J. Phys. Chem. 1995,99, 10181-10185
10181
Ab Initio Quantum Chemical Calculations on Uranyl UOz2+,Plutonyl PuO~~',and Their Nitrates and Sulfates J. Simon Craw, Mark A. Vincent, and Ian H. Hillier" Department of Chemistry, University of Manchester, Manchester MI3 9PL, United Kingdom
Andrew L. Wallwork BNFL Sellafeld Research and Development, Seascale, Cumbria CA20 1PG, United Kingdom Received: January 20, 1995; In Final Fonn: March 28, 1995@
A b initio molecular orbital calculations on the structure and stability of the nitrate and sulfate complexes of + ) effective core potentials are reported. It is found that the binding uranyl (UOz2+)and plutonyl ( P u O ~ ~ using energy of sulfate is greater than that of nitrate to both uranyl and plutonyl, with a slight preference for plutonyl. A method of decomposing the binding energy into electrostatic, Pauli repulsion, polarization, and chargetransfer components is described which predicts that electrostatic forces are dominant. A simple molecular mechanics potential is developed by using this finding, which is successful in reproducing the a b initio results.
I. Introduction Uranium and plutonium are the two most important elements in nuclear power generation, and their separation from other fission products, and each other, in irradiated fuel, is one of the nuclear industry's most important processes. Although there exists an abundance of empirical data concerning the chemistry of these two elements, there has been very little quantitative theoretical work on their coordination chemistry. We report here the results of ab initio molecular orbital calculations on some uranium and plutonium complexes. We first discuss the dioxides of uranium and plutonium, U02*+ and PuOz2+, the actinyls, which are extremely stable entities and form the basis for many coordination complexes. We investigate the electronic structure of these molecules using a variety of ab initio procedures, with the primary aim of understanding the gross features of their bonding and charge distribution. We then report calculations on the nitrate and sulfate complexes of U022+ and Pu0z2+. These complexes are of particular interest to the nuclear industry, as most actinide separation is achieved through solvent extraction, in which the aqueous phase is nitric acid.' Clearly, the ability to predict the strength and nature of the binding of a particular ligand to an actinyl is of great importance and has the potential to eliminate costly experiments on toxic compounds. Actinide complexes present formidable problems for quantum chemistry. The elements are large with high nuclear charge so that they must be treated in a relativistic manner. Actinides exhibit a wide range of oxidation states, a result of their 5f valence electrons being more weakly bound than the 4f electrons of the lanthanides. Uranyl has been the subject of several previous theoretical including relativistic extended Hiicke14 and relativistic (all electron) Hartree-Fock-Slater calc~lations.~ A study of relativistic (mass-velocity and Danvin) effects on geometry has shown that slightly longer bond lengths are predicted compared with nonrelativistic calculations. This has been ascribed to the semicore nature of the 6p shelL5 There are no previous ab initio calculations on plutonyl, although Wadt6 has reported calculations on h F 6 and noted that the nominal 5fz occupation of Pu6+gives rise to a multitude of lowlying excited states. @
Abstract published in Advance ACS Absrructs, June 1, 1995.
