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The interaction natures between Pu and different ligands in several plutonyl VI complexes are investigated by performing topological analyses of electron density. The geometrical structures in both gaseous and aqueous phases are obtained with B3LYP functional, and are generally in agreement with available theoretical and experimental results when combined with all-electron segmented all-electron relativistic contracted SARC basis set.

The Pu—O y l bond orders show significant linear dependence on bond length and the charge of oxygen atoms in plutonyl moiety. The closed-shell interactions were identified for Pu-Ligand bonds in most complexes with quantum theory of atoms in molecules QTAIM analyses. The interactive nature of Pu—ligand bonds were revealed based on the interaction quantum atom IQA energy decomposition approach, and our results indicate that all Pu—Ligand interactions is dominated by the electrostatic attraction interaction as expected.

Meanwhile it is also important to note that the quantum mechanical exchange-correlation contributions can not be ignored. By means of the non-covalent interaction NCI approach it has been found that some weak and repulsion interactions existed in plutonyl VI complexes, which can not be distinguished by QTAIM, can be successfully identified. Plutonium is the only element in the periodic table that can have appreciable amounts of four different oxidation states existing in aqueous acidic solutions simultaneously.

Therefore, the structures, redox, and interactive features of plutonium complexes are needed to be investigated by using experimental or theoretical methods. Plutonium complexes were significantly less studied in experiments relative to its thorium and uranium counterparts for reasons of toxicity and radioactivity. Raman spectra of plutonyl VI ions in solution were measured by Madic et al. A number of novel and unexpected behaviors were observed in their experiment.

The chloride plutonyl VI complexes were investigated in NaCl solutions using conventional absorption spectrophotometry [ 3 ]. The experiment suggests that mixed chloro-hydroxo or chloro-carbonate may form at high pH due to strong affinity of plutonyl VI for chloride. Recently, the crystal structure of the plutonyl VI dinitrate complex was characterized with spectroscopic methods by Gaunt et al.

They found the dominant formation of mononitrate and negligible formation of the dinitrate in aqueous solutions. In contrast to experiments, tremendous theoretical works with quantum mechanical methods have been performed to probe the structural and electronic properties and spectra of plutonyl complexes. There are a number of theoretical data on plutonyl aquo [ 567 ], carbonate [ 8 ], hydroxo [ 9 ] and fluoride [ 1011 ] complexes. The chemical bonding natures of Uranyl VI complexes have been recently identified by Vallet et al.

These reports significantly indicate that QTAIM is an appropriate and successful tool to access the bonding characters of actinides complexes. Based on the gaseous electron density, the QTAIM and interaction quantum atom IQA [ 16 ] analyses were performed to identify the interaction nature between Pu and coordinating ligands.

In addition, another density-based analytical tool, the electron localization function ELF [ 17 ], was utilized to study the chemical bondings. Calculations were performed with two different packages, ORCA The geometry structures of titled complexes in gaseous and aqueous phases were optimized without symmetry constrain, and were identified to be one minimum in potential energy surface through vibrational frequency calculations.

All solution phase calculations were carried out in water. In G09 calculations, the B3LYP functional in conjunction with relativistic effective core pseudopotential RECP was utilized for optimization, the Stuttgart small-core relativistic effective core potential RECP with 60 core electrons frozen and assisted basis set [ 2627 ] were employed to describe Pu atom, and aug-cc-PVDZ basis set [ vomplexes ] for light elements.

The ultrafine grid 99, was bipyridin in the integral calculations to accurately describe heavy element Pu.

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The solution phase calculations in G09 were carried out with the polarizable continuum model PCM [ 29 ].

The wave-function used in these bonding biyridine were obtained at the B3LYP level of theory.

The calculated structural parameters are listed in Table 1. The consideration of solution effect results in the elongation of Pu—O y l bonds and the shortening of Pu—OH 2 bonds for all studied aquo maceocyclic.

Similar effects have been observed in recent DFT calculations [ 12 ]. It is worth noting that the Pu—O y l and Pu—OH 2 bond lengths are closer to the experimental values when the solution effect is considered. On the whole, the Pu—O y l bond lengths calculated with solution effect considered are slightly larger than those obtained in macricyclic calculation for all studied complexes.

On the other hand, the consideration of solution results in contraction of the Pu—OH 2 bond lengths and elongation of of Pu—Ligand bonds. We also investigated the geometry properties of mono- and bis-chloro complexes which have been identified in NaCl solutions by Runde et al.

Energetically, two isomers of PuO 2 Cl 2 H 2 O 2 are very close to each other, with the difference of 4. Both isomers show the similar structural features as shown in Table 2. There also exists no great difference in geometry parameters for both isomers. This result agrees well with previous experimental [ 3 ] and theoretical investigations [ 12 ]. In experiments, Gaunt et al.

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The typical bonds in trinitrate complex are not significantly different from that in dinitrate complex. The Pu atoms with about 1 e lost act as electron donor in all complexes as expected.

One can moeetiy from Table 3 that the Pu atoms possess 5 f 5 6 d 1. Comparing with the population of free Pu atom, 5 f 6 7 s 2there are significant electron promotions from 7 s orbital to 6 d and 7 p orbitals when Pu atoms are involved in chemical bonds. On the other hand, the oxygen atoms in plutonyl moiety and coordinating atoms in studied Lewis acid ligands accept electrons from Pu compldxes. In the peroxide, nitrate and carbontrate complexes, the bond orders between Pu and coordinating oxygen are significantly smaller than other Pu—L bonds.

This can be easily understood that the bigger the bond orders, the shorter the Pu—O y l bond lengths. The oxygen atoms in Pu—O y l chemical bonds which accept fewer electrons generally possess the larger bond orders. Therefore, the Pu—O y l chemical bonds bipyrodine not mainly derived from charge transfer mechanism.

This implies that the interactive nature between Pu and O y l is covalent predominantly, not ionic features. However, Plutonyl peroxide and carbonate complexes do not show similar interaction feature to Uranyl analogies. Electron density in the equatorial plane of studied plutonyl VI complexes, the BCPs are shown as the small blue sphere; the thin green lines represent the counter map of density; thin blue lines represent the interatomic paths that separate the atom electron density basins; the values of the density at the BCPs are also shown.

However, this criterion has been proved to be not sufficient to describe the bond natures of heavy atoms in previous work [ 36 ]. Another property, the total energy density H r defined as the sum of local kinetic energy density G r and the local potential energy density V r proposed by Cremer [ 37 ] was proved to be very appropriate to characterize the degree of covalency of a bond, the more negative the H r value, the more stabilizing the interaction.

The topological parameters a. To interpret the physical nature of the Pu—L interatomic interaction and its local contribution to the overall energy of one Plutonyl VI complex, the computationally expensive but highly useful IQA energy decomposition scheme is considered. IQA defines the interaction E i n t A B between two atoms as a competing contribution made by classical components interaction energy between electrons and nuclei as well as coulombic interaction between electrons of atom A and B conveniently combined as V c l A Band quantum-mechanical contribution, as V x c A Macrocyxlic.

These quantities can not be fomplexes and also make a significant contribution to total interaction energy. Therefore, the quantum-exchange term is also important for the stabilizing of interaction. For the rest of the Pu—Ligand bonds, the stabilizing interactions are characterized with dominant electrostatic term and non-ignorable quantum-exchange contribution. According to the ELF criterion, for covalent bonding, a typical maximum coomplexes of the ELF connecting two atoms is closer to 1.

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Moreover, one can find the variation of ELF in these Pu—O y l bifurcation points is very small, indicating that different ligand fields do not significantly affect the Pu—O y l interaction—for the ELF values associated with Pu and ligands suggest the weak interaction between them.

NCI index based on the relation between the electron density and reduced density gradient RDG was proposed by Yang et al. The RDG is defined as follows:. Compleexes allows for distinguishing which of the weak interactions are attractive or repulsive, and it can reveal those interactions that are not easily detected by QTAIM.

Moreover, the Pu—O y l and Pu—OH 2 bond lengths are more close to the experimental values when solution effect is considered. NPA analyses showed that there are significant electron promotions from 7 s orbital to 6 d and 7 bipyridne orbitals when Pu atoms are involved in chemical bonds.

The Pu—O y l chemical bonds, bipyrieine which oxygen atoms accept fewer electrons, correspond to the stronger bond strength. The interesting linear dependence between Pu—O y l bond orders and the charge of oxygen atoms revealed the covalent characters of Pu—O y l bonds. Our QTAIM analyses indicate that the significant closed-shell interactions are suggested for Pu—Ligand bonds in almost all complexes studied. It is worth noting that the Pu—Ligand interaction shows a slight degree of bipyricine in some complexes.

Moreover, the ligand affinity of plutonyl VI is different from uranyl VI analogies.

IQA energy decomposition scheme was utilized to probe into the interaction between Pu and ligands, the electrostatic attractive interaction is characterized as the dominate stabilizing contribution, but the quantum-exchange term is also non-ignorable, making obvious contribution to total interaction energy. Moreover, the steric repulsion can be found with NCI analyses. We hope our electron density topological analyses are useful for a deep understanding of Pu—Ligand chemical bondings in plutonyl VI complexes.

Jiguang Du performed the computations, analyzed the results and wrote the manuscript; Xiyuan Sun helped in preparing the manuscript; Gang Jiang reviewed the manuscript. National Center for Biotechnology InformationU.

Int J Mol Sci. Published online Apr Francesc Illas, Macricyclic Editor. Author information Article notes Copyright and License information Disclaimer. Received Nov 15; Accepted Jan This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution CC-BY license http: Abstract The interaction natures between Pu and different ligands in several plutonyl VI complexes are investigated by performing topological analyses of comllexes density.

Introduction Plutonium is the only element in the periodic table that can have appreciable amounts of four different oxidation states existing in aqueous acidic solutions simultaneously. Results and Discussion 3.

Open in a separate window. Table 4 The topological parameters a. Interaction Quantum Atom IQA Analyses To interpret the physical nature of the Pu—L interatomic interaction and bipyrkdine local contribution to the overall energy of one Plutonyl VI complex, the computationally expensive but highly useful IQA energy decomposition scheme is considered.

The RDG is defined as follows: Author Contributions Jiguang Du performed the computations, analyzed the results and wrote the manuscript; Xiyuan Sun helped in preparing the manuscript; Gang Jiang reviewed the manuscript.

Conflicts of Interest The authors declare no conflict of interest. Raman spectroscopy of neptunyl and plutonyl Ions in aqueous solution: Higher order speciation effects on plutonium L bipyeidine X-ray absorption near edge spectra. Structural and spectroscopic characterization of Plutonyl VI Nitrate under acidic conditions. Density functional studies of actinyl aquo complexes studied using small-core effective core potentials and a scalar four-component relativistic method.

Strong correlations in actinide redox reactions. Electronic structure and spectra of plutonyl complexes and their hydrated forms: