ISSN 00360244, Russian Journal of Physical Chemistry A, 2012, Vol. 86, No. 11, pp. 1659–1663. © Pleiades Publishing, Ltd., 2012. Published in Russian in Zhurnal Fizicheskoi Khimii, 2012, Vol. 86, No. 11, pp. 1781–1785.
CHEMICAL KINETICS AND CATALYSIS
Computerized Modeling of Intermediate Compounds Formed During Thermal Decomposition of Gadolinium Nitrate Hydrate1 P. P. Melnikov, V. A. Nascimento, and L. Z. Zanoni Consolo Federal University of Mato Grosso do Sul/UFMS, Caixa Postal 549, Campo Grande/MS, Brazil email:
[email protected] Received September 28, 2011
Abstract—It was shown that after partial dehydration occurs a simultaneous condensation of four mol of ini tial monomer Gd(NO3)3 · 6H2O into a tetramer Gd4O4(NO3)4. The heterocycle containing 4 gadolinium atoms gradually loses N2O5 and, through the formation of unstable oxynitrates, is transformed into Gd2O3. The interatomic distances and angles were calculated using the molecular mechanics method. The compar ison of the potential energies of consecutive oxyphosphates permitted an evaluation of their stability. The models of intermediate oxynitrates represent a reasonably good approximation to the real structures and a proper interpretation of experimental data. Keywords: thermal analysis, gadolinium, molecular mechanics. DOI: 10.1134/S0036024412110180 1
INTRODUCTION
Gadolinium nitrate serves as a starting reagent for a series of chelates. In particular, several studies have been stimulated by the application of these complexes as magnetic imaging contrasts in medicine [1]. The pure compound is being employed as neutron absorber with effective capture neutron cross section used to control nuclear reactions at atomic power stations. Its solution in heavy water is used for emergency shutting down the reactor in a very short time [2]. Processes of recovery are extremely complex and expensive, so the factual information concerning Ln(NO3)3 · 6H2O thermal behavior should be brought up to date. As known the decomposition of rare earth nitrates Ln(NO3)3 · 6H2O is not a simple process of water loss. The stability of octahedral complex cations [Ln(H2O)6]3+ is so high that part of NO3− groups present in these compounds are eliminated before a complete dehydration is achieved or at least simulta neously with this process. In this context, the conclu sions drawn from the thermal decomposition kinetics of the alleged “dehydrated” nitrates [3] are question able when the compounds are obtained using a method that, in principle, would not have allowed pre paring anhydrous salts. It seems paradoxical, but in an earlier investigation [4] thermal dehydration of gadolinium nitrate hydrate has been described in an almost correct way. In particular, it was suggested that during heating Gd(NO3)3 ⋅ хH2O forms 2 basic salts, GdONO3 and Gd4O3(NO3)2, 1 The article was translated by the authors.
and decomposes totally to Gd2O3 at 582°С. However, the proposed mechanism of transformation has not been convincingly demonstrated. In a recent work, the formation of tetrameric structures was proved for the case of gallium nitrate [5]. The present study was undertaken to revert to the thermolysis of the above hydrate, in order to test this hypothesis for the case of rare earth elements, offering a new, more realistic scheme of transformation for Gd(NO3)3 ⋅ 6H2O. As an additional tool we have used the molecular mechanics method in order to build up feasible models of unstable compounds forming during Gd(NO3)3 ⋅ 6H2O thermolysis. Recently progress has been made toward development of force fields which allow calcu lating minimal potential energy of these compounds and making (with due caution) comparison of the sta bilities of what we regard as the true intermediates. EXPERIMENTAL The starting reagent used was gadolinium nitrate hexahydrate Gd(NO3)3 ⋅ 6H2O, of analytical grade purity (99.9%) purchased from Aldrich. Direct heat ing of the commercial reagent resulted in mass loss of 58.41% confirming the water number 6. Thermogravi metric analysis (TGA) was carried out under nitrogen flux ramping 5 K min–1, employing 50H Shimadzu Instrumentation. Mass losses during heating were ana lyzed and compared to previously calculated values. Compounds were simulated using AVOGADRO, an opensource molecular builder and visualization tool, version 0.9.9. All models were submitted to a rig orous conformational analysis using the Tripos 5.2 and
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Δm, % 100
80
60
40
200
400
600
800 T, °C
Fig. 1. TG curve of Gd(NO3)3 ⋅ 6H2O.
