Theoretical Study on Structures and Bond Properties of NpO2m+ Ions

Mar 9, 2015 - The equilibrium structures, vibrational frequencies, and bond characteristics of NpO2m+ ions and NpO2(H2O)nm+ (m = 1–2, n = 1–6) com...
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Theoretical Study on Structures and Bond Properties of NpO2m+ Ions and NpO2(H2O)nm+ (m = 1−2, n = 1−6) Complexes in the Gas Phase and Aqueous Solution Yao-Peng Yin, Chen-Zhong Dong,* and Xiao-Bin Ding Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China ABSTRACT: The equilibrium structures, vibrational frequencies, and bond characteristics of NpO2m+ ions and NpO2(H2O)nm+ (m = 1−2, n = 1−6) complexes have been studied by carrying out ab initio calculations in the gas phase and aqueous solution. The geometries have been obtained at the B3LYP level with the use of the polarized continuum model (PCM). The computed structural parameters that are in reasonably good agreement with the available data show that the solvation effect leads to a red shift of the IR spectra and the weakness of interaction strengths in neptunyl ions. By comparing the structural properties and the density of states (DOS) of these aqua complexes in the gas phase and aqueous solution, it is found that the solvation effect can be simulated approximately with the calculations of these aqua complexes in the gas phase. In addition, the DOS of these aqua complexes together with the binding energies between the neptunyl ion and water molecule reveal that the penta-aqua complex is preferred for neptunyl ions in aqueous solution.

1. INTRODUCTION Nuclear power has been regarded as one of the most important sources for the production of electricity in the future. However, a mass of fissile waste with high-level radioactivity has been produced during the process, which can give rise to a threat to human health and the environment. Therefore, the safe disposal of those radioactive nuclear waste materials has become a key factor affecting the sustainable development of nuclear energy. Actinyl ions existing in high-level liquid wastes during the extraction process are very stable under different oxidation states and are able to coordinate with water molecules to form moderately strong aqua complexes. Accordingly, a systematic understanding about the properties of actinyl ions in aqueous solution is very crucial for nuclear waste treatment. During the past decade, actinyl ions in solution have been the subject of numerous experimental and theoretical studies.1−23 However, most of these studies just focused on the uranyl ions and their hydrated complexes in aqueous solution, and there is very limited work about neptunyl ions. To the best of our knowledge, in experimental studies, only the structures and vibrational frequencies of neptunyl ions in solution1,2,4−6,9 were measured via the EXAFS spectra, X-ray powder diffraction, and electron diffraction. In theoretical studies, for the equilibrium structure, vibrational frequencies of neptunyl ions were investigated through extensive ab initio calculations in the gas phase and in solution,11,14,20,21 and the geochemistry behavior in the different oxidation states was studied by calculating the reduction potentials and solubility.22,23 Yet, some of these studies with theories and experiments are not always consistent. © 2015 American Chemical Society

Up to now, the electronic structural properties and chemical bond characteristics of neptunyl ions are still open, and there are uncertain questions because of the complicated behavior of the 5f orbital electrons in the neptunium element. Consequently, there is a compelling need to make systematic studies of these complexes in the gas phase and particularly in aqueous solution. In the present work, the equilibrium structures, vibrational frequencies, and bond characteristics of neptunyl ions NpO2m+ and their hydrated forms NpO2(H2O)nm+ (m = 1−2, n = 1−6) have been systematically studied by carrying out ab initio calculations in the gas phase and aqueous solution. The geometries have been fully obtained by employing relativistic effective core potentials (RECP) at the B3LYP level, and the solvation effect has been studied by using a polarized continuum model (PCM). On the basis of the above studies, the total and projected density of states (TDOS and PDOS) and binding energies have been calculated to further study the stability of these aqua complexes. The computational details are presented in section 2, the results and discussion are provided in section 3, and the conclusions are summarized in section 4.