0022-365419512099-10181$09.00/0
Experimental crystal data have been reported for uranium nitrate and ~ u l f a t e . ~ - 'Although ~ structures with various numbers of water molecules have been reported'' we here chose to study the basic unit common to all of these, uranyl nitrate with two nitrate groups and two closely bound water molecules, depicted in Figure 1 (Ac = U). The overall symmetry of the complex is D2h; Le., the two nitrate ions and two water molecules lie in the same plane. X-ray diffraction studies of uranyl sulfate indicate that the structure is polymeric with two sulfate ions forming bridges between two uranyl ions, each of which also has three closely bound water molecule^.^^^^ In this paper we model uranyl sulfate containing three water molecules and one sulfate ion bound to uranyl in a bidentate manner (Figure 2, Ac = U), yielding a C2" structure. In all cases overall electrical neutrality is preserved. We choose to model the P u 0 ~ complexes ~+ using the same basic geometry as the U022+ complexes. 11. Computational Methods Of major importance when performing calculations on actinide systems is the question of how to treat the "core" electrons, i.e., the inner shell electrons that play no role in the bonding. A recently published review discusses this point with specific reference to actinide complexes.I2 In this paper we adopt the conventional approach of replacing the core with an effective one-electron pseudopotential (ECP) optimized to reproduce the orbital energies obtained from relativistic calculations on atomic states. The valence electrons of the actinide and all the electrons associated with other ligands are treated by using conventional Hartree-Fock (HF) and post-HartreeFock methods. We use the relativistic ECP of Hay to replace the core, viz; [Xe]4f45dI0, electrons of the actinide, in conjunction with a [3s3p2d2fl contracted Gaussian basis set, to describe the valence e1ectr0ns.I~ A double-l; basis (DZ) set is used for H, N, O,I4 and S,I5with one set of 3d functions (&,j= 0.6) on the sulfur. These basis sets were primarily chosen for their combination of flexibility and compactness. Only the spherical harmonic d and f functions are used. Many systems containing d or f block elements possess lowlying excited states.I6 Indeed Wadt6 has already noted that this is the case in due to the partial occupancy of the 5f shell. 0 1995 American Chemical Society
10182 J. Phys. Chem., Vol. 99, No. 25, 1995
Craw et al. Bagus's starts from the same antisymmetrized wavefunction, but suffers from the charge transfer and polarization energies depending on the order in which the various orbital rotations are performed.** In this paper the binding energy AEO is decomposed into the following additive components
+
+
AEB = EEL ERE' I?'
L
Figure 1. Schematic representation of AcOz(N0;)2(Hz0)2.
y
H Figure 2. Schematic representation of AcOz(SO4)(HzO);.
For PuO>?+there are two electrons which must be distributed among the seven molecular orbitals arising from the 5f atomic orbitals. Of these, the 6, orbitals, which correlate with thef,,, and j&-,?) atomic functions are the lowest lying in D-h. Occupancy of these orbitals with two electrons lead to 'Z,+, 3C,-, and Ir, states. To study these states CASSCF calculations were carried out involving 12 or 14 active electrons in 10 or 11 orbitals, for U0z2+ and PuO~", respectively. The active orbitals included the 40,,, In,,2n,, and 40, molecular orbitals of mainly oxygen 2p character and the 6, orbitals. We denote these calculations as CASSCF(m,n), where m and n refer to the number of active electrons and the number of orbitals, respectively. To assess the likely impact of dynamic electron correlation, post-HF calculations are also performed. The first and simplest method is second-order Moller-Plesset perturbation theory (MP2), which we apply to the I C,' state of U022f. The second method is configuration interaction with single and double excitations (CISD). Lastly, we use density functional theory," again only for U022+, employing the Becke exchange functional'* and the correlation functional of Lee, Yang, and ParrI9 (B-LYP). The charge distribution is interpreted in terms of Mulliken population analysis" and atomic charges q M E P designed to reproduce the molecular electrostatic potential (MEP).*' The charge distribution calculations are based on the ground-state HF wavefunctions. One of the major aims of the paper is to understand the nature of the binding between the actinyl and the nitrate or sulfate ligand. The total gas-phase binding energy, AEB,is defined as the energy of the following reaction
where A represent the actinyl and B the rest of the complex. In order to obtain a better understanding of the binding mechanism, we decompose AEB into various additive components. There are several schemes in the literature to accomplish this; the method of Morokuma22-24 starts with the antisymmetrized product of the fragment wavefunctions and selectively blocks out various elements of the Fock matrix. It has been arguedZ5 that this method overestimates the electrostatic interaction and underestimates the charge transfer, due to the orthogonalization of fragments A's occupied orbitals to fragment B's virtual orbitals.
+ c,",+
(2) L
where L denotes either A or B. EEL is the electrostatic component of the binding energy, EREPthe Pauli (exchange) repulsion energy due the overlap of occupied molecular orbitals on A with occupied molecular orbitals on B, .lFTthe chargetransfer energy, i.e., the mixing of occupied orbitals on A with virtuals on B and vice versa, EPoL the polarization, and EDEF the deformation energies. Binding energies calculated with a finite basis contain a basis set superposition error (BSSE), Le., the (artificial) energy lowering of the dimer's energy due to the individual monomers being able to use each other's basis functions. The magnitude of the BSSE is estimated by using the counterpoise correction procedure of Boys and BemardLz9 All calculations were performed by using the Gaussian 9230 program, running on an IBM 6000/590 of the Manchester Computing Centre.
