By assuming a constant relative differential Raman scattering cross-section for the n ŽCC. modes of all complexed species, the peak areas are proportional to the concentration of the related structure. The calculated concentrations by PSEQUAD for the stoichio-
metric complexes were used and combined in order to calculate the concentrations of the bidentate and the pseudobridging forms. UO 2 AcOq is assumed pseudobridging because the line at 943 cmy1 is intense when the complex UO 2 AcOq is in high quantity. Three further hypotheses are proposed: Ža. one ligand of UO 2 ŽAcO. 2 and of UO 2 ŽAcO.y 3 is pseudobridging, the others being bidentate; Žb. UO 2 ŽAcO. 2 is like in Ža. and UO 2 ŽAcO.y 3 is entirely bidentate; Žc. all acetates in UO 2 ŽAcO. 2 and
Fig. 8. Quantitative analysis of the n ŽCC. range in the Raman spectra of species AcO, either free or complexed with uranyl.
F. Quiles, ` A. Burneaur Vibrational Spectroscopy 18 (1998) 61–75
74
UO 2 ŽAcO.y 3 are bidentate. We can then calculate y 1 and y 2 as:
Ž a. Ž b. Ž c.
½ ½ ½
y1 s x 1 q x 2 q x 3 y2 s x 2 q 2 x 3 y1 s x 1 q x 2 y2 s x 2 q 3 x 3 y1 s x 1 y2 s 2 x 2 q 3 x 3
and compare the ratio y 1ry 2 to the one obtained by areas measurements A 943rA 951 , where A 943 and A 951 are the measured areas of the pseudobridging acetate at 943 cmy1 and of the bidentate form at 952 cmy1 , respectively. The results for the three proposed configurations are given in Table 5. It is concluded that both hypotheses Ža. and Žb. are possible. Anyway, there is little doubt that one acetate ligand of UO 2 ŽAcO. 2 has a pseudobridging structure, the other one being in the bidentate structure. This conclusion does not agree with the one of Kakihana et al. w14x who state a unique bidentate structure for UO 2 ŽAcO. 2 . But if we have a precise look to their spectra, they have neglected Ž1. two weak bands near 1390 and 1330 cmy1 on their IR spectra Ž2. the asymmetry of the COO peak near 190 ppm in their 13 C NMR spectra. The short repetition time makes this last peak analysis difficult. For UO 2 ŽAcO.y 3, there is still some uncertainty. But it is quite possible that the solvated complex does not display three bidentate acetates as it was shown for solid NaUO 2 ŽCH 3 COO. 3 by X-Ray w26x and EXAFS w27x. The hypothesis of different structures in the same complex was already perceptible regarding the uranyl signals. The pseudobridging form leads to the release of only one water molecule Žand an 8 cmy1 red shift. which could be thermodynamically favourable for the formation of the complex from free hydrated uranyl cation. After that, by steric considerations, the bidentate structure could be better, leads to the release of two water molecules and it is traduced by a mean red shift of 22 cmy1 . 4. Conclusions In this work, we have studied the interactions of uranyl with the hydroxide and acetate molecules
their probable structures in diluted aqueous solutions. We have shown that IR and Raman spectroscopies are sensitive methods to characterize different structures of dioxouraniumŽVI. complexes in aqueous diluted solutions. For example they allow us to show that for the same complex the ligation could be realised with different structural modes. It appears that for UO 22q as for Cu2q w15x, the first complexation step in water involves a pseudobridging structure of acetate, rather than a bidentate complexation. This fact suggests that this step keeps a hydration number of the cation as high as possible. Further research is needed to generalise this trend which is governed by both energy and entropy contributions.
Acknowledgements The authors are grateful to the Laboratoire de Geologie et de Gestion des Ressources Minerales et ´ ´ Energetiques, Nancy, France for the gracious furni´ ture of the synthesized UO 3 , 2 H 2 O4 .
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