2. COMPUTATIONAL DETAILS The density functional theory (DFT) methods have been considered as a practical and effective computational tool for the heavy actinide complexes,24−26 which can include the Received: November 10, 2014 Revised: March 6, 2015 Published: March 9, 2015 3253

DOI: 10.1021/jp511265j J. Phys. Chem. A 2015, 119, 3253−3260

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The Journal of Physical Chemistry A electron correlation effects partly.27−30 Our calculations are carried out by using one of the DFT methods, which is a hybrid Hartree−Fork method incorporating Becke’s three-parameter functional (B3) with the Lee, Yang, and Parr (LYP) correlation functional (abbreviated as B3LYP).27,31 In our calculations, the RECP and the corresponding orbital basis sets developed by the Stuttgart group are used for the neptunium atom.32−34 In the approximation, the large-core (15 valence electrons) RECP replaces 78 electrons in the inner shells by a pseudopotential adjusted to produce valence energies, and the 6s, 6p, 6d, 5f, and 7s electrons are treated as valence electrons; this corresponds to contracted Gaussian-type orbitals basis sets [7s6p2d4f]/ [3s3p2d2f].35 The 6-31+G* basis sets that added the diffuse functions are used for hydrogen and oxygen atoms.36 The solvation effect of the water is further studied by using the polarizable continuum model (PCM) in the self-consistent reaction field (SCRF) method, which considers the solvent as a continuum dielectric.37−40 In the PCM model, the solute is embedded in a shape-adapted cavity defined radius.41 Furthermore, on the basis of the optimized structures, the natural atomic charge distributions are obtained by natural bond orbital (NBO) analysis at the same level of theory.42−47 In order to compare the interaction strengths among all aqua complexes, the binding energies between the neptunyl ion and the water molecule have been calculated as the following formula

= 5 complex, the four water molecules still stay on the equatorial plane, while the fifth water molecule orientates out of the plane. In the case of the n = 6 complex, the four water molecules are also on the equatorial plane, while the other two water molecules lie above and below the plane. We also note that the geometrical structures of NpO2(H2O)n2+ (n = 1−6) complexes of Np(VI) are similar to those of NpO2(H2O)n+ (n = 1−6) complexes of Np(V) both in the gas phase and in aqueous solution. Tables 1 and 2 summarize the total energies, structural parameters of neptunyl ions, and these aqua complexes in the gas phase and aqueous solution; the available experimental data of these geometries are also contained in the tables for comparison.5,6,9 As clearly shown in the tables, the total energies of NpO2(H2O)n+ (n = 1−6) complexes are lower by roughly 48.4−143.9 kcal/mol in aqueous solution compared to the ones in the gas phase; a similar trend is also observed for NpO2(H2O)n2+ (n = 1−6) complexes. It is also found that the NpO bond lengths and Np−H2O distances in these aqua complexes become larger with increasing number of combined water molecules. Such a behavior indicates that the short-range interactions between the neptunyl ion and the water molecule in the aqua complexes have a significant influence on geometrical properties of neptunyl ions. It is primarily due to the direct coordination of the water molecules. On the other hand, the difference of the NpO bond lengths becomes unobvious with the number of water molecules running up to n = 4−6; for example, the NpO bond lengths increase by 0.013 Å from the NpO2+ ion to the NpO2(H2O)+ complex, while the difference between the NpO2(H2O)5+ and NpO2(H2O)6+ complexes is just 0.003 Å. This means that the number of water molecules reaches a limit in the hydrated sphere. A similar trend is also observed in aqueous solution and will be discussed in the later parts. As can be seen from the tables, there is an appreciable change of the NpO bond lengths for neptunyl ions in aqueous solution. The NpO bond lengths are elongated by 0.056 Å for NpO2+ ions in aqueous solution. The evident changes of the NpO bond lengths indicate that the long-range interactions from the continuum medium play an important role in calculations of the bond lengths of neptunyl ions; it is primarily attributed to the polarization effects between neptunyl ions and solvent water molecules. However, the change of NpO bond lengths for these aqua complexes is unobvious in aqueous solution, and the difference becomes relatively small with the increasing number of the water molecules. For example, the NpO bond lengths increase by 0.034 Å for the NpO2(H2O)2+ complex, while the ones increase just by 0.002 Å for the NpO2(H2O)5+ complex. A similar trend is also observed for Np−H2O distances; the distances increase by 0.055 Å for the NpO2(H2O)2+ complex, as presented in Table 1, yet the difference is just 0.004 Å for the NpO2(H2O)6+ complex. This suggests that the long-range interactions have an obviously weak influence on these aqua complexes. It is due to the water molecules in the hydrated sphere already being considered directly in these aqua complexes. As clearly shown in the tables, the ONpO axial bonds tend to remain linear in the whole series of NpO2(H2O)nm+ (n = 1−6, m = 1−2) complexes both in the gas phase and in aqueous solution. From the tables, we can see that the angles are about 180° in the gas phase and aqueous solution. This suggests that the ONpO bond angles are not sensitive in these aqua complexes; therefore, we would not pay attention to

ΔE[NpO2 (H 2O)nm + ] = E[NpO2 (H 2O)nm + ] − E[NpO2 m +] − nE[H 2O] + E[BSSE]

In the above calculations, the basis set superposition error (BSSE) has also been calculated for these aqua complexes by employing the method from Boys and Bernardi.48 All above calculations are carried out by using Gaussian 03 package for quantum chemistry.49

3. RESULTS AND DISCUSSION a. Structures and Electronic Properties of NpO2(H2O)nm+ (n = 0−6, m = 1−2) Complexes. Figure 1

Figure 1. Geometrical structures of NpO2(H2O)nm+ (n = 1−6, m = 1− 2) in the gas phase and aqueous solution.

shows the optimized geometrical structures of neptunyl aqua complexes both in the gas phase and in aqueous solution. It is found that the geometrical structures of these aqua complexes in aqueous solution are similar to the ones in the gas phase, besides the fact that the total energies of these aqua complexes are lower in aqueous solution. As expected, the water molecules are bonded on an equatorial plane, and the two hydrogen atoms lie symmetrically on both sides of the plane in mono-, di-, tri-, and tetra-aqua complexes. However, in the case of the n 3254

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Table 1. Total Energies (Hartree), Bond Lengths (Å), and Bond Angles (degrees) for the NpO2+ Ion and NpO2(H2O)n+ (n = 1− 6) Complexes in the Gas Phase and Aqueous Solution (in parentheses) at the B3LYP Method Etotal

molecules NpO2+ NpO2(H2O)+ NpO2(H2O)2+ NpO2(H2O)3+ NpO2(H2O)4+ NpO2(H2O)5+ NpO2(H2O)6+ exp a

−211.2228 −287.7025 −364.1744 −440.6122 −517.1014 −593.5421 −669.9883

(−211.4522) (−287.8891) (−364.3250) (−440.6957) (−517.1837) (−593.6222) (−670.0654)

R3(Oeq−H)

R1(NpOax)

R2(Np−Oeq)

1.738 1.751 1.762 1.778 1.784 1.789 1.792 1.83a

2.475 (2.568) 2.519 (2.574) 2.535 (2.578) 2.555 (2.582) 2.614 (2.625) 2.691 (2.695) 2.50a/2.52b

(1.794) (1.796) (1.796) (1.797) (1.797) (1.801) (1.802)

0.975 0.974 0.973 0.972 0.972 0.972

(θ1) ∠Oax−Np−Oax

(0.972) (0.972) (0.972) (0.972) (0.972) (0.975)

180.0 180.0 180.0 180.0 180.0 179.5

(178.0) (180.0) (179.6) (180.0) (179.6) (178.9)

Reference 6. bReference 5.

Table 2. Total Energies (Hartree), Bond Lengths (Å), and Bond Angles (degrees) for the NpO22+ Ion and NpO2(H2O)n2+ (n = 1−6) Complexes in the Gas Phase and Aqueous Solution (in parentheses) at the B3LYP Method Etotal

molecules NpO22+ NpO2(H2O)2+ NpO2(H2O)22+ NpO2(H2O)32+ NpO2(H2O)42+ NpO2(H2O)52+ NpO2(H2O)62+ exp a

−210.6285 −287.1562 −363.6675 −440.1637 −516.6585 −593.1182 −669.5929

(−211.2013) (−287.6425) (−364.0817) (−440.5184) (−516.9556) (−593.3781) (−669.8361)

R1(NpOax)

R2(Np−Oeq)

1.708 1.720 1.727 1.734 1.744 1.745 1.753 1.75a

2.366 2.406 2.435 2.454 2.517 2.579 2.42a

(1.749) (1.750) (1.753) (1.757) (1.758) (1.759) (1.761)

(2.470) (2.478) (2.486) (2.490) (2.527) (2.610)

R3(Oeq−H) 0.987 0.983 0.980 0.978 0.976 0.974

(θ1) ∠Oax−Np−Oax

(0.975) (0.974) (0.974) (0.974) (0.976) (0.974)

180.0 180.0 180.0 180.0 180.0 180.0

(175.8) (179.0) (177.1) (180.0) (179.8) (179.6)

Reference 9.

Table 3. Mulliken Charges (Electrons) on Atoms for NpO2m+ Ions and NpO2(H2O)nm+ (m = 1−2, n = 1−6) Complexes in the Gas Phase and Aqueous Solution (in parentheses) at the B3LYP Method m=1 q (Np) NpO2m+ NpO2(H2O)m+ NpO2(H2O)2m+ NpO2(H2O)3m+ NpO2(H2O)4m+ NpO2(H2O)5m+ NpO2(H2O)6m+

1.68 1.67 1.68 1.67 1.65 1.63 1.62

(2.21) (2.14) (2.08) (1.98) (1.86) (1.75) (1.69)

q (Oax) −0.34 −0.38 −0.43 −0.43 −0.45 −0.48 −0.58

(−0.60) (−0.62) (−0.65) (−0.59) (−0.58) (−0.56) (−0.64)

m=2 Δq [NpO2+] −0.09 −0.18 −0.19 −0.25 −0.33 −0.54

(−0.10) (−0.22) (−0.20) (−0.28) (−0.37) (−0.59)

the angular data in the later discussions. In addition, it is noted that the bond lengths of the water molecules in these aqua complexes are elongated compared to the ones in the separate water molecule. In Table 3, the electronic charge distributions for neptunium and axial oxygen atoms are presented both in the gas phase and in aqueous solution, and the electronic charge differences for neptunyl ions are also included for a comparison. As can be seen from the table, due to the short-range interactions, the electronic charges for neptunium, axial oxygen atoms, and neptunyl ions become more negative in these aqua complexes. Especially for axial oxygen atoms, the electronic charges decrease by 0.24e from neptunyl ions to the hexa-aqua complex in the gas phase. There is also a significant difference in electronic charge distributions for neptunyl ions with an increasing number of water molecules in the hydrated sphere; the electronic charge decreases by 0.45e from the mono-aqua complex to the hexa-aqua complex in the gas phase. The significant change indicates that the short-range interactions have a strong influence on the electronic charge distributions for neptunyl ions. A similar conclusion can also be obtained for these aqua complexes in aqueous solution.

q (Np) 2.01 1.91 1.85 1.84 1.80 1.78 1.74

(2.51) (2.39) (2.29) (2.17) (1.96) (1.85) (1.77)

q (Oax) −0.01 −0.07 −0.12 −0.14 −0.19 −0.20 −0.29

(−0.25) (−0.32) (−0.36) (−0.31) (−0.28) (−0.25) (−0.32)

Δq [NpO22+] −0.23 −0.39 −0.44 −0.58 −0.62 −0.84

(−0.25) (−0.43) (−0.45) (−0.60) (−0.65) (−0.87)

Furthermore, the influence of the long-range interactions on the electronic charge distributions has also been examined by comparing the ones in the gas phase and those in aqueous solution. As shown in the table, there is a significant difference in electronic charge distributions for the neptunium atom, axial oxygen, and neptunyl ions from the gas phase to aqueous solution, while the difference becomes unobvious with an increasing number of water molecules. For example, the difference is 0.53e and 0.26e for neptunium and axial oxygen atoms in NpO2+ ions, respectively. However, the electronic charges just increase by 0.12e, 0.06e, and 0.04e for neptunium, axial oxygen, and neptunyl ions in the NpO2(H2O)5+ complex, respectively. It is also found that the electronic charge of neptunyl ions Δq[NpO2+] just decreases by 0.01−0.05e for all of the aqua complexes in aqueous solution. This indicates that the long-rang interactions have an obvious influence only on the electronic charge of neptunyl ions but a very small influence on the aqua complexes in aqueous solution. b. Vibrational Frequencies of NpO2(H2O)nm+ (n = 1−6, m = 1−2) Complexes. The symmetric (νss) and asymmetric (νas) stretch modes are two of the most important modes in all kinds of vibrational modes. Therefore, we also study the νss and 3255

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Table 4. Vibrational Frequencies of Symmetric (νss) and Asymmetric (νas) Stretch Modes (in cm−1) for NpO2m+ Ions and NpO2(H2O)nm+ (m = 1−2, n = 1−6) Complexes in the Gas Phase and Aqueous Solution (in parentheses) at the B3LYP Method m=1

m=2

ours

exp/ref

ss NpO2m+ NpO2(H2O)m+ NpO2(H2O)2m+ NpO2(H2O)3m+ NpO2(H2O)4m+ NpO2(H2O)5m+ NpO2(H2O)6m+ a

915 895 875 843 838 829 815

(796) (795) (793) (792) (790) (786) (785)

as 991 968 948 932 910 902 892

(850) (843) (842) (841) (838) (835) (830)

ss 767

ours as

a

ss b

824

821d 806d 826d

850d 833d 864d

926 904 881 865 847 876 840

(839) (823) (822) (822) (820) (815) (803)

exp/ref as 999(942) 990(938) 984(935) 983 (933) 981 (930) 974 (923) 969 (921)

ss c

854 /863

854e

as a

969b

983e

Reference 2. bReference 1. cReference 4. dReference 20. eReference 35.

νas vibrational frequencies for neptunyl ions and these aqua complexes. As usual, the value of 1.0 for the scaling factor is taken for the vibrational frequencies calculation in the present work. Table 4 summaries the symmetric and asymmetric vibrational frequencies for neptunyl ions and these aqua complexes in the gas phase and aqueous solution. As a comparison, the available results from experiments1,2,4 and other calculations20,35 are also included in the table. From the table, we can see that our calculations of O−Np−O symmetric and asymmetric vibrational frequencies are very close to the experimental and other calculated results. For example, in NpO2+ ions, the values of O−Np−O stretch frequencies are 796 (symmetric) and 850 cm−1 (asymmetric) in aqueous solution, which are in very good agreement with the experimental values of 767 (symmetric) and 824 cm −1 (asymmetric).1,2 In the NpO2(H2O)5+ complex, the values of O−Np−O stretch frequencies are 786 (symmetric) and 835 cm−1 (asymmetric) in aqueous solution, which are in accordance with the other calculated values of 806 (symmetric) and 833 cm−1 (asymmetric).20 The consistency of the available data provides convincing evidence to testify to our calculations of vibrational frequencies for these aqua complexes. As expected, due to the short-range interactions in these aqua complexes, the νss and νas vibrational frequencies become lower gradually with an increasing number of water molecules. It means that the short-range interactions have a strong influence on the calculations of νss and νas vibrational frequencies. In addition, it is found that the vibrational frequencies tend to be lower in aqueous solution, and the difference of the vibrational frequencies between the gas phase and aqueous solution becomes unobvious for the n = 4−6 complexes. It suggests that the long-range interactions from the continuum medium become weak for these n = 4−6 aqua complexes. Figures 2 and 3 show the calculated IR spectra for all of the aqua complexes in the gas phase and in aqueous solution. As clearly shown in the figures, the IR spectra in the gas phase are very different from the counterparts in aqueous solution for these aqua complexes. There are more lines (red lines) that appear in aqueous solution, especially in the frequency range of 0−600 cm−1; it is primarily caused by the splitting of lines due to the solvent effects. We also note that the lines in the frequency range of 0−1000 cm−1, which in involved in vibrational modes of neptunyl ions, have a little red shift in aqueous solution for all series of aqua complexes; it is quite consistent with the trend of increase of NpO bond lengths. While the lines around 3700 cm−1 that come from the water

Figure 2. IR spectra of NpO2(H2O)n+ (n = 1−6) complexes in the gas phase (black line) and aqueous solution (red line) at the B3LYP method.

Figure 3. IR spectra of NpO2(H2O)n2+ (n = 1−6) complexes in the gas phase (black line) and aqueous solution (red line) at the B3LYP method.

molecule vibrations have a slight blue shift in aqueous solution, it is also consistent with the trend of decrease of the O−H bond length in the water molecules. As can be seen from the figures, the differences of IR spectra between the gas phase and aqueous solution become small for the n = 4−6 aqua complexes. It suggests that the influence of the long-range interactions becomes weak in these aqua complexes. The Np−H2O vibrational frequencies in NpO2(H2O)nm+ (n = 1−6, m = 1−2) complexes have also been calculated. The vibrational frequencies have a little red shift with the increasing number of water molecules in the gas phase and aqueous solution. For example, in the series of NpO2(H2O)n+ (n = 1−6) complexes, the vibrational frequencies are about in the range of 264−317, 260−314, 251−292, 244−288, and 232−252 cm−1 from n = 1 to n = 6 complexes in the gas phase, respectively. 3256

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On the basis of the above statements, neptunyl ions tend to form the aqua complexes with the coordination of water molecules in a hydrated sphere. Therefore, it is necessary to study the chemical bond characteristics for these aqua complexes. Here, the TDOS of NpO2(H2O)nm+ (n = 0−6, m = 1−2) complexes have been calculated both in the gas phase and in aqueous solution. As an example, the results of NpO2(H2O)n+ (n = 0−6) complexes are shown in Figure 5.

The decrease of the vibrational frequencies is consistent with the trend of the increase of the distances between neptunyl ions and water molecules. Furthermore, we find that all of the frequency differences between the gas phase and aqueous solution become unobvious with an increasing number of the water molecules. This suggests that the long-range interactions from the continuum medium have only a small influence on the vibrational frequencies in these aqua complexes. c. Chemical Bond Characteristics of NpO2(H2O)nm+(n = 0−6, m = 1−2) Complexes. In order to further understand the properties of neptunyl ions in aqueous solution, the chemical bond characteristics of neptunyl ions and NpO2(H2O)nm+ (n = 0−6, m = 1−2) complexes are also studied both in the gas phase and in aqueous solution. Here, the chemical bond characteristics are demonstrated clearly by calculating the TDOS and PDOS for neptunyl ions and these aqua complexes. Figure 4 shows the TDOS and PDOS of outer-shell electrons of neptunium and axial oxygen atoms in NpO2+ ions both in

Figure 5. TDOS of NpO2(H2O)n+ (n = 0−6) complexes in the gas phase (short dash) and aqueous solution (solid).

As can be seen from the figure, due to the short-ranges interactions, the orbital energies of all electrons become higher with the increasing number of the water molecules in these aqua complexes. There are obvious differences of the TDOS in mono-, di-, and tri-aqua complexes, while the difference becomes unobvious in n = 4−6 aqua complexes. On the other hand, due to the weak influences of long-range interactions on these aqua complexes in aqueous solution, the differences of the TDOS among these aqua complexes are very small and very similar to the ones in the gas phase. Especially, the DOS of the n = 4−6 complexes in the gas phase are very similar to the counterparts in aqueous solution; the differences of orbital energies are just about 4.0 eV between the gas phase and aqueous solution. Furthermore, it is also found that the DOS of the n = 4−6 aqua complexes in the gas phase are close to the DOS of neptunyl ions in aqueous solution; the differences of orbital energies are just about 3.0 eV. A similar trend is also observed in NpO2(H2O)n2+ (n = 0−6) complexes. This means that the influence of long-range interactions is very small on the chemical bond characteristics for these aqua complexes. As a result, in further studies, we can use the results of these aqua complexes in the gas phase to approximately simulate the solvation effects of neptunyl ions in aqueous solution. Figure 6 shows the TDOS and PDOS of the outer-shell electrons in the n = 3−6 aqua complexes. As can be seen from the figure, there is a partial overlap between the 5f, 6d, and 7s orbitals and the 2p orbital in the tri-aqua complex. While the 2p orbital of the axial oxygen atoms occurs obviously split in the n = 4−6 complexes, most of the 2p orbital of the axial oxygen atoms lies away from the 5f, 6d, and 7s orbitals of neptunium in these n = 4−6 aqua complexes; there is just a little overlap among these orbitals. These small overlaps revel that the interaction between neptunium and the axial oxygen becomes weaker in these aqua complexes; it is primary due to the association of water molecules in the hydrated sphere. A similar

Figure 4. TDOS and PDOS for neptunium 5f, 6s, 6p, 6d, and 7s and axial oxygen 2s and 2p electrons of NpO2+ ions in the gas phase (short dash) and aqueous solution (solid).

the gas phase (short dash lines) and in aqueous solution (solid lines), in which the orbital energies are in the ranges of −60 to 0 eV. As clearly shown in the figure, in the low-energy ranges, the DOS are contributed from the 6s, 6p electrons of neptunium and the 2s electrons of axial oxygen atoms, respectively. For example, in the energy range from −60 to −50 eV, the DOS are mainly contributed from the 6s electrons of neptunium, while in the energy ranges from −40 to −20 eV, the DOS are contributed from the 6p electrons of neptunium and the 2s electrons of the axial oxygen atoms, respectively. However, in the case of high-energy ranges, the DOS are contributed from the 5f, 6d, and 7s electrons of neptunium together with the 2p electrons of the axial oxygen atoms. It is found that the orbital energies of the 5f, 6d, and 7s electrons of the Np element are very close to the energies of the 2p electrons of the axial oxygen atoms, and there are some overlaps among these orbitals. As can be seen, there are partial overlaps between the 5f, 6d, and 7s orbitals and the 2p orbital from −20 to 0 eV in the gas phase; this suggests that the 5f, 6d, and 7s orbitals of the neptunium bond with the 2p orbital of the axial oxygen atoms in neptunyl ions. In aqueous solution, a similar trend is observed, but the overlaps between the 5f orbital and 2p orbital become unobvious, and the energy ranges of bonding orbitals become higher evidently compared to the ones in the gas phase. This shows that the long-range interactions have a great effect on chemical bond properties of neptunyl ions without considering the coordination of water molecules. 3257

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considered the influence of the BSS; it is found that there is only a small influence of the BSSE in our calculations. For example, the binding energies increase by 12 kcal/mol when considering the influence of the BSSE for the penta-aqua complex. In fact, Zhiji Cao and K. Balasubramanian have already obtained similar conclusions according to their calculated binding energies of NpO2(H2O)n+ (n = 4−6) complexes for different methods.20 They found that only five-coordinated species are preferred for the neptunyl ion in aqueous solution. Even though the binding energy of the hexa-aqua complex is the lowest in their calculations, the geometry of the hexa-aqua complex has five water molecules in the first coordination shell and the sixth water hydrogen bonded to two of the other water molecules in a bridge configuration. It means that the n = 5 complex is the most stable; it is consist with our conclusion completely. Furthermore, in order to understand the stability of these aqua complexes, the TDOS and PDOS of the neptunium element and the equatorial oxygen atoms in these aqua complexes are calculated and shown in Figure 8.

Figure 6. TDOS and PDOS for neptunium 5f, 6s, 6p, 6d, and 7s and axial oxygen 2s and 2p electrons of NpO2(H2O)n+ (n = 3−6) complexes in the gas phase.

conclusion can also be found in NpO2(H2O)n2+ (n = 3−6) complexes. d. Binding Energies for NpO2(H2O)nm+ (n = 1−6, m = 1−2) Complexes. In order to discuss the interaction strength between the neptunyl ion and the water molecule in these aqua complexes, the binding energies between the neptunyl ion and the equatorial water molecule have been calculated. In Figure 7, the binding energies for NpO2(H2O)nm+ (m = 1−2, n = 1−6) complexes are displayed along with the available theoretical calculations for comparison. Figure 8. TDOS and PDOS of neptunium 5f, 6d, and 7s and equatorial oxygen 2s and 2p electrons in NpO2(H2O)n+ (n = 1−6) complexes in the gas phase.

As can be seen from the figure, in the n = 3 complex, the 5f, 6d, and 7s orbitals lie away from the 2p orbital of the equatorial oxygen atoms; there is just slight overlap between the 6d and the 2p orbitals. It means that the interaction between the neptunyl ion and the water molecule is relatively weak. However, in the n = 4−6 complexes, there is a partial overlap between 5f, 6d, and 7s orbitals and the 2p orbital. In particular, in the n = 5 complex, there is another complete overlap between the 5f orbital and the 2p orbital in the energy ranges from −11 to −10 eV. It suggests that there is also a strong interaction between neptunium and the equatorial oxygen atoms. As a result, the n = 5 complex has the lowest binding energies. In NpO2(H2O)n2+ (n = 3−6) complexes, similar results of the TDOS and PDOS for these aqua complexes have also been obtained.

Figure 7. Variation trends of binding energy of NpO2(H2O)nm+ (m = 1−2, n = 1−6) complexes.

As presented in the figure, the binding energies are in the ranges from −25 to −35 kcal/mol for these aqua complexes except for the n = 5 complex. It is interesting that the binding energies for the penta-aqua complex become very low rapidly; it is about −145 kcal/mol for the NpO2(H2O)5+ complex and −245 kcal/mol for the NpO2(H2O)52+ complex. The behavior indicates that the interactions between the neptunyl ion and water molecule is the strongest in the penta-aqua complex among all of these aqua complexes. It means that neptunyl ions prefer to exist in aqueous solution with combining five water molecules in a hydrated sphere. In addition, we have also

4. CONCLUSIONS In the present work, we have investigated the geometrical structures, charge distributions, vibrational frequencies, and IR spectra of NpO2(H2O)nm+ (m = 1−2, n = 0−6) complexes in the gas phase and aqueous solution. Our calculated structural parameters of neptunyl ions and these aqua complexes are in reasonably good agreement with the experimental data and other calculations. The significant difference of the structural properties indicates that the short-range interactions in the 3258

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aqua complexes have a strong influence on the central neptunyl ions. Furthermore, we reoptimized the structures of these aqua complexes in aqueous solution. It is found that the long-range interactions from the continuum medium only have a strong influence on structural properties of neptunyl ions but a weak influence on those of these aqua complexes. In order to understand the bond characteristics of these complexes, we have further systematically calculated their DOS in the gas phase and aqueous solution. The calculations show that the solvation effects can be simulated approximately by the results of the aqua complexes in the gas phase. It is also found that the interactions between neptunium and the axial oxygen atoms become weak due to the association of water molecules in these aqua complexes. Finally, the stability of these aqua complexes has been discussed by calculating the binding energies between the neptunyl ion and water molecule in a hydrated sphere. The results reveal that the penta-aqua complex is the most stable in these aqua complexes. It is mainly due to the orbital overlaps between the 5f electrons of neptunium and the 2p electrons of the equatorial oxygen atom.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 91126007). REFERENCES

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