111. Results A. Uranyl and Plutonyl. The relative ordering of the 40, and 40, in uranyl has been the subject of some discussion.I2 Most empirical method^^,^' place the 40, above the 40, and argue that this is due to the effect of the 6p shell. DeKock et aL3>also note that the 40, may be raised above the 40, by way of mixing with the 5f shell. It is known that relativistic calculations place the 5f shell at a higher energy compared to nonrelativistic calculation^,^ and it is suggested by DeKock et ~ 1 that. this ~ may ~ be the reason for the 40, lying above the 4u,. Our calculations do little to resolve this debate and place the 40, above the 40, for uranyl, although we note that the orbital energies are close to each other (eigenvalues of - 1.052 Eh and - 1.085 Eh), so the relative ordering may be basis-set or ECP dependent. On going from LQ2+ to P u O ~ ~a pair + of electrons are put into a degenerate pair of d,, orbitals constructed from the f atomic orbitals (fn, and fZ+,2j). CAS(2,2) calculations (in a Slater determinant basis), i.e., two electrons in the two 6, orbitals, which correspond to two configuration HF calculations for the singlet states, predict the 3Es- state to be of lowest energy followed by the 'rgstate and the (Table 1). The larger CAS( 14,ll) and the CISD calculations maintain this ordering, although with a decreased energy between the 3Z,- and the ITg. It is interesting to speculate as to whether this is the actual ordering observed experimentally, or merely a result of this particular combination of basis set and ECP. As the size of the nitrate and sulfate complexes precludes the use of a larger more flexible one-particle basis set, it is appropriate to examine whether increasing the size and quality of the basis set has an effect on the geometry on these systems. Calculations on U022+ at the HF level using the larger TZP basis of Dunning33on oxygen has essentially no effect on the geometry, nor does addition of a set of optimized 5g functions ( < 5 g = 0.456) to U. The DZ basis set on 0, in combination with the ECPhalence basis of Hay on the actinide is thus capable of reproducing the results of more extensive basis sets at considerably less computational effort.
Ab Initio Quantum Chemical Calculations
J. Phys. Chem., Vol. 99, No. 25, 1995 10183
TABLE 1: Energies, Actinide Oxygen Bond Lengths (R) and Vibrational Frequencies of Uranyl and Plutonyl (Eh, and cm-9 HF CASSCFa MP2 CISDb B-LYP
A,
UOz2+('Zg+) E -199.638 12 -199.668 21 -200.313 21 R 1.663 1.663 1.783 V I 1183 933 vz 264 154 v3 1254 962
-200.163 63 -201.255 30 1.700 1.779 1082 218 1157
v2 ~3
h022+('$+)
-219.436 67 1.659
For U0z2+ CASSCF(12,10), h 0 z 2 + CASSCF(14,ll) see text for details. bThe CISD calculations on Pu022+ omit the lowest eight occupied and highest eight virtual orbitals.
TABLE 2: Mulliken Orbital Populations and Atomic Charges of Uranyl and Plutonyl (e) species
ns
np
nd
nf
2.1
5.8 4.2
1.5
2.6
5.8
1.5
0 3.9 Pu~~~+(~X,-)
h 0
2.1 3.9
4.1
9MuIl
OMEP
2.0
2.8 -0.4
0.0 4.7
2.0
0.0
Rq
1.72 (1.76) 3.04 (2.95) 2.49 (2.40) 1.31
R5
1.20
e2
112.5 (114.6) 115.0
R3
1.68 3.02 2.45 1.31 1.20 112.2 115.2
1.74 (1.75) 3.08 (3.68)c 2.64 (2.40) 1.55 1.44 94.7 112.9
1.70 3.04 2.47 1.55 1.44 95.2 111.7
See Figure 1. See Figure 2. Distance between Ac and a bridging
-219.456 71 1.659
U022+('E,+) U
R1 Rz
sulfur.
1288
E -219.138 31 -219.252 43 R 1.630 1.654 V I 1186 v2 347 ~3 1273
(1
el
PUO~~+(~$-) E -219.195 78 -219.298 14 -219.476 12 R 1.628 1.628 1.628
puoz2+(lr,) E -219.167 02 -219.267 61 R 1.630 1.654 VI 1164
TABLE 3: RHF Optimized Geometries for the Uranyl and Pluton 1 Nitrate and Sulfat Complexes, Experimental Values7-lo in Parentheses and ded nitraten sulfateb U Pu U h
2.1 -0.4
From Table 1 it can be seen that electron correlation (Lowdin's d e f i n i t i ~ n plays ~ ~ ) a minor role in determining the geometry of these molecules. Our results for U022+ compare favorably with those in the literature; for example, Wadt2reports a U-0 bond length of 1.63 8, optimized at the HF level. Wezenbeek's et al.5 nonrelativistic Hartree-Fock-Slater calculations gave 1.67 8, and a lengthening of 0.03 A, associated with relativistic effects. Pyykkij et ale3have investigated how different ECPibasis-set combinations effect the U-0 bond length at the HF level, obtaining values ranging from 1.66 to 1.72 A. The vibrational frequencies of U022+ are not significantly effected by electron correlation. Pyykko et al.3 also report harmonic vibrational frequencies for U022f calculated at the HF level, the agreement with our values in Table 1 being very good. When compared with experimental values of the symmetric stretch" V I , viz, 860 cm-' and 830 cm-' for U022+ and Pu0z2+,respectively, the calculated values would appear to be too large. However, the experimental values refer to measurements made on solid-phase samples and so may differ from the gas-phase values. The Mulliken population of each shell is given in Table 2. The bonding mechanism between oxygen and U or Pu is primarily via donation from the oxygen p orbitals into the (formally) empty d and f orbitals of U or the empty d and the partially filled f orbitals of Pu. The np shell occupancy of the oxygens is close to four, indicating that two electrons have been delocalized onto the actinide. Formally uranium VI+ is 6d05p and plutonium VI+ 6d05fz,and so the four 2p electrons (two from each 0) are associated with the d and f shells of the
actinide. As with previous calculations5 the 6p is essentially fully occupied, i.e., can be regarded as a semicore shell. The two extra f electrons in P u 0 2 2 + residing in 6, orbitals have no significant overlap with the oxygen orbitals. Interestingly, the Mulliken charges for oxygen in both cases are zero, which would argue for no quadrupole moment; however, although there are no experimentaldata concerning the quadrupole of these species, calculated values from the HF wavefunctions are -7.8 au and -6.7 au for U022+ and PuOz2+, respectively. Clearly the Mulliken population analysis is underestimating the actinide oxygen polarization. A more reliable method of obtaining atomic charges is via fitting to the MEP and gives values of qMEP (Table 2) which reproduce the quadrupole moment almost exactly. Calculated bond orders, using the method of Villar and D u p ~ i s of , ~2.4 ~ for both U0z2+ and Pu0z2+ confirm that the extra 5f electrons in Pu do not participate in the bonding. B. Structures of Nitrates and Sulfates. The geometries for these complexes have been optimized at the RHF level, assuming single-determinant closed-shell ground states, and within the particular symmetry constraint (Dzh or Czvfor nitrate and sulfate, respectively). To ascertain whether the DZh arrangement is the lowest energy structure for the nitrate we have performed calculations on U02(N03)2(H20)2 with the plane of the water molecules placed perpendicular to the plane of the nitrates. The energy of this structure was found to be 45.7 ldl mol higher than that of the planar structure. A vibrational frequency calculation on this nonplanar structure also produced two imaginary frequencies, the normal coordinates of which correspond to motions toward the planar structure. In Table 3 we report the bond lengths and angles along with the experimental values, where known. The agreement with experiment is generally good. The calculated actinyl bonds are generally shorter than experiment, particularly in the uranyl sulfate complex. However, in this complex the comparison is not strictly valid, as in the crystal structure the sulfate ions are monodentate and bridging, and so a longer U-S distance might be anticipated. The valence angle for water appears to be quite large in these complexes but in actual fact is a consequence of this basis set, the RHF/DZ value for gas-phase water being 112.5'. A comparison of the uranyl and plutonyl structures shows that there is very little difference between the two. As with the isolated actinyls the Ac-0 distances are slightly shorter in the plutonyl complexes, a result of the increase nuclear charge and the inability of the 5f electrons to shield this. Atomic charges derived from the MEP are given in Table 4,along with Mulliken values, for comparison. An interesting point is that the actinide charges are very similar to those of the isolated species (cf. Table 2). There is a considerable amount of charge transfer from the ligands to the Ac0z2+ moiety, about 0.6e in all four cases. This charge is mostly localized onto the oxygen
Craw et al.