UFF [6, 7] force fields. Structures were found by min imizing energy values with respect to all geometrical variables, no additional assumptions being made. It was possible to build 3d images as well as evaluate angles and interatomic distances by using special fea tures of the program. We are fully aware that the simulation of the struc ture of solid compounds by the molecular mechanics method can sometimes lead to very confusing results. And yet, a better understanding of such aspects of the problem as the stress in inorganic rings, the ratio of the sizes of functional groups, possibilities of intramolec ular rotations, etc can be extremely useful when eval uating the structures of real compounds and for assess ing how stable they are likely to be. RESULTS AND DISCUSSION As established by visual observations, the com pound becomes liquid at around 50°C. It is well known that the decomposition onset for the nitrates of transition metals is generally below 100°C due to a backdonation of electrons from the nitrate ions to an unfilled dorbital of the cations [8, 9]. In this respect gadolinium nitrate is not an exception. The TG curve of Gd(NO3)3 · 6H2O is given in Fig. 1. According to this curve, it melts in its own water of crystallization at 57°C and, between 60 and 235.7°C, loses 19.80% of mass. The next loss of mass (6.09%) takes place between 236 and 323°C. A substantial reduction of mass (21.18%) occurs between 419 and 392°C. There follows a loss of 6.66% between 419 and 500°C. Finally, the remaining mass diminishes slowly and gradually, losing 4.33% of volatile products. It is obvious that the loss of mass which takes place during the first two stages, a total of 51.81%, cannot possibly result from the disintegration of the single mol of Gd(NO3)3 · 6H2O, if only because its formula unit
contains no more than one atom of metal, whereas at least two are required for the formation of Gd2O3. Consequently, we must take into consideration the processes of condensation, characteristic of the chem istry of cations with the charge +3. Here, it is worth referring to the existing published data on the elements whose properties are most close to gadolinium. For yttrium and scandium [4, 7], for example, it has been shown that such condensation enhances the formation of stable groups containing four metal atoms. As to gadolinium itself, a cyclic cluster containing 4Gd has been isolated in crystalline form [10]. Calculations show that the hypothesis concerning cluster preexistence in the solid state is quite applica ble to the present case of gadolinium nitrate hexahy drate. Indeed, that suggests that at least four mol of Gd(NO3)3 · 6H2O are involved in the condensation pro cess, and the total decomposition can be described as 4[Gd(NO3)3 ⋅ 6H2O] = 2Gd2O3 + 6N2O5 + 24H2O. This gives a total mass loss of volatile products of 59.87%, which corresponds well to the experimental value of 58.47% obtained after treating gadolinium nitrate at 800°С for 2 h. Naturally, we might start con sidering the condensation of 2[Gd(NO3)3 ⋅ 6H2O], but in this case, at the end of the process, we would have to resort to fractional values of stoichiometric coeffi cients for N2O5. Another possibility is that the tetrameric clusters precede thermal treatment, and that 6H2O stabilize them through a system of hydrogen bonds. In any case, as no mass loss for cluster formation is taking place, it does not affect the interpretation of DTG results. As for the individual stages of thermal decomposi tion, the DTG curve can be explained as follows. At the first stage, during the process of the initial melting and dehydration of the compound and immediately after wards, 20 mol of looselybound water are removed. This produces a mass loss of: calc. 19.94%; exp. 19.80% with the formation of Gd4(NO3)12 ⋅ 4H2O. Thermal analysis fails to detect intermediate hydrates Gd(NO3)3 ⋅ 5H2O and Gd(NO3)3 ⋅ 3.5H2O, which were prepared by Chinese researchers by dehydration of Gd(NO3)3 ⋅ 6H2O with conc. H2SO4 or P4O10 [11]. Up to this point, there has been no interaction between individual molecules of Gd(NO3)3, although the stability of clusters may have decreased owing to the alterations in the system of intermolecular hydro gen bonds. At the second stage, one mol of N2O5 is removed, producing a mass loss of: calc. 5.98%; exp. 6.09%. The removal of N2O5 leads to the creation of oxygen bridges between the gadolinium atoms, obliging 4Gd(NO3)3, to condense into a tetrameric structure Gd4О(NO3)10 (scheme Fig. 2). At the third stage, 3HNO3 and 3N2O5 are removed, producing a mass loss of: calc. 20.99%; exp. 21.18%
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Gd O
O O
NO2
NO2
O
Gd
O O NO2
NO2
NO2
NO2
NO2
NO2
–N2O5
–N2O5
NO2
NO2
O
O
Gd
O
NO2
NO2
O O O Gd
–N2O5 –4N2O5
O
Gd
Gd O NO2
O
O NO 2 NO2
O
NO2 O Gd
O
Gd
O
Fig. 2. Scheme showing the condensation process of 4[Gd(NO3)3 ⋅ 6H2O].
with the formation of Gd4О4(NO3)4 ⋅ H2O. At the fourth stage, one mol of H2O and one mol of N2O5 are eliminated. This gives a mass loss of: calc. 6.98%, exp. 6.55% with the formation of Gd4O5(NO3)2. Finally, the last mol of N2O5 is removed (calc. 5.98%; exp. 4.33%). As a result, the second oxygencontaining bridge is unable to withstand the strain and compound collapses, leading to the formation of 2Gd2O3. In
N2 Gd2
total, the partial losses comprise 59.87%, which is close to the 58.47% reached by heating directly to 800°C. The models obtained by using the molecular mechanics technique are shown in Figs. 3 and 4. As can be seen, the base of the tetramer is formed by an inorganic heterocycle composed of four atoms of gad olinium, alternating with four atoms of oxygen, which, to simplify things, are numbered from 1 to 4. The interatomic distances and bond angles are pre sented in Tables 1 and 2. At the first glance, this cycle seems symmetric as all distances Gd–O and angles Gd–O–Gd are identical: 2.252 Å and 104.5°, respec tively. The comparison shows that the distances are of the same order as those refined from Xray diffraction data for the high temperature modification of Gd2O3, i.e., in the range 2.223–2.387 Å [12]. However, the distances between the opposite gado linium atoms in the cycle, e.g., Gd(1)–Gd(3) and Gd(2)–Gd(4), are not equal: 5.285 and 4.755 Å, respectively, so the gadolinium and oxygen atoms are not located in the same planes as can appear from the Fig. 3. Since the bonds Gd–O forming the cycles are not covalent, but rather ionic in nature, it becomes quite clear that, in contrast to the planar benzene ring, these cycles can easily become corrugated. Moreover, as the angle of this “folding” may vary within certain limits, one can expect to find the presence of confor mational isomers with slightly differing values of potential energy. In a real solid, this degree of freedom is unlikely to be preserved due to the requirements imposed by the densest packing. At this stage, we can only suppose that this model corresponds to a dis torted chair conformation. The same is true of the degree of freedom of rota tion about the bonds between gadolinium and oxygen of NO3− groups attached to gadolinium atoms. These groups behave as normal nitrate anions with N–O dis tances in the range 1.191–1.314 Å [12], that is slightly larger than the interatomic distances in the NO3−
N3
N3 O3
O4
N4
Gd3
Gd3
Gd4
O3 O2
O4 N2
O1 N1
O1
Gd2
Gd4
Gd1
Gd1
O2 N1
N4 −
Fig. 3. Model of tetramer structure containing 4NO3 , front and profile views. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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Gd1
Gd2
O2
O3
O1 Gd4 O4
Gd3
N2
−
Fig. 4. Model of tetramer structure containing 2NO3 .
