A Systematic Theoretical Study of UC6: Structure, Bonding Nature

Nov 2, 2017 - The study of uranium carbides has received renewed attention in recent years due to the potential use of these compounds as fuels in new...
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A Systematic Theoretical Study of UC6: Structure, Bonding Nature, and Spectroscopy Jiguang Du*,† and Gang Jiang‡ †

College of Physical Science and Technology and ‡Insitute of Atomic and Molecular Physics, Sichuan University, Chengdu 610064, People’s Republic of China S Supporting Information *

ABSTRACT: The study of uranium carbides has received renewed attention in recent years due to the potential use of these compounds as fuels in new generations of nuclear reactors. The isomers of the UC6 cluster were determined by DFT and ab initio methods. The structures obtained using SCRECP for U were generally consistent with those obtained using an all-electron basis set (ZORA-SARC). The CCSD(T) calculations indicated that two isomers had similar energies and may coexist in laser evaporation experiments. The nature of the U−C bonds in the different isomers was examined via a topological analysis of the electron density, and the results indicated that the U−C bonds are predominantly closed-shell (ionic) interactions with a certain degree of covalent character in all cases, particularly in the linear species. The IR and UV−vis spectra of the isomers were theoretically simulated to provide information that can be used to identify the isomers of UC6 in future experiments. tricarbide.10 Pogány et al.11 investigated the molecular structures and bonding characters of uranium tetracarbides. A planar fan structure with C2v symmetry, in which the U atom is connected to a bent C4 unit, was predicted to be the lowestlying isomer of UC4. In the present work, we examine the molecular structures and bonding characters of the gas-phase UC6 molecule, which were also observed with high intensity in the mass spectrum of a UC4 sample.7 To the best of our knowledge, only two isomers of UC6 have been previously investigated at the density functional theory (DFT) level.6 The main purpose of the present work is to clarify the growing patterns of UCn clusters. The isomers of UC6 were determined by DFT and ab initio calculations in the present work. The bonding characters of the different isomers were determined using the quantum theory of atoms in molecules (QTAIM), and IR and ultraviolet−visible (UV−vis) spectra were simulated to provide useful information for identifying the isomers of UC6 in future experiments.

1. INTRODUCTION Uranium carbides in the solid phase, which can potentially be used as fuels in the new generation of nuclear reactors, are very relevant in the nuclear industry.1,2 The vaporization of solidphase uranium−carbon compounds is very important due to the high temperatures in the nuclear reactors. Therefore, it is necessary to understand the properties of gas-phase compounds in detail. Moreover, the study of molecular actinide carbides provides a basic understanding of the 5f-electron characteristics of actinide atoms. Mass spectrometry studies of gaseous uranium carbides3,4 provided information about the stoichiometry of the compounds formed in the gas phase, their relative abundances, and several thermodynamic parameters. Wang et al.5,6 obtained gaseous uranium−carbon compounds through laser evaporation of carbon-rich uranium/carbon alloys followed by reaction of the atoms in a solid argon matrix. Infrared (IR) absorption bands related to UC and UC2 were observed in their experiments. More recently, gas-phase molecular thorium and uranium carbide cluster cations were produced from the laser ionization of AnC4 alloys (An = Th, U), and the products were detected by Fourier transform ion cyclotron resonance mass spectrometry.7 The molecular structures of UC2 have been investigated by high-level multiconfigurational methods.8,9 A triangular species corresponding to a 5A2 electronic state was predicted to be more stable than the linear CUC species observed in a recent infrared spectroscopy experiment.5 In a theoretical study, a fan geometry in which the uranium atom is bonded to a quasilinear C3 unit was identified as the most stable isomer of uranium © XXXX American Chemical Society

2. COMPUTATIONAL METHODS All calculations were carried out using the ORCA-3.0.3 package.12 The molecular geometries of different isomers were optimized using the hybrid HF/DFT B3LYP functional.13 The segmented all-electron relativistically contracted (SARC)14 Gaussian-type basis set was employed to describe the U atom, and a relativistically recontracted version of the triple-ζ valence basis set (ZORA-def2-TZVPP)15 was used for the light C atoms. The scalar relativistic effect was examined Received: July 21, 2017

A

DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Key structural parameters for the relaxed structures of the different isomers of UC6 obtained at the B3LYP-ZORA/def2-TZVPP-SARC and B3LYP/def-TZVPP-RECP (in parentheses) levels of theory, respectively. using the zero-order regular approximation (ZORA).16 For the sake of comparison, the small-core scalar relativistic effective core potential (SC-RECP) was used in combination with the [10s9p5d4f3g] basis set17−19 to describe the U atoms, and the triple-ζ valence basis set defTZVPP20 was used for the C atoms. These two calculational schemes are defined as B3LYP-ZORA/def2-TZVPP-SARC and B3LYP/defTZVPP-RECP, respectively, for the following discussion. Harmonic vibrational frequency calculations were performed at the same level of theory to ensure that the structures we obtained were at the real local minima on the potential energy surface. For heavy atoms, the size of the integration grid in a numerical integration is important for obtaining reasonable results; therefore, an integration grid size of 12 was chosen to describe the U atoms in all calculations. The electronic states with different spin multiplicities were calculated for each system to determine the ground electronic state. The single-point energies were also calculated using coupled cluster theory with single and double excitations and a perturbative triples correction (CCSD(T))21 to obtain a more reasonable energy order for the different isomers. Previous works10,22 have indicated that CCCD(T) can predict energy order similarly to the multireference configuration interaction (MRCI), and these works have also shown that the spin−orbit coupling (SOC) produces low incremental energy difference. The SOC was not considered to evaluate the energy order for the different isomers in the present work. The bonding natures of different UC6 isomers were analyzed using QTAIM methods.23 Topological analyses were performed with the MULTIWFN program24 based on the all-electron wave functions obtained at the B3LYP-ZORA/def2-TZVPP-SARC level of theory. Time-dependent density functional theory (TD-DFT) was utilized to study the electronic spectra. The excitation energies of several selected isomers were calculated with the CAM-B3LYP functional25 in conjunction with the all-electron basis set (SARC for U, ZORAdef2-TZVPP for C). A total of 80 transitions were included in the excited state calculations to represent the absorption spectra up to approximately 5 eV. In the analysis of the cofactor spectra, we made extensive use of natural transition orbitals (NTOs),26 which give the most compact description of the orbital space involved in the electronic excitation.

3. RESULTS AND DISCUSSION 3.1. Isomers of UC6. Numerous initial structures were optimized to find the low-lying isomers of UC6. After extensive structure searching, eight stable isomers (shown in Figure 1) which have the lowest relative energy were selected for detailed analysis. For the eight isomers, the different spin multiplicities were considered using two schemes, B3LYP-ZORA/def2TZVPP-SARC and B3LYP/def-TZVPP-RECP, to obtain the lowest-lying spin state. The relative energies (in eV) of the different spin multiplicities of each isomer are shown in Table S1 of the Supporting Information. All of the isomers except for VIII prefer an open-shell electronic state (either triplet or quintuplet). The SC-RECP calculations showed excellent consistency with the results of the all-electron basis set (ZORA-SARC), and they generally predicted the same ground state except in the case of the VII isomer. The relaxed structures of the most relevant isomers in their ground states are depicted in Figure 1, and the structural parameters predicted at the B3LYP level of theory are also shown in Figure 1. The detailed Cartesian coordinates of the isomers are given in Table S2 of the Supporting Information. All isomers shown in Figure 1 were determined to be local minima on the potential energy surface by assessing the vibrational frequency. A fan-type structure (Figure 1) with a quintet state, which contains one U atom connected to a bent C6 unit, was found to have the lowest total energy. The average distance between U and the bonding carbon atoms was 2.572 Å in the prediction by ZORA-SARC, which is slightly larger than that of SC-RECP. The fuzzy bond order (FBO)27 indicates that single-bond character is favored for U−C bonding. The C−C interactions in the fan isomer are the strongest among all isomers of UC6, as revealed by its largest C−C FBO values. We should mention that the U−C bond length in the fan isomer of UC6 is significantly larger than that in a UC3 system reported in B

DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry previous work.10 This is because the fan isomer of UC6 has a higher U−C coordination number than the UC3 species. Isomer II of UC6, which is formed by adding a C2 unit to the outer plane of the fan structure of UC4 (predicted to be the lowest-lying isomer of UC4),11 has the next lowest energy. Both the U−C and C−C bond lengths in isomer II are slightly shorter than those in the fan structure. The linear structure that has the U atom bonded at the end of the C chain shows the third highest stability at the B3LYP level of theory. The U−C bond distance is very close to that in the linear isomer of UC3. The high strength of the U−C bond in the linear isomer is revealed by its high FBO value of 1.78, which indicates doublebond character. By adding one C atom to the outer plane of the quasi-fan structure of UC5, isomer IV is obtained, and the C−C bond lengths in this isomer are significantly longer than those in the first three isomers. Isomer V is constructed by placing a quasi-linear C3 unit on the perpendicular plane of the fan structure of UC3. The binding of two C3 rings to the U atom forms isomer VI. For isomer VII, the U atom is bonded to a highly deformed C6 ring. By binding of three C2 units to the U atom, the tricyclic isomer VII can be formed. It should be mentioned that the U−C interaction in this isomer is very strong, as indicated by its short U−C bond distance and high FBO values of the U−C bonds. The Hirshfeld28 and Voronoi deformation density (VDD)29 charges were calculated and are presented in Table 1. The

The dissociation energies for all the possible dissociation pathways were calculated for isomer I of UC6. All six dissociation channels and corresponding energies are given in Table S3 in the Supporting Information. It can be seen that the loss of a C3 fragment needs the smallest energies and is likely to be the dominant dissociation pathway. This can be explained as the special structural stability of the C3 fragment. The dissociation energy for the loss of a UC2 or UC4 fragment is also low because the high stability of UC2, UC4 fragment, as evidenced in the mass spectrum of the UC4 alloy sample.7 The relative energy was calculated at different levels of theory, including B3LYP/def-TZVPP-RECP, B3LYP-ZORA/ def2-TZVPP-SARC, and CCSD(T)/def-TZVPP-RECP with Hartree−Fock (HF) and DFT (B3LYP) initial orbitals, respectively. The predicted values are collected in Table 2, Table 2. Relative Energies of the Eight Most Relevant Isomers of UC6 Predicted at Different Levels of Theory ΔE (eV) I 0.00 0.00 0.16

Table 1. Total Fuzzy Bond Order (FBO) Values for U−C and C−C Bonds in the Isomers of UC6 and the Hirfsheld and VDD Charges of the U Atom species I II III IV V VI VII VIII a b

FBOU−C

FBOC−C

QH(U)

QV(U)

5.40a 5.49b 7.77 7.76 1.78 1.77 6.76 6.58 7.97 7.88 5.66 5.60 5.80 5.73 10.49 10.47

9.82 7.96 6.72 6.71 9.48 9.48 7.07 7.10 6.10 6.10 7.28 7.27 7.82 7.80 5.17 5.14

0.61

0.46

0.94

0.79

0.69

0.68

0.84

0.76

0.93

0.72

0.98

0.83

0.74

0.71

0.98

0.83

0.21

II

III

IV

V

VI

VII

B3LYP/def-TZVPP-RECP 0.74 1.02 1.53 3.13 3.17 3.92 B3LYP-ZORA/def2-TZVPP-SARC 0.65 0.90 1.50 3.03 3.07 3.73 CCSD(T)/def-TZVPP-RECP (HF orbital) 0.00 1.81 1.94 2.61 2.86 3.71 CCSD(T)/def-TZVPP-RECP (B3LYP orbital) 0.00 1.73 1.59 2.91 3.00 3.81

VIII 5.16 4.35 3.20 3.34

from which one can observe that at the B3LYP level SC-RECP predicted the same energy order as the all-electron basis set (ZORA-SARC). In the CCSD(T) calculation, two different starting orbitals, Hartree−Fock (HF) and B3LYP, were considered. The T1 diagnostic30 shown in Table S4 in the Supporting Information was used to assess the validity of the single reference method. In most cases, the T1 values do not indicate a strong multireference character of the wave functions, as shown in Table S4. Previous studies10,22 on actinide carbide compounds have also shown that the results of multiconfigurational calculations are consistent with those of CCSD(T). Our results indicate that the CCSD(T) method predicted a different energy order in comparison to B3LYP. At the CCSD(T) level, the fan-type isomer is predicted to be less stable than isomer II. With HF initial orbitals, the energy differences between I and II is 0.16 eV, and for DFT (B3LYP) starting orbitals, the energy difference is 0.21 eV. According to the results of the CCSD(T) calculation, we can conclude that the fan structure and isomer II (a C2 unit on the outer plane of the UC4 fan structure) are comparable in energy and may coexist in a laser evaporation experiment. The CCSD(T) calculation using DFT (B3LYP) starting orbitals predicted that the linear isomer is less stable than isomer IV. The relative stability of tricyclic isomer VIII is slightly enhanced at the CCSD(T) level. Moreover, from Table S4, one can see that the T1 diagnostic is reduced when DFT orbitals are used. This indicates the usefulness of using DFT orbitals in CCSD(T) calculations. More recently, Fang et al.31 also found that the use of DFT orbitals in CCSD(T) calculations improved the predictions for diatomic transitionmetal compounds which have large T1 diagnostics at the HFCCSD(T) level. 3.2. Chemical Bonding. To reveal the nature of the interaction between U and the coordinating C atoms in the

Values calculated at the B3LYP-ZORA/def2-TZVPP-SARC level. Values calculated at the B3LYP/def-TZVPP-RECP level (in italics).

charge of the U atom in the isomers predicted by VDD population is generally smaller than that of the Hirshfeld prediction. The U atom in all of the isomers acts as an electron donor due to its low first ionization energy. The transfer of charge from U to the neighboring C atoms suggests the presence of an electrostatic interaction in the UC6 isomers. From Table 1, one can note that the atomic charges of U in the VI and VIII isomers are very large (close to 1). From a structural perspective, the U atom in these two isomers is bonded to a highly stable C fragment, either C2 (VIII) or a C3 ring (VI). This implies that a strong electrostatic interaction exists between U and the C2 (C3) fragment in these isomers. C

DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Molecular graphs of the different UC6 species. Color scheme identifying critical points: cyan spheres indicate attractors, blue spheres are used for bond critical points (BCPs), and red spheres indicate ring critical points (RCPs).

Table 3. Topological Parameters for the U−C Bond Critical Points (BCPs) in the Studied Species species I II III IV V VI VII VIII

bond

ρ(r)

∇2ρ

H(r)

G(r)

V(r)

−V(r)/G(r)

ELF

LOL

U−C C−C U−C C−C U−C C−C U−C C−C U−C C−C U−C C−C U−C C−C U−C C−C

0.061 0.390 0.082 0.402 0.134 0.357 0.077 0.322 0.076 0.360 0.106 0.289 0.109 0.275 0.115 0.395

0.124 −1.111 0.162 −1.262 0.169 −1.094 0.151 −0.895 0.137 −1.106 0.209 −0.576 0.163 −0.647 0.220 −1.225

−0.011 −0.525 −0.022 −0.543 −0.065 −0.440 −0.019 −0.360 −0.020 −0.451 −0.038 −0.297 −0.043 −0.276 −0.038 −0.527

0.042 0.267 0.063 0.227 0.107 0.166 0.057 0.136 0.054 0.174 0.090 0.153 0.084 0.114 0.093 0.221

−0.053 −0.772 −0.085 −0.770 −0.172 −0.606 −0.077 −0.496 −0.074 −0.625 −0.128 −0.450 −0.127 −0.390 −0.132 −0.748

1.262 2.891 1.349 3.392 1.607 3.651 1.351 3.647 1.370 3.592 1.422 2.941 1.512 3.421 1.419 3.385

0.30 0.85 0.34 0.88 0.47 0.91 0.33 0.91 0.36 0.90 0.36 0.85 0.42 0.90 0.41 0.88

0.39 0.71 0.42 0.73 0.49 0.76 0.41 0.76 0.43 0.75 0.43 0.70 0.46 0.75 0.46 0.73

heavy atoms. Cremer and Kraka33 proposed the total energy density H(r) (defined as the sum of the local kinetic energy density G(r) and the local potential energy density V(r)) as an appropriate parameter for characterizing the degree of covalency of a bond. Generally, the more negative the H(r) value, the more stable the interaction. One can also employ the −V(r)/G(r) ratio as another useful description; −V(r)/G(r) < 1 is characteristic of a typical ionic bond, −V(r)/G(r) > 2 is diagnostic of a classical covalent interaction, and ratios between 1 and 2 suggest partially covalent bonds. Table 3 collects the topological parameters relevant to U−C and C−C BCPs. Overall, the C−C bonds in the isomers are typical of shared (covalent) interactions, which can be concluded from the high electron density (ρ(r)) values, the negative Laplacian ∇2ρ(r) value, −V(r)/G(r) values >2, and the low negative values of the total energy densities, H(r). The values for the electron localization function (ELF)34−36 and localized orbital locator (LOL)37 at the BCPs were also calculated. The covalent nature of the C−C bonds is also confirmed by the high ELF (larger than 0.8) and LOL (larger than 0.7) values, as shown in Table 3. For the U−C bond, a low ρ(r) value and a positive ∇2ρ(r) value confirm its closed-shell interaction. Meanwhile, we note that the U−C bonds have negative H(r) values with small absolute quantities, indicating that the U−C bonds have a certain degree of covalent character. The shortest U−C bond, which is in linear isomer III, shows a high degree of covalent

relevant isomers of UC6, QTAIM analyses were performed on the basis of the wave function obtained at the B3LYP-ZORA/ def2-TZVPP-SARC level of theory. The bond paths and locations of the bond critical points (BCPs) are depicted in Figure 2. In the fan isomer, in addition to the two peripheral U−C bond critical points, one BCP located between U and the central C−C BCP was found, resulting in the formation of a bicyclic species with two ring critical points (RCPs). In isomer II, three U−C BCPs and one five-membered ring exist in the fan plane of UC4. Only BCPs between adjacent atoms were found in linear isomer III; therefore, we do not show its molecular graph in Figure 2. The molecular graph clearly shows a four-membered ring in isomers IV and V. In addition to C− C−C rings, two three-membered U−C−C rings are also found in isomer VI, which is consistent with its short U−C bond lengths, as shown in Table 1. In the case of isomer VII, two U− C BCPs, three- and five-membered C rings, and a U−C−C ring are observed. Isomer VIII does not show any RCPs; only three C−C and U−C BCPs connecting U and C2 units are found. This isomer can be described as a U atom strongly π-bonded to a C2 moiety [9], which is consistent with its short U−C bond length and high FBO value. According to the criteria used for the QTAIM analysis, a covalent interaction corresponds to a negative ∇2ρ value at the critical point (CP). However, previous work32 indicated that this criterion is not sufficient to describe the bond natures of D

DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry character, as revealed by its high ρ(r) value, more negative H(r) value, and −V(r)/G(r) ratio close to 2. In general, our QTAIM results indicate that the U−C bonds have a predominantly ionic (closed-shell) bonding nature with a small degree of covalency, which is also supported by the results of the ELF and LOL calculations, as shown in Table 3. We also performed density of states (DOS) and molecular orbital (MO) analyses to understand the bonding properties. In the DOS analysis, the discrete energy levels are broadened to curve by a Gaussian function and the full width at halfmaximum (fwhm) is 0.8 eV. This analysis has been successfully employed in recent computational studies involving 5f-electron complexes.38−40 Figure 3 shows the partial density of states

bonds, as shown in HOMO-18 and HOMO-19. This phenomenon was also found in another U−C system (uranium dicarbide on a defective graphene surface).41 The bonding characteristics of isomer II are very similar to those of isomer I. A positive OPDOS is found at approximately −10 eV, and those bond overlaps are derived from U 6d and C 2p orbital interactions, as shown in the bonding MOs HOMO-11, HOMO-8, and HOMO-5. The 5f-character bonding orbitals HOMO-2 and HOMO are responsible for the small OPDOS peak near the HOMO energy level (vertical dashed line in Figure 3b). The 6p electrons of the U atom are also coupled with the 2s electrons of the C atoms in the low-energy region, which is confirmed in the HOMO-16 and HOMO-18 orbitals. From Figure S1 of the Supporting Information, the OPDOS between U and C in isomer II is more expansive than that in isomer I; moreover, the bonding overlap population of isomer II occurred at an energy lower than that for isomer I. Therefore, the interaction between U and the C atoms in isomer II is stronger than that in isomer I. This is in line with the results from QTAIM, in which the U−C bonds in isomer II generally have larger ρ(r) values and more negative H(r) values relative to those in isomer I. As Figure S1 shows, in addition to forming bonding orbitals, the 6p electrons of U can participate in antibonding interactions with the 2p electrons of the C atoms. 3.3. Spectra of Low-Lying Isomers. Infrared (IR) and ultraviolet−visible (UV−vis) spectra were theoretically simulated to provide information that can be used to identify the isomers of UC6 in future experiments. The IR spectra of the first four isomers obtained at the B3LYP-ZORA/def2-TZVPPSARC level of theory are depicted in Figure 4. The IR spectra

Figure 3. Partial density of states (PDOS) of U and C atoms in isomers I (a) and II (b) of the UC6 cluster, the OPDOS between orbitals of U and C atoms, and the molecular orbitals (MOs) (α-spin) relevant to the U−C interactions. The vertical dashed line indicates the level of the HOMO.

(PDOS), the bond overlaps DOS (OPDOS) between the U and C atoms, and the MOs relevant to the U−C bonds for the fan structure (I) and isomer II. From Figure 3a, one can see that there are clear interactions (positive OPDOS values) between U and the C atoms in the region of −12 to −9 eV for isomer I. The interactions are mainly derived from the orbital overlap between the 6d electrons of U and the 2p electrons of the C atoms, as evidenced by the bonding MOs HOMO-7, HOMO-9, and HOMO-11 (insert in Figure 3a), and the detailed orbital compositions of those MOs are provided in Table S5 of the Supporting Information. The 5f electrons of the U atom are mainly distributed in higher energy levels relative to the 6d electrons and are also involved in bonding with C 2p. The contribution from U 5f is approximately 12% in the characteristic 5f bonding orbitals (HOMO-6 and HOMO-5). It is very interesting to note that there are obvious orbital overlaps between the 6p electrons of the U atom and the 2s electrons of the C atoms in the very low energy region, resulting in σ-type

Figure 4. IR spectra of the first four low-lying isomers of UC6.

were simulated by broadening discrete lines with Lorentzian function, and the fwhm is 20 cm−1. In addition, detailed vibrational frequency and IR intensity data are given in Table S6 of the Supporting Information. The fan-type isomer I shows a somewhat complicated spectrum with many peaks due to the high coordination number of this species. The peak at 228 cm−1 can be identified as the antisymmetric stretching vibration of peripheral U−C bonds. The C−C bending vibration produces an intense peak at 551 cm−1. As expected, the C−C stretching vibration shows a peak in the high-frequency region. The lowest vibrational frequency for isomer I is 125 cm−1, which is E

DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

4. CONCLUSION The isomers of the UC6 cluster were investigated by DFT and ab initio calculations. At the B3LYP level of theory, a fan-type structure (I) was predicted to be the most stable isomer, and the structures and energy orders of the isomers obtained using SC-RECP for U were generally consistent with those from the all-electron basis set (ZORA-SARC). Isomer II of UC6, which is constructed by adding a C2 unit to the outer plane of the fivemembered, lowest-lying fan structure of UC4, is the next lowest in energy and is approximately 0.7 eV higher than that of fan structure I. In comparison to B3LYP, the energy order of the studied isomers changed when the high-level CCSD(T) calculation was employed. Isomer I is approximately 0.16 eV less stable than isomer II at the CCSD(T) level. According to the CCSD(T) results, isomers I and II are comparable in energy and may coexist in laser evaporation experiments. Topological analyses of the electron density were performed for all isomers to understand the U−C bonding nature. It was determined that the U−C bonds are predominantly closed-shell (ionic) interactions. Nevertheless, a partial covalent character was also observed in all cases and particularly in the linear species that has only one U−C bond. IR and UV−vis spectra were theoretically simulated to provide information that could be used to identify the isomers of UC6 in future experiments. One clear peak corresponding to U−C2 stretching in the IR spectrum of isomer II can be used to distinguish isomers I and II, which have comparable energies at the CCSD(T) level. Different absorption features were also observed in the UV−vis spectra of the low-lying isomers.

sufficiently large to meet a stability criterion suggested by Hoffmann et al.42 Isomer II, which has a energy comparable to that of the fan structure (I), exhibits a very different peak feature in comparison to the fan isomer. Only one clear peak is observed in the IR spectrum of isomer II at 529 cm−1, which can be assigned to the stretching between U and the C2 unit. This fingerprint of U−C2 bonding may be used to distinguish the comparable isomers (I and II) in future experiments. The linear isomer III shows a clear peak at 2190 cm−1 from the C− C stretching vibration. In the case of isomer IV, U−C and C−C stretching vibrations produce intense peaks at 298 and 1041 cm−1, respectively. The excitation energies were calculated within the TDDFT framework at the CAM-B3LYP-ZORA/def2-TZVPP-SARC level of theory to simulate the UV−vis spectra. The CAMB3LYP functional has been proven to be appropriate for quantitative studies of actinide spectra.43 The natural transition orbitals (NTOs), which best describe the electronic excitation from one donor (hole) orbital to one acceptor (electron) orbital, are analyzed. In the simulation of UV−vis spectra, the Lorentzian function was selected for broadening discrete lines as curves with an fwhm of 15 nm. Figure 5 shows the simulated

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01871. Relative energy of different spin multiplicities, Cartesian coordinates predicted at the B3LYP level and calculated T1 values at the CCSD(T) level for isomers of UC6, dissociation energy of isomer I, orbital compositions relative to U−C bonds in Figure 3 and OPDOS for isomers I and II, and vibrational frequencies and electronic transition properties for the first four lowlying isomers of UC6 (PDF)

Figure 5. UV−vis spectra of the first four low-lying isomers of UC6.

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AUTHOR INFORMATION

Corresponding Author

spectra of the first four isomers, and the important absorption features, including absorption wavelength (λ) in nm, oscillator strength (f), and major NTOs, are given in Table S7 of the Supporting Information. For fan-type isomer I, we can find two clearly discrete absorption bands in the UV region. One corresponds to the π(C) → d(U) transition and the other to the d(U)→ p(U) transition. One wide absorption band is found for isomer II, and the main transition can be assigned as either σ(C) → f(U) or π(C) → f(U), as shown in Table S6 of the Supporting Information. There are two discrete bands in the spectrum of linear isomer III. One intense peak at 405 nm can be assigned as the π(C) → d(U) transition. For isomer IV, one intense band, attributed to the π(C) → f(U) transition, is found in the region 200−300 nm. In addition, one peak located at 468 nm corresponding to the f(U)→ d(U) transition is also found.

*E-mail for J.D.: [email protected]. ORCID

Jiguang Du: 0000-0001-6669-0989 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 (NO.11204193).

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REFERENCES

(1) Utton, C. A.; De Bruycker, F.; Boboridis, K.; Jardin, R.; Noel, H.; Guéneau, C.; Manara, D. Laser melting of uranium carbides. J. Nucl. Mater. 2009, 385, 443−448.

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DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b01871 Inorg. Chem. XXXX, XXX, XXX−XXX