Unique Spectroscopic Signature of Nearly Degenerate Isomers of Au

LETTER pubs.acs.org/JPCL

Unique Spectroscopic Signature of Nearly Degenerate Isomers of Au(CN)3 Anion Devleena Samanta,†,‡ Miao Miao Wu,§,† and Puru Jena*,† †

Department of Physics and ‡Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, United States § Department of Materials Science & Engineering, Peking University, Beijing 100871, China

bS Supporting Information ABSTRACT: Distinguishing between nearly degenerate isomers is difficult not only because there are no current experimental techniques that can probe their geometries unambiguously but also because their properties may be very similar. We show that the Au(CN)3 anion is an exception. Although its two energetically nearly degenerate isomers are only 0.08 eV apart, their spectroscopic signatures are unlike any other isomers known. The vertical detachment energy of the isomer with three CN ligands is 5.44 eV, while it is only 3.28 eV for the second isomer consisting of a CN and cyanogen (NCCN) ligand. The former isomer is stabilized by its superhalogen behavior, while the latter draws its stability from the unusually large binding energy of the NCCN moiety. Comparison of these results with the AuF3 anion demonstrates the limitations of CN pseudohalogens as building blocks of superhalogens. SECTION: Dynamics, Clusters, Excited States

T

he importance of negative ions in the chemical industry as constituents of salts and oxidizing and purifying agents has led to a constant search over the past 50 years for molecules and clusters whose electron affinities can exceed 3.6 eV,1,2 the highest value attributed to Cl in the periodic table of elements. This search has led to the discovery of many superhalogens3 and hyperhalogens4,5 with electron affinities as high as 14 eV.6 While originally the superhalogens were found to consist of a metal atom at the core surrounded by halogen atoms, later studies have shown that they can also be created with O2 and H as ligands.7 10 Recently, the pool of superhalogens has been further extended by using pseudohalogens such as CN as building blocks.11 During the course of our investigation to examine the limitations of pseudohalogens in forming superhalogens, we unraveled an unusual property of Au(CN)3; its anion was found to possess two energetically nearly degenerate isomers that are only 0.08 eV apart, while the resulting neutral isomers lie 2.69 eV apart. In addition, the anion isomers have strikingly different spectroscopic properties. The vertical detachment energy (VDE) of one isomer is 5.44 eV, while it is only 3.28 eV for the other. In the former isomer, the Au atom is attached to three individual CN ligands in a T-shaped configuration, while in the latter isomer, Au binds to cyanogen (NCCN) and a separate CN moiety. The vast difference in the VDE arises because the stabilities of the two anion isomers are governed by very different mechanisms. The former isomer is stabilized by its superhalogen behavior, while the latter draws its stability from the unusually large binding energy of cyanogen. Following electron detachment, the degeneracy in the corresponding neutrals is lifted as the anions relax toward their nearest equilibrium configuration. Despite the large energy difference (2.69 eV) between the two neutral isomers, the r 2011 American Chemical Society

photoelectron spectra resulting from these anion isomers are expected to be sharp. This is because chemical bonds need to break for the higher-energy neutral isomer to reach its groundstate structure, and this would involve crossing large energy barriers. Photoelectron spectroscopy experiments should be able to verify these predictions. It is well-known that structure and properties of matter are intertwined, and a fundamental understanding of these relationships is important for the synthesis of nanomaterials with tailored properties. Clusters are the ultimate nanoparticles where every atom and every electron count. Unfortunately, there are no current experimental techniques that can unambiguously determine the structures of clusters without the benefit of theoretical input. This becomes an even more difficult task when the energy differences between the nearly degenerate isomers are beyond the accuracy of theoretical methods. We find Au(CN)3 to be a unique example of such a cluster where photoelectron spectra in combination with theory can unmistakably distinguish between the nearly degenerate anion isomers. Our reasons for focusing on Au cyanide complexes are two-fold. First, Au is the most electronegative metal atom in the periodic table and is known to exhibit oxidation states from 1 to +5, with the +1 and +3 states being most widely prevalent.12 Because CN is a pseudohalogen and mimics the chemistry of halogen atoms, we expect Au(CN)3 to be a superhalogen just as AuF3, with an electron affinity of 5.15 eV, is a known superhalogen.13 Second, gold has a rich coordination chemistry with cyanide,14,15 cyanidation being Received: October 27, 2011 Accepted: November 16, 2011 Published: November 16, 2011 3027

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Figure 1. Geometries of low-lying isomers of anionic (left) and neutral (right) Au(CN)3 isomers (a d). The bond lengths are in Å. The energies are measured with respect to the ground states of the anion and neutral in (a). Blue represents N, gray represents C, and yellow represents Au.

one of the major methods for extraction of gold from its ore.16 Also, gold cyanide complexes are known to exhibit supramolecular behavior.17 Our calculations are based on density functional theory with the B3LYP18,19 hybrid functional accounting for the exchange correlation potential. We have used the Gaussian 03 code20 with the 6-311+G*21,22 basis set for C and N and the Stuttgart pseudopotential SDD23,24 basis for Au. This choice has already been shown to yield results in good agreement with experiments.4,25 To obtain the ground-state geometry of the Au(CN)3 anion, we used 22 initial structures where CN moieties are bound to a Au atom either individually or in dimerized/ trimerized form. In the former case, Au has a choice to bind to either C or N. For the later configurations, the choices are more complex because (CN)2 and (CN)3 can have many isomers of their own.26 28 For the dimerized configuration, we used NC Au NCCN, NC Au NCNC, NC Au CNCN, NC Au CNNC, as well as CN Au NCCN, CN Au NCNC, CN Au CNCN, and CN Au CNNC. Similarly, there are also a number of ways that (CN)3 can attach to Au, and one such example is Au NCCNCN. All of the different isomers for the various different ways of attachment of CN with Au are shown in Figures S1 S4 (Supporting Information). For each configuration, the geometries were fully optimized without any symmetry constraint. The convergence in total energy and forces was set to 1  10 6 eV and 1  10 2 eV/Å, respectively. The geometries of the neutral clusters were obtained by using anion geometries as starting points and optimizing the geometries following electron detachment with the same constraints outlined above. The vertical detachment energy (VDE) was calculated by taking the difference in the total energy of the anion and its neutral at the anion geometry. The adiabatic detachment energy (ADE) was computed by further relaxing the neutral geometry to its nearest equilibrium position in the potential energy surface. In Figure 1, we show the geometries of the low-lying isomers of Au(CN)3 for different modes of CN attachment. We begin with the geometries of the Au(CN)3 . The ground-state geometry has the NC Au NCCN configuration. Here, two of the CN moieties dimerize and bind to Au while the third CN moiety binds to Au on the opposite side with C pointing toward Au.

The structure has a pseudolinear form, with the NCCN moiety having a bent structure. We note that this has the same form as (CN)2 . Au exists in the oxidation state of +1, and the added electron is distributed over the NCCN moiety. The next higher energy structure lying only 0.08 eV above the ground state has three CN moieties attached to Au separately. In this configuration, Au has an oxidation state of +2. With the extra electron in the anion, there are enough electrons to break the NC CN bond. The near degeneracy of these two structures results from separate mechanisms. The stability of the ground-state structure arises because the binding energy of (CN)2 is 5.84 eV.29 The stability of the next higher energy structure arises because the extra electron is distributed over three CN moieties and hence leads to high binding energy, namely, 5.07 eV. Thus, this isomer behaves as a superhalogen. Calculations indicate that both of the isomers are stable with respect to fragmentation into AuCN and NCCN (fragmentation energies are 1.53 and 1.61 eV for Au(CN)3 and NC Au NCCN isomers, respectively). Furthermore, we note that the lowest-energy structure where the trimerized form of (CN)3 attaches to the Au atom is significantly higher in energy than the isomers discussed above. Next, we discuss the geometries of the neutral isomers. Here, the ground state is a linear chain with Au bonded to C of CN and N of NCCN moieties. The oxidation state of Au is +2, and the stability of the structure is due to the large binding energy of NCCN. The structure where the CN moieties bind separately is 2.69 eV higher in energy than the ground state. Here, Au exists in the +3 oxidation state, but the cluster does not gain the same energy that it did in the anion due to distribution of the added electron over all three ligands. Hence, we see that while the linear isomer is bound against fragmentation into AuCN and NCCN (by 1.26 eV), the T-shaped neutral isomer is not (by 1.42 eV). However, there are other linear structures of Au(CN)3 whose energies are closer to the ground state. Here, (CN)2 binds either as CNCN and NCCN or CNNC. Once again, Au bound to (CN)3, namely, Au NCCNCN, has much higher energy. To confirm the validity of our results, we have repeated the calculations for ADE and VDE for the two nearly degenerate Au(CN)3 anions at different levels of theory to study the effect of changing the method as well as the basis set. The results are 3028

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Table 1. ADE and VDE of the Two Lowest-Energy Au(CN)3 Clusters Calculated at Different Theoretical Levels method cluster

Au basis

Au(CN)3 NC Au NCCN

B3LYP

C,N

SDD

LANL2DZ

B3PW91

M06

SDD

SDD

6-311+G*

aug-cc-pvtz

6-311+G*

aug-cc-pvtz

ADE

5.07

5.00

5.07

4.99

4.91

4.89

VDE

5.44

5.41

5.47

5.36

5.26

5.29

ADE

2.46

2.40

2.44

2.40

2.42

2.20

VDE

3.28

3.20

3.26

3.19

3.16

2.79

given below in Table 1. At the B3LYP18,19 level, we used LANL2DZ30 and SDD23,24 pseudopotentials for Au and 6-311 +G*21,22 and aug-cc-pVTZ31 basis sets for C and N. At the B3PW9118,32 and M0633 levels, we used the SDD basis set for Au (as it is superior to LANL2DZ) and aug-cc-pVTZ for C and N. As can be seen, the ADE and VDE values lie within 0.2 eV of each other for the different methods and different basis sets tested here. This is within the error limit associated with these calculations. We did not include corrections for zero-point vibrational energy or basis set superposition error as these do not significantly alter the ADE and VDE. In all cases, geometries were optimized. The energy difference between the isomers varied between 2.56 and 2.91 eV for the neutral and 0.05 and 0.22 eV for the anion depending on the theoretical method used, the energy being lower for the isomers where CN moieties were dimerized. For extensive calculations, we have chosen the B3LYP hybrid functional with the SDD basis for Au and the 6-311+G* basis for C and N because this method has been found to provide results very close to experimental values, particularly for gold cyanide clusters.34 For example, the ADE and VDE of AuCN predicted using this method are 2.12 and 2.25 eV, respectively, which matches very well with the experimentally determined values (2.07 and 2.19 eV, respectively).32 UCCSD(T) calculations done using the aug-cc-pVTZ basis set for C and N and the augcc-pVTZ-pp basis for Au predict these values to be 1.97 and 2.08 eV, respectively.35 Similarly, the calculated VDE value for Au(CN)2 using the above B3LYP level of theory is 6.10 eV. This also compares very well with the CASSCF/CCSD(T)36 value of 6.02 eV and the experimental value of 6.09 eV. Therefore, to save computation time, we have not performed CCSD(T) calculations for the Au(CN)3 complexes. One of the ideal experiments to probe the electronic properties of clusters is photoelectron spectroscopy (PES). Here, a mass-isolated anion is interjected with a fixed frequency laser, and the energy of the photoejected electron is measured. The resulting PES carries information on the vertical and adiabatic detachment energies as well as electron affinity. In fact, several superhalogens have been studied using PES experiments, and VDEs as large as about 7 eV have been measured.5,36 38 In anion clusters that do not possess nearly degenerate isomers, interpretation of the PES is simple. Sharp spectra correspond to the geometries of the neutral, which is very similar to that of its anion. Broad spectra, on the other hand, reflect the fact that the neutral and anion ground-state geometries are significantly different. In the event that nearly degenerate isomers of the anion exist, interpretation of the PES becomes complex, particularly if the isomers have very different geometries. This is the case here. The VDE and ADE of the isomer in Figure 1a yields 3.28 and 2.46 eV, respectively, while for those corresponding to Figure 1b, the

aug-cc-pvtz

aug-cc-pvtz

Figure 2. Vertical detachment energies of the two nearly degenerate Au(CN)3 anions.

energies are 5.44 and 5.07 eV. Because the geometries of the anions and the corresponding neutrals in Figure 1a and b are similar, the PES spectra of both isomers are expected to be sharp. Going from the anion geometry in Figure 1a to the neutral geometry in Figure 1b or from the anion geometry in Figure 1b to neutral geometry in Figure 1a will be difficult as it would require breaking chemical bonds and hence encountering large energy barriers. The PES contains more information than just the VDE, ADE, and EA. The many peaks in the spectrum reflect the density of states of the neutral cluster. These are commonly studied in theory by broadening the discrete molecular orbital energy levels. Because most calculations are carried out by using density functional theory, one should recall that single-particle energy levels in DFT do not have any formal meaning. Consequently, Gutsev et al.39 suggested a different procedure where more information can be gleaned from DFT calculations to compare with the PES experiment. This deals with energy gaps between the two lowest peaks in the PES. Note that when an electron is removed from an anion with spin multiplicity M, the corresponding spin multiplicity of the neutral can be either M + 1 or M 1, depending upon whether the electron is removed from the spin-down or spin-up state. The difference between these two energies yields the energy separation between the two lowest peaks in the PES. In Figure 2, we provide the vertical detachment energies associated with the two nearly degenerate isomers from the doublet spin state to the singlet and triplet states of the corresponding neutrals. These would correspond to the two low-energy peaks in the PES and provide further data against which experiment can be compared. 3029

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The Journal of Physical Chemistry Letters In conclusion, we show that Au(CN)3 anions possess two nearly degenerate isomers whose energetic stabilities are derived from entirely different mechanisms. This degeneracy is lifted when the extra electron is photodetached. Unlike most nearly degenerate isomers, the spectroscopic signatures of Au(CN)3 isomers are very different. It should be noted that the two Au(CN)3 isomers are structural isomers, and hence, the differences in their spectroscopic properties are not unexpected. However, if these anionic clusters are produced and mass separated, it will be observed that, depending on the source conditions, they have different spectroscopic properties. The vertical detachment energies (adiabatic detachment energies) of the isomers are 5.44 (5.07) and 3.28 (2.46) eV. Similarly, the energy difference between the singlet and triplet states of the neutrals resulting from the anion isomers are 1.47 and 2.80 eV. These vast energy differences would be easy to measure experimentally and would establish the existence of these isomers. We should also note that the electron affinity, defined as the energy difference between the ground states of the anion and neutral in Figure 1a, is 2.46 eV. If this is used as a criterion for defining the superhalogen (and it should), Au(CN)3 is not a superhalogen! This is in sharp contrast with AuF3, which is a superhalogen with an electron affinity of 5.15 eV. Thus, even if pseudohalogens mimic the chemistry of halogens, they may not form superhalogens the same way halogens do. This is the limitation!

’ ASSOCIATED CONTENT

bS

Supporting Information. Structures, geometries, and bond lengths of all 22 isomers investigated. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

’ ACKNOWLEDGMENT This work was supported in part by grants from the Department of Energy. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. ’ REFERENCES (1) Hotop, H.; Lineberger, W. C. Binding-Energies in Atomic Negative-Ions 0.2. J. Phys. Chem. Ref. Data 1985, 14, 731–750. (2) Bartlett, N. Xenon Hexafluoroplatinate(v) Xe+[PtF] . Proc. Chem. Soc. 1962, 218. (3) Gutsev, G. L.; Boldyrev, A. I. DVM-Xα Calculations on the Ionization Potentials of MXk+1 Complex Anions and the Electron Affinities of MXk+1 “Superhalogens”. Chem. Phys. 1981, 56, 277–283. (4) Willis, M.; Gotz, M; Kandalam, A. K.; Gantefor, G; Jena, P. Hyperhalogens: Discovery of a New Class of Highly Electronegative Species. Angew. Chem., Int. Ed. 2010, 49, 8966–8970. (5) Feng, Y. A.; Xu, H. G.; Zheng, W. J.; Zhao, H. M.; Kandalam, A. K.; Jena, P. Structures and Photoelectron Spectroscopy of

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Cun(BO2)m (n, m = 1, 2) Clusters: Observation of Hyperhalogen Behavior. J. Chem. Phys. 2011, 134, 094309. (6) Freza, S.; Skurski, P. Enormously Large (Approaching 14 eV!) Electron Binding Energies of [HnFn+1] (n = 1 5, 7, 9, 12) Anions. Chem. Phys. Lett. 2010, 487, 19–23. (7) Pradhan, K.; Gutsev, G. L.; Weatherford, C. A.; Jena, P. A Systematic Study of Neutral and Charged 3d-Metal Trioxides and Tetraoxides. J. Chem. Phys. 2011, 134, 144305. (8) Gutsev, G. L.; Rao, B. K.; Jena, P.; Wang, X. B.; Wang, L. S. Origin of the Unusual Stability of MnO4. Chem. Phys. Lett. 1999, 312, 598–605. (9) Boldyrev, A. I.; Niessen, W. V. The First Ionization Potentials of Some MHk+1 and M2H2k+1 Anions Calculated by a Green’s function method. Chem. Phys. 1991, 155, 71–78. (10) Boldyrev, A. I.; Simons, J. Vertical and Adiabatical Ionization Potentials of MHK+1 Anions. Ab Initio Study of The Structure and Stability Of Hypervalent MHk+1 Molecules. J. Chem. Phys. 1993, 99, 4628–4637. (11) Smuczynska, S.; Skurski, P. Halogenoids as Ligands in Superhalogen Anions. Inorg. Chem. 2009, 48, 10231–10238. (12) Bergendahl, T. J. Oxidation-States of Gold. J. Chem. Educ. 1975, 52, 731–732. (13) Koirala, P.; Willis, M.; Kiran, B.; Kandalam, A. K.; Jena, P. Superhalogen Properties of Fluorinated Coinage Metal Clusters. J. Phys. Chem. C 2010, 114, 16018–16024. (14) Leznoff, D.; Lefebvre, J. Coordination Polymers with Cyanoaurate Building Blocks: Potential New Industrial Applications for Gold. Gold Bull. 2005, 38, 47–54. (15) Ahmad, S. The Chemistry of Cyano Complexes of Gold(I) With Emphasis on the Ligand Scrambling Reactions. Coord. Chem. Rev. 2004, 248, 231–243. (16) Rubo, A.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. Alkali Metal Cyanides; Wiley-VCH Verlag GmbH & Co. KGaA: New York, 2006. (17) Shorrock, C.; Jong, H.; Batchelor, R.; Leznoff, D. [Au(CN)4] as a Supramolecular Building Block for Heterobimetallic Coordination Polymers. Inorg. Chem. 2003, 42, 3917–3924. (18) Becke, A. D. Density-Functional Thermochemistry. 3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648–5652. (19) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle Salvetti Conelation Energy Formula into A Functional of the Electron Density. Phys. Rev. B 1988, 37, 785–789. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al. Gaussian 03, revision D.02; Gaussian, Inc.: Wallingford CT, 2004. (21) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650–654. (22) Mclean, A. D.; Chandler, G. S. Contracted Gaussian Basis Sets for Molecular Calculations. I. Second Row Atoms, Z = 11 18. J. Chem. Phys. 1980, 72, 5639–5648. (23) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P. D. W. Relativistic Effects in Gold Chemistry. I. Diatomic Gold Compounds. J. Chem. Phys. 1989, 91, 1762–1774. (24) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. Energy-Adjusted Ab Initio Pseudopotentials for the First Row Transition Elements. J. Chem. Phys. 1987, 86, 866–872. (25) Wu, M. M.; Wang, H. P.; Ko, Y. J.; Wang, Q. A.; Sun, Q. A.; Kiran, B.; Kandalam, A. K.; Bowen, K. H.; Jena, P. Manganese-Based Magnetic Superhalogens. Angew. Chem., Int. Ed. 2011, 50, 2568–2572. (26) Botschwina, P.; Flugge, J. Ab Initio Vibration—Rotation Coupling Constants and the Equilibrium Geometries of NCCN and CNCN. Chem. Phys. Lett. 1991, 180, 589–593. (27) Ding, Y.; Huang, X.; Li, Z.; Sun, C. Theoretical Study on Potential-Energy Surface of C2N2. J. Chem. Phys. 1998, 108, 2024–2027. (28) Jiang, Z.; Xu, X.; Wu, H.; Jin, Z. Theoretical Study of Structures and Stabilities of CmN2 (m = 1 14) Ions. Int. J. Mass. Spectrom 2003, 230, 33–39. 3030

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(29) Wannenmacher, E. A. J.; Lin, H.; Jackson, W. M. Photodissociation Dynamics of Cyanogen (C2N2) in the Threshold Region for Dissociation. J. Phys. Chem. 1990, 94, 6608–6615. (30) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299–310. (31) Kendall, R. A.; Dunning, T. H.; Harrison, R. J. Electron Affinities of the First-Row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96, 6796–6806. (32) Perdew, L. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244–13249. (33) Zhao, Y.; Truhlar, D. G. The M06 Suite Of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215–241. (34) Samanta, D.; Wu, M. M.; Jena, P. Au(CN)n Complexes: Superhalogens with Pseudohalogen as Building Blocks. Inorg. Chem. 2011, 50, 8918–8925. (35) Wu, X.; Qin, Z.; Xie, H.; Cong, R.; Wu, X.; Tang, Z.; Fan, H. Photoelectron Imaging and Theoretical Studies of Group 11 Cyanides MCN (M = Cu, Ag, Au). J. Phys. Chem. A 2010, 114, 12839–12844. (36) Wang, X. B.; Wang, Y.-L.; Yang, J.; Xing, X. P.; Li, J.; Wang, L. S. Evidence of Significant Covalent Bonding in Au(CN)2 . J. Am. Chem. Soc. 2009, 131, 16368–16370. (37) Willis, M; Gotz, M; Kandalam, A. K.; Gantefor, G. F.; Jena, P. Hyperhalogens: Discovery of a New Class of Highly Electronegative Species. Angew. Chem., Int. Ed. 2010, 49, 8966–8970. (38) Yang, J.; Wang, X. B.; Xing, X. P.; Wang, L. S. Photoelectron Spectroscopy of Anions at 118.2 nm: Observation of High Electron Binding Energies in Superhalogens MCl4 (M = Sc, Y, La). J. Chem. Phys. 2008, 128, 201102. (39) Gutsev, G. L.; Khanna, S. N.; Jena, P. Unambiguous Assignment of the Ground State of a Nearly Degenerate Cluster. Phys. Rev. B 2000, 62, 1604–1606.

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