10184 J. Phys. Chem., Vol. 99, No. 25, 1995 TABLE 4: Atomic Charges (QI\?EP) for the Uranyl and Plutonyl Nitrate and Sulfate Complexes, Mulliken Values in Parentheses (e) nitrate sulfate Ac 0 L"H?O
U
Pu
U
Pu
2.7 (1.9) -0.6 (-0.3) -0.8 (-0.7) 0.1 (0.1)
2.6 (1.8) -0.6 (-0.3) -0.8 (-0.7) 0.1 (0.1)
2.6 (1.9) -0.6 (-0.4) -1.5 (-1.4) 0.1 (0.1)
2.6 (1.8) -0.6 (-0.3) -1.6 (-1.4) 0.0 (0.1)
TABLE 5: Decomposition of the Total Bindin Energies (BB) into Electrostatic (EEL), Polarization (EpgL), Deformation (EDEF), Pauli Repulsion (EREP), Charge Transfer (ECT)Energies for the Uranyl and Plutonyl Nitrate and Sulfate Complexes, Counterpoise Corrections (E:' and Ef)9-ind CP Corrected Binding Enerp (A&), Enthalpies A W , and Gibbs Free Energies AQ9 (kJ/mol) nitrate EEL EPOL 4
E?L EDEF 4
EDEF B
EREP
AEB
ECT
Ec,p
GP AEB,P
AH?98 AG298
sulfate
the interaction energy EEL,we choose to use a simple scheme recently proposed by H ~ f m a n n In . ~ this ~ method each fragment is replaced in tum by point charges and the total energy (including the point charges) calculated. If the energy of A at the geometry found in the complex, interacting with point charges at B, is EL and the Coulombic self energy of these charges is EC,
(7) Then the electrostatic interaction energy of A with the point charges representing B is
E? = EL - E*, - Ec
(8)
The electrostatic component of the binding energy is taken to be the average of the two individual electrostatic energies,
U
Pu
U
Pu
-2209 -47 -213 25 424 50 -2278 -308 16 60 -2208 -2182 -2008
-2238 -47 -213 22 432 57 -2297 -310 19 60 -2219 -2192 -201 1
-2286 -69 -207 43 318 -16 -2594 -377 14 63 -2517 -2493 -2338
-2327 -72 -219 153 254 108 -2657 -554 17 64 -2516 -2546 -2382
atoms which are ca. 0.2e more negative in the complexes than in isolated Ac02'+. The Mulliken charges exhibit similar behavior, predicting a similar amount of charge transfer. The water molecules appear to play no part in the charge transfer and remain neutral overall. C. Actinyl-Ligand Binding Energy. The total gas-phase binding energies, representing the reaction
where Ln- represents either N03- or S042-, are given in Table 5. We divide the complexes into two fragments, A c O ~ ~and + (N03)2(H20)22- or (SOd)(H20)3*-, denoted A and B, respectively (cf. eq l), and decompose the interaction energy into the various components in eq 2. We start with the antisymmetrized product wavefunction for the complex formed from the two constituents wavefunctions,
(4) V: and YE are optimized closed-shell HF wavefunctions of A
and B, respectively. The energy of this wavefunction yields the zeroth-order energy,
where AB is the Hamiltonian for the whole system and accounts for the electrostatic and Pauli (exchange) repulsion energies between the two fragments,
where E*A is the energy of A at the geometry found in the complex. In order to estimate the electrostatic component of
EEL= '/*[E?
+E
3
(9)
The deformation energy is the energy difference between that of fragment A at the geometry found in the complex and the closed-shell HF optimized energy of A, Le.,
The polarization energy of A is the difference between the energy of A with the point charges at B allowing the wavefunction to polarize E: and the energy of A, Le.,
E F=
e,
E? - E*A
(11)
With a knowledge of from eq 5 and EELfrom eq 9, the repulsion between the two charge densities can be calculated. Implicit in this is the assumption that the electrostatic energies in eqs. 6 and 9 are equivalent, which may not be the case. The charge-transfer energy is, as with other decomposition methods, not well defined in this scheme. We calculate ECT as the difference between AEB and the sum of the other contributions. The results of this decomposition scheme are given in Table 5, using the qMEP point charges. The total binding energies are dominated by the electrostatic contribution, which is essentially due to the ionic nature of these complexes. The electrostatic contribution makes up a larger proportion of the total binding energy in the case of the nitrate complexes, although the total binding energies of the nitrates are less than those of the sulfates. The greater charge-transfer energy for the sulfate complexes also contribute to their increased binding energies. The deformation energy reflects the structural change in the NO3and S042- ligands on complexation. The geometries of N03and Sod2- in the complexes are given in Table 3, which can be compared to the optimized values of the isolated species, viz; RN0 = 1.27 A, RSO = 1.50 ti,e O N O = 120', and OoSo = 109.47'. Sulfate is more distorted than nitrate by the presence of the A c O ~ ~moiety, + leading to a larger deformation energy for this ligand. The repulsion term EREP is small in three cases and actually negative for UO,(SOd)(H20)3. This anomalous value probably arises from the electrostatic energies in eqs 6 and 9 being slightly different as already mentioned. However, of more importance is that the magnitude of EREp in all cases is very small. The values of EREPwould be expected to mirror ST, as the charge transfer essentially comes from the antisymmetrization of @AB, and indeed this is the case for the three complexes with positive values of EREP.The charge-transfer
Ab Initio Quantum Chemical Calculations
J. Phys. Chem., Vol. 99, No. 25, 1995 10185
TABLE 6: Quantum Mechanics (QM) and Molecular Mechanics (MM) Binding Energies and Geometries for the Uranyl and Plutonyl Nitrate and Sulfate Complexes (kJ/mol, A) nitrate AEB(QM) AEB(MM) RI(QM) R I( M M ) R2(QM) R2(MM)
sulfate
U
Pu
U
Pu
-2278 -1921 1.72
-2297 -1976 1.68 1.69 3.02 3.OO
-2594 -21 17 1.74 1.72 3.08 3.02
-2657 -2265 1.70 1.70 3.04 3.20
3.04 3 .OO
energies are also small compared with the overall binding energies, although they are only approximate because of the way in which they are calculated. Bearing this in mind, it does appear that sulfate is a better “donor” than nitrate and that the higher binding energy of sulfate is in part due to this. Counterpoise corrected values of the binding energy (AEEp) are also given in Table 5 , along with the individual and BSSE estimated in this way is about corrections 70 kJ/mol in the nitrate complexes and ca. 80 kJ/mol in the sulfate complexes. Most of the BSSE arises from the nitrate or sulfate, a not unexpected result in view of the problem of choosing appropriate basis sets for anionic species. The harmonic frequencies enable us to calculate the thermodynamic quantities @98 and AG298for the general reaction 3. D. Molecular Mechanics. We have used the fact that these complexes are essentially bonded ionically to develop a molecular mechanics model based on the MMz3’ program. In our model there are no covalent forces between the metal center and the ligands, the only interaction being the van der Waals and Coulomb terms. The parameters for the former, not present in MM2, were taken as rv =2.2 A, EU =0.04 k c d m o l and rp, =2.1 A, cpU =0.038 kcal/mol; the atomic charges needed for the Coulomb terms are qMEP. This model allows us to predict both the geometry and the binding energy of the complexes, which we have previously obtained using the full quantum mechanical treatment. The results of these two treatments are compared in Table 6, where there is encouraging agreement both in the structures and the trends in the binding energies. Clearly the molecular mechanics model performs well, correctly predicting that Pu binds more strongly than U and that sulfate binds more strongly than nitrate.
e e.
IV. Conclusions The binding energies and structures of complexes of nitrate and sulfate ligands bound to uranyl and plutonyl have been predicted by using ab initio methods including effective core potentials. The binding is found to be mainly electrostatic in nature, with the sulfate ligand being more strongly bound than the nitrate. A molecular mechanics model using the formal atomic charges derived from fitting to the molecular electrostatic potential was successful in predicting both structures and binding
energies of these complexes. Such a model might be improved by a more sophisticated treatment of the electrostatic interacti~n.~~ Acknowledgment. We thank BNFL for financial support and Dr. P. J. Hay for helpful discussions and use of his ECPs prior to publication. References and Notes (1) Campbell, D. 0.; Burch, W. D. J . Radioanal. Nucl. Chem. Art. 1990, 142, 303.
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