Ground States Of Clusters Research Articles

Probing the structural and electronic properties of Al-doped small niobium clusters

The structural evolution and bonding of a series of bimetallic cluster of metal cluster anions of niobium with an external aluminum atom, NbnAl- (n=2–10), are investigated using earlier photoelectron spectroscopy and theoretical calculations. For each niobium aluminum alloy cluster, many low-lying isomers are generated using the Saunders ‘Kick’ global minimum stochastic search method. Theoretical electron detachment energies are compared with the experimental measurements to verify the ground states of the bimetallic clusters obtained from DFT calculations. The current Letter shows that the dopant atom always occupies the surface of bimetallic clusters and structural transition does not take place until n=10.

Cluster radioactivity using various versions of nuclear proximity potentials

The systematic study of doubly closed shell ${}^{208}\mathrm{Pb}$-daughter cluster radioactivity using different versions of proximity potentials within the preformed cluster-decay model is extended to daughter nuclei other than ${}^{208}\mathrm{Pb}$. The nuclear proximity potentials used in the calculation of total interaction potential cover a wide range of barrier characteristics. The decay half-life of the cluster changes significantly with the use of these different potentials because of the corresponding change in the preformation probability (${P}_{0}$) and penetration probability ($P$) of the cluster. The cluster radioactivity is worked out at touching as well as at an elongated neck configuration by taking the neck-length parameter $\ensuremath{\Delta}R=0.5$ fm. In order to observe the effect of deformation, the spherical fragmentation of clusters is also taken into account and the standard root-mean-square (rms) deviation is also compared for the various nuclear proximity potentials. It is observed that Prox 1977 reproduces the experimental half-lives very well at $\ensuremath{\Delta}R=0.5$ fm and further improves upon the value obtained in previous work. Thus Prox 1977 turns out to be a better option for studying the cluster decay of radioactive nuclei; however, the calculated half-lives of ${}^{14}\mathrm{C}$ clusters are still poor. With the use of mod-Prox 1988, having stronger isospin and asymmetry dependence, the comparison for ${}^{14}\mathrm{C}$ clusters improves significantly and the standard rms deviation from experimental half-lives comes out to be 1.01 and 1.46 for spherical and deformed choices of fragmentation, respectively. The standard rms deviation reported for various proximity potentials can be further improved if the precise experimental half-lives of 15, out of the 45 clusters used in present study, are made available. Till now only lower limits are available for these clusters. The barrier characteristics of various interaction potentials obtained using different nuclear proximity potentials are also explored in the context of ground-state cluster radioactivity. The new cases of ${}^{34}\mathrm{Si}$ decay of ${}^{238}\mathrm{U}$ and ${}^{14}\mathrm{C}$ and ${}^{15}\mathrm{N}$ decays of ${}^{223}\mathrm{Ac}$ are also included in the present study of 45 clusters. Predictions of half-lives of possible cluster decays in the trans-lead region are also made using Prox 1977. The deformation effect is included with optimum cold orientations.

Interplay between Magnetism and Magicness in Nanoclusters

Our experimental and theoretical investigation focuses on the role of magnetism on the structures and energetics of nanoclusters of the important 3d antiferromagnetic (AFM) semiconductor, manganese sulphide (MnS). Our cluster beam mass spectra for both (MnS)n and (MnSe)n show a pronounced “magic” abundance, at n = 13, in accord with results for other nonmagnetic chalcogenide clusters (e.g., CdS, CdSe, ZnS, ZnSe). Using global optimization and ab initio calculations we focus on cluster isomers of (MnS)13 and its two neighboring compositions, (MnS)12 and (MnS)14, and show that the magic status of the former is fully compatible with the high relative energetic stability of the n = 13 ground state with respect to those for n ± 1. We demonstrate a strong symbiosis between cluster structure and spin ordering, which leads to a particular type of AFM spin-ordered ground-state cluster structure being energetically selected for all cluster sizes and which enhances the magic stability of (MnS)13. This nanomagnetostructural effect highlights a fundamental link between magnetism and cluster structure having potential implications for (nano)technologies based on magnetostructural transitions (e.g., data storage, magnetic refrigeration).

Experimental and computational evidence of octa- and nona-coordinated planar iron-doped boron clusters: Fe©B8− and Fe©B9−

We report the observation of two Fe-doped boron clusters with wheel-type structures, containing an octa-coordinate (C8v-Fe©B8−) and a nona-coordinate (D9h-Fe©B9−) Fe atom. The clusters were produced in a laser vaporization supersonic molecular beam and characterized by combined photoelectron spectroscopy and ab initio studies. Chemical bonding analyses revealed that in the ground state both clusters are doubly (σ + π) aromatic and that the iron atom interacts with the peripheral boron ring exclusively through delocalized bonds. These findings provide further support to the design principle for metal-doped boron clusters with highly symmetric molecular wheel-type structures.

Comparison of molecular graphs of Lin, Nanand Cun(n= 2–5) clusters obtained from the density and the molecular electrostatic potential

AbstractThe analysis of the molecular graphs of the density and the molecular electrostatic potential (MEP) of small lithium, sodium and copper ground state cluster structures up to the pentamer is presented. For the underlying topological analysis all‐electron calculations were performed within the framework of Kohn–Sham density functional theory. The calculations are performed with the local level of theory in combination with all‐electron double zeta valence polarization basis sets. For this analysis, the ground state cluster structures obtained at the same level of theory were used. Molecular graphs of the density as well as the MEP are reported. The differences in the topology of the electronic density, MEP, and corresponding molecular graphs of the studied small lithium, sodium, and copper clusters are discussed. © 2012 Wiley Periodicals, Inc.

On the Electronic Structures of Au25(SR)18 Clusters Studied by Magnetic Circular Dichroism Spectroscopy.

The first magnetic circular dichroism (MCD) spectra are reported at room temperature for well-defined thiolate-protected Au25 clusters (Au25(SG)18 and Au25(PET)18, where SG and PET denote glutathione and 2-phenylethanethiolate, respectively). MCD essentially corresponds to electronic transitions in the absorption spectrum, so the electronic structures of the Au25 clusters are explored based on a simultaneous deconvolution analysis of both the electronic absorption and MCD spectra, giving enhanced spectral resolution. We then find that the observed MCD responses are entirely interpreted in terms of the Faraday B terms, representing strict nondegeneracies of the excited and ground states of the Au25 clusters that correspond to so-called superatom D and P orbitals, respectively.

LARGE-SCALE FIRST PRINCIPLES CONFIGURATION INTERACTION CALCULATIONS OF OPTICAL ABSORPTION IN BORON CLUSTERS

We have performed systematic large-scale all-electron correlated calculations on boron clusters B n(n = 2 - 5), to study their linear optical absorption spectra. Several possible isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles doubles (CCSD) level of theory. Using the optimized ground-state geometries, the excited states of different clusters were computed using the multi-reference singles-doubles configuration–interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wave functions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, eventually leading to their linear absorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion were carefully examined. The contribution of configurations to many body wave-function of various excited states suggests that the excitations involved are collective, plasmonic type.

Doping golden clusters: MAu−19 and M2Au−18 (M = Cu and Na)

The structural and electronic properties of MAu−19 and M2Au−18 (M=Cu and Na) have been studied by the relativistic density-functional calculations. It is found that the most stable configurations of CuAu−19 and Cu2Au−18