Peng Li*a,
Wenxia Niu*b and
Tao Gaoc
aCollege of Physics and Electronic Engineering, Shanxi University, Taiyuan, 030006, China. E-mail: lip@sxu.edu.cn
bDepartment of Physics, Taiyuan Normal University, Taiyuan, 030031, China. E-mail: nwx_ky@163.com
cInstitute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, China
First published on 16th January 2017
The equilibrium structure, stabilities, electronic structures, chemical bonding and topological properties of PuO2(H2O)n2+ (n = 1–6) complexes in the gas phase have been systematically investigated by different levels of theory. The results indicate that all the ground states of these complexes are triplet. The five water molecules of PuO2(H2O)m2+ (m = 1–5) are arranged on the equatorial plane of plutonyl. Reactivity analysis of PuO2(H2O)52+ shows that the oxygen atom of the sixth water molecule is connected by hydrogen-bonds to the two water molecules which are on the equatorial plane of PuO2(H2O)52+. The optimized geometries are in agreement with available theoretical and experimental results. The weak covalent interactions of Pu–ligand bonds were evaluated by the electron localization function and atoms in molecules analyses. The orbital interactions were investigated by analysis of total, partial, and overlap population density of state diagrams. Besides, a reduced density gradient approach was implemented to analyze the weak interactions and steric repulsions present in PuO2(H2O)52+ and PuO2(H2O)62+ complexes.
Theoretical research in this area is more common than experimental; this may be due to the high activity and radioactivity which causes difficulties for experimental studies. In a recent experimental study,11 the hydration and oxidation of gas-phase plutonyl monovalent ions were investigated by electrospray ionization mass spectrometry (ESI-MS), and the ESI mass spectra of PuO2(H2O)n+ (n = 0–5) were obtained. In contrast, tremendous theoretical works have been performed to exploring the properties of PuO2(H2O)n2+ complexes.10,12,16,17,21–24 Cao et al. have utilized density functional theory (DFT) methods to investigate the structural properties and vibrational frequencies of PuO2(H2O)m2+ complexes (m = 4–6).10 Most of the previous studies predicted that plutonyl(VI) ions will be the favorable structure, therefore, the equilibrium structure of PuO2(H2O)52+ has been well studied.12,16,17,20–23 On the other hand, these theoretical studies significantly suggest that DFT is an appropriate and successful method to access the nature of PuO2(H2O)n2+ complexes.
As seen from the above survey of PuO2(H2O)n2+ complexes, there are uncertainties of the bonds characterization, bonding mechanism and the effect of plutonyl orbitals in the interaction. Therefore, detailed interaction mechanisms and the topological properties as well as orbitals information need to be systematically examined.
The main focus of the present work is to perform a new thorough investigation of the PuO2(H2O)n2+ complexes (n = 1–6). The optimized geometries and electronic state properties of these complexes are reported. The covalent interactions were evaluated for Pu–ligand bonds in these complexes with electron localization function (ELF) and atoms in molecules (AIM) analyses. The roles of plutonyl orbitals were analyzed by total and partial density of state (TDOS and PDOS), and overlap population density of state (OPDOS) analysis. In addition, the weak interactions and steric repulsions exist in these complexes were studied by reduced density gradient (RDG) approach.
With the aim of deeply investigate the nature of the bonding, a systematic topological description of these complexes was performed. The wavefunction files (.wfn) which obtained from geometry optimization and frequency calculation were used as input files of the Multiwfn41 package to perform the ELF42,43 and AIM44 analysis. In order to gathering insights about the participation of plutonyl orbitals in the chemical bonds of complexes, TDOS, PDOS, and OPDOS45,46 analysis were also performed. In addition, the reduced density gradient proposed by Yang et al.47 was used to obtain a deep insight into the weak interactions and steric repulsions in titled complexes.
Methods | Methods | r(Pu–O) | Vibrational frequencies |
---|---|---|---|
a This work, SDD for Pu and 6-311++G(d,p) for O atoms.b Ref. 48.c Ref. 49.d Ref. 50.e Ref. 10.f Ref. 51.g Ref. 52. | |||
PuO | B3LYP/SDDa | 1.832 | 792.73 |
B3PW91/SDDa | 1.817 | 814.97 | |
PBE0/SDDa | 1.813 | 816.41 | |
PW91/SDDa | 1.832 | 768.92 | |
CASPT2b | 1.818 | ||
PW91/SDDc | 1.828 | ||
Expt.d | 822.28 (Ar), 817.27 (Kr) | ||
PuO2 | B3LYP/SDDa | 1.816 | 808.41, 757.01, 160.11 |
B3PW91/SDDa | 1.801 | 833.03, 778.24, 170.32 | |
PBE0/SDDa | 1.794 | 844.90, 789.74, 173.52 | |
PW91/SDDa | 1.804 | 823.57, 709.59, 173.61 | |
M06/SDDc | 1.786 | ||
PW91/SDDc | 1.802 | ||
Expt.d | 794.2 (Ar), 786.8 (Kr) | ||
PuO22+ | B3LYP/SDDa | 1.673 | 1101.36, 237.98, 250.20, 980.20 |
B3PW91/SDDa | 1.662 | 1127.62, 244.23, 255.84, 1010.96 | |
PBE0/SDDa | 1.653 | 1153.99, 257.02, 263.77, 1041.14 | |
PW91/SDDa | 1.695 | 1039.55, 207.96, 210.55, 915.67 | |
B3LYP/RECP78e | 1.663 | ||
MP2/RECP78e | 1.686 | ||
Expt. | 1.74f | 1043, 218 × 2, 1020g |
As can been seen from Fig. 1, the optimized geometries show little dependence on the level of theory. Our results indicate that the ground electronic state of these complexes are triplet. In terms of geometry, we can see that water molecules of PuO2(H2O)m2+ (m = 1–5) are arranged on the equatorial plane of the plutonyl group. In particular, water molecules wherein PuO2(H2O)32+ and PuO2(H2O)42+ are almost in the equivalent position.
To evaluate the sites of six water molecules in PuO2(H2O)62+, we performed the reactivity analysis by using electrostatic potential (ESP) on molecular vdW surface of PuO2(H2O)52+. ESP on vdW surface has been proved as a critical for studying intermolecular interaction, and widely used for prediction of nucleophilic and electrophilic sites. More negative ESP signifies this region has the stronger ability to attract electrophiles, on the contrary, more positive ESP site has the stronger ability to attract nucleophiles.54 The ESP-mapped vdW surface of PuO2(H2O)52+ is shown in Fig. 2. It can be seen that, the surface maximum of ESP present at the H atoms of water molecule. Therefore, from ESP point of view, these H atoms of the PuO2(H2O)52+ are the primary bonding site of O atom. After optimization, as seen from Fig. 1, we found that the oxygen atom of sixth water molecule is connected to the two water molecules which on the equatorial plane of PuO2(H2O)52+ by hydrogen-bonds.
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Fig. 2 ESP-mapped molecular vdW surface of PuO2(H2O)52+. The unit is in kcal mol−1. Surface local minima and maxima of ESP are represented as green and orange spheres, respectively. |
In terms of bond length, with the increase of water molecules, the bond length of Pu–Oyl and PuO increased slightly. Taking the calculation result of B3LYP/SDD as an example, for bond length of Pu–Oyl, 2.293 Å for PuO2(H2O)2+ and 2.469 Å for PuO2(H2O)62+; for bond length of Pu
O, 1.673 Å for PuO2(H2O)2+ and 1.705 Å for PuO2(H2O)62+, respectively.
The ELF shaded-surface-projection map of the titled complexes are displayed in Fig. 3. As can be seen, these are disynaptic valence basins between the Pu and Oyl atoms, which indicate that there are covalent bonds formation between plutonyl and H2O molecules.
In order to further exploring the chemical-bonding natures of these complexes, we employ a more quantitative method, the atoms in molecules (AIM) analysis. The different topological properties including electron density ρ(r) and its Laplacian ∇2ρ(r), kinetic electron energy density G(r), potential energy density V(r) and the total energy density H(r) of the (3, −1) bond critical points (BCP) were calculated. The corresponding results are shown in Table 2.
Complexes | Species | ρ(r) | ∇2ρ(r) | G(r) | V(r) | H(r) |
---|---|---|---|---|---|---|
a Two special water molecules of PuO2(H2O)52+, which connected with the sixth water molecule.b Hydrogen-bonds. | ||||||
PuO2H2O2+ | Pu–O | 0.388 | 0.211 | 0.476 | −0.900 | −0.424 |
Pu–Oyl | 0.074 | 0.314 | 0.085 | −0.093 | −0.008 | |
PuO2(H2O)22+ | Pu–O | 0.384 | 0.187 | 0.466 | −0.885 | −0.419 |
Pu–Oyl | 0.072 | 0.302 | 0.081 | −0.087 | −0.006 | |
PuO2(H2O)32+ | Pu–O | 0.375 | 0.191 | 0.448 | −0.849 | −0.401 |
Pu–Oyl | 0.065 | 0.280 | 0.074 | −0.077 | −0.003 | |
PuO2(H2O)42+ | Pu–O | 0.366 | 0.199 | 0.432 | −0.815 | −0.383 |
Pu–Oyl | 0.060 | 0.257 | 0.066 | −0.068 | −0.002 | |
PuO2(H2O)52+ | Pu–O | 0.364 | 0.200 | 0.428 | −0.806 | −0.378 |
Pu–Oyl | 0.051 | 0.215 | 0.054 | −0.0543 | −0.0003 | |
PuO2(H2O)62+ | Pu–O | 0.323 | 0.326 | 0.377 | −0.673 | −0.296 |
Pu–Oyl | 0.049 | 0.203 | 0.0511 | −0.0513 | −0.0002 | |
Pu–Oyla | 0.054 | 0.212 | 0.054 | −0.056 | −0.002 | |
H–Ob | 0.039 | 0.121 | 0.032 | −0.034 | −0.002 |
As the criterion proposed by Cremer and Kraka:57 |V(r)| > G(r), H(r) is negative, corresponds to covalent interactions; conversely, for closed-shell interactions, |V(r)| < G(r), H(r) is positive. This criterion was proved to be very sufficient to explore the bonding characteristics for actinide system.37–39 As shown in Table 2, the H(r) values of all calculated BCPs in the PuO2(H2O)n2+ are negative, the corresponding |V(r)|/G(r) ratio exceeds 1.0. The H(r) values of Pu–Oyl bonds are relatively small (around −0.003), and with the increase of water molecules, this value is decreasing. These quantities suggest that the Pu–Oyl bonds belong to weak covalent interaction. This conclusion is consistent with the ELF analysis. In term of the connection of sixth water molecule, there are two (3, −1) BCPs between the oxygen atom of sixth water molecule and two water molecules in the vertical plane of PuO2(H2O)52+. The corresponding ρ(r) = 0.039, H(r) = −0.002, indicating that these two bonds are hydrogen bonds.
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Fig. 4 The TDOS, PDOS, and OPDOS curves of PuO2(H2O)52+ and PuO2(H2O)62+ at the B3PW91/SDD levels of theory. |
As seen in Fig. 4, at the position of the HOMO level, the H2O orbitals approached the TDOS line, which means that most of the contributions to the HOMO came from the H2O orbitals. The OPDOS values of plutonyl and H2O orbitals are almost zero, which means that there are weak covalent characters in PuO2(H2O)52+ and PuO2(H2O)62+ complex. Comparing the two subgraphs, we found that the addition of the sixth water molecule had little effect on the overall DOS.
Fig. 5 shows the 3D isosurfaces (s = 0.5) and scatter plots of the RDG vs. ρ(r)sign(λ2) were generated for PuO2(H2O)52+ and PuO2(H2O)62+ complexes. From the isosurfaces, we can see that the Pu–Oyl interaction in PuO2(H2O)62+ is slightly weaker than that in the PuO2(H2O)52+. The ρ(r)·sgn(λ2) value of Pu–Oyl interaction in the PuO2(H2O)52+ is −0.0335 a.u., and the corresponding value in PuO2(H2O)62+ is −0.034 a.u. In addition, there are two spikes in RDG vs. ρ(r)sign(λ2) of PuO2(H2O)52+, one is around −0.01 a.u. corresponding the van der Waals interaction between the adjacent water molecules; the other one is around 0.01 a.u., represents the steric repulsion effects between Pu and H2O molecules which on both sides of Pu–Oyl. Differently, the RDG vs. ρ(r)sign(λ2) of PuO2(H2O)62+ is one more spike than that of PuO2(H2O)52+ in the region of 0 to −0.4 a.u. This spike around −0.034 is corresponds to the hydrogen-bond interaction of the sixth H2O molecule and two water molecules in the vertical plane of PuO2(H2O)52+.
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Fig. 5 3D isosurfaces (s = 0.5) and scatter plots of the RDG vs. ρ(r)sign(λ2) were generated for PuO2(H2O)52+ and PuO2(H2O)62+ complexes. |
In order to quantitatively gauge the bond strengths, in Table 3, we computed bond dissociation energies (BDE) of Pu–Oyl bond for PuO2(H2O)32+, PuO2(H2O)42+ and PuO2(H2O)52+ as well as hydrogen-bond for PuO2(H2O)62+ complexes. It can be clearly seen that, as the H2O number increases, the BDE of Pu–Oyl bond decrease. Taking PW91/SDD results as an example, the BDE of Pu–Oyl for PuO2(H2O)32+ as much as 49.788 kcal mol−1, which is larger than that computed for the PuO2(H2O)42+ (36.675 kcal mol−1) and more larger than that for the PuO2(H2O)52+ (27.145 kcal mol−1). In the case of O–H hydrogen-bond for PuO2(H2O)62+, by contrast, the BDE is smaller (12.287 kcal mol−1 in PW91/SDD method).
Complex | B3LYP/SDD | B3PW91/SDD | PBE0/SDD | PW91/SDD | |
---|---|---|---|---|---|
PuO2(H2O)32+ | r(Pu–Oyl) | 2.351 | 2.310 | 2.302 | 2.331 |
BDE | 42.586 | 49.903 | 45.973 | 49.788 | |
PuO2(H2O)42+ | r(Pu–Oyl) | 2.379 | 2.381 | 2.375 | 2.382 |
BDE | 40.330 | 39.143 | 40.941 | 36.675 | |
PuO2(H2O)52+ | r(Pu–Oyl) | 2.467 | 2.450 | 2.442 | 2.463 |
BDE | 21.150 | 20.413 | 22.967 | 27.145 | |
PuO2(H2O)62+ | r(O6yl–H′) | 1.836 | 1.819 | 1.810 | 1.751 |
BDE | 11.133 | 10.794 | 11.637 | 12.287 |
(1) The ground state geometry of these complexes was found, and our results show that all the ground states of these complexes are triplet. The water molecules of PuO2(H2O)m2+ (m = 1–5) are arranged on the equatorial plane of plutonyl. Reactivity analysis and optimization results of PuO2(H2O)62+ show that, the oxygen atom of sixth water molecule connected with two water molecules in the vertical plane of PuO2(H2O)52+ by hydrogen-bonds. These optimized geometries are in agreement with available theoretical and experimental results.
(2) The properties of the chemical-bonding of these complexes were evaluated with ELF, AIM and TDOS, PDOS and OPDOS analyses. The ELF and AIM analyses indicate the Pu–Oyl bonds have weak covalent interaction. This conclusion is consistent with the OPDOS analysis. The TDOS results show that most of the contributions to the HOMO of PuO2(H2O)52+ and PuO2(H2O)62+ came from the H2O orbitals.
(3) The RDG approach was implemented to analyze the weak interactions and steric repulsions existed in PuO2(H2O)52+ and PuO2(H2O)62+ complexes. In addition to explaining weak covalent interactions of Pu–Oyl, the RDG results also suggest that the interactions of the sixth H2O molecule and two water molecules in the vertical plane of PuO2(H2O)52+ are hydrogen-bonds.
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