groups of solid nitrates established by Xray methods (1.20–1.22 Å) [13]. The large, but different distances between nitrogen atoms in transpositions (Table 1) illustrate the nonequivalence between the NO3− groups present. Meanwhile, the bond angles O–N–O found for the model are equal to the angles in solid nitrates (all 120.0°) in accordance with the actual Table 1. Interatomic distances (Å) calculated for tetrame ric models Distances
Gd4O4(NO3)4
Gd4O5(NO3)2
O1–Gd1 O2–Gd2 O3–Gd3 O4–Gd4 Gd1–O2 Gd2–O3 Gd3–O4 Gd4–O1 Gd1–Gd2 Gd2–Gd3 Gd3–Gd4 Gd4–Gd1 Gd1–Gd3 Gd2–Gd4 N1–N3 N2–N4 N1–N2
2.252 2.252 2.252 2.252 2.252 2.252 2.252 2.252 3.562 3.561 3.562 3.562 5.285 4.755 9.813 10.484 –
2.264 2.254 2.263 2.253 2.263 2.253 2.263 2.254 3.577 3.571 3.571 3.577 6.288 3.396 – – 10.409
Table 2. Bond angles (deg) calculated for tetrameric models Angles
Gd4O4(NO3)4
Gd4O5(NO3)2
Gd1–O2–Gd2 Gd2–O3–Gd3 Gd3–O4–Gd4 Gd4–O1–Gd1 Gd4–O5–Gd2
104.50 104.50 104.50 104.50 No bridge
104.70 104.50 104.50 104.70 93.30
structure of the polyatomic ion, which has trigonal planar geometry [14]. Figure 4 shows the structure of the oxynitrate com position Gd4O5(NO3)2 that is to be formed after the removal of the penultimate mol of N2O5. Here, the numeration of the cycle is the same as for the previous model. In this instance, a newformed crossbridge Gd–O–Gd connects gadolinium atoms 2 and 6. In view of the exceptional narrowness of this space (the distance between Gd(2) and Gd(6) is about 3.388 Å) the oxygen atoms are located far outside the cycle. The “legs” of the bridge are at an angle of 93.3° in relation ship to each other, which is lower than the angles Gd– O–Gd within the cycle, where there is an average of 104.6°. At the same time, the remaining two anions NO3− have attached themselves to the distant atoms Gd(1) and Gd(3) and the formation of a second bridge does not seem probable. That is why the elimination of the last N2O5 instead of metastable Gd4O6 will pro duce 2 mol of Gd2O3, in agreement with the experi mental data. The calculation of minimal potential energies for the aforementioned models shows that the difference between their numerical values for both tetramers, Gd4O4(NO3)4 and Gd4O5(NO2)2, is, in an arbitrary scale, very small, i.e., 3.606 kJ mol–1 (algebraic sum ⎯5.742 + 9.347 kJ mol–1). This is a clear indication that the corresponding compounds may be formed easily and possess a certain stability, conclusion that is in agreement with the experimental curve of thermal analysis. The exceptionally high level of potential energy for a hypothetical Gd4O6 (298.080 kJ mol–1) shows that the existence of a second bridge is practi cally impossible. As to the final Gd2O3, as expected, its potential energy is the lowest of all compounds con sidered, that is—0.055 kJ mol–1. It is clear that the levels of potential energy, calcu lated by means of molecular mechanics must not nec essarily have any definite physical meaning in them selves. However, when considering a series of related structures, the method may be of help when interpret ing the experimental findings in absence of Xray data for the interatomic distances and bond angles, as in the case of amorphous compounds. CONCLUSIONS The thermal decomposition of gadolinium nitrate occurs through simultaneous condensation of four mol of the initial monomer Gd(NO3)3 ⋅ 6H2O into a tetramer. The resulting inorganic cycle Gd4О4(NO3)4 ⋅ H2O gradually loses H2O and N2O5, and, through the formation of unstable oxynitrates, is transformed into gadolinium oxide. The molecular mechanics method permitted to construct the models of intermediate oxynitrates and to determine their structural parame ters. The models of intermediate oxynitrates represent
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a reasonably good approximation to the real struc tures. ACKNOWLEDGMENTS The authors are indebted to CNPq and FUN DECT/MS (Brazilian agencies) for financial support and to Dr. R. Sewell for fruitful discussions. REFERENCES 1. A. E. Merbach and E. Toth, The Chemistry of Contrast Agents in Medical Magnetic Resonance Imaging (Wiley, New York, 2001). 2. Karanam, Anal. Lett. 42, 2141 (2009). 3. C. A. Strydom and C. P. J. Van Vuuren, Thermochim. Acta 129, 335 (1988). 4. J. Perelman, Russ. J. Inorg. Chem. 11, 1817 (1966). 5. P. Melnikov, V. A. Nascimento, and L. Z. Zanoni Con solo, J. Therm. Anal. Calor. 107 (3), 1117 (2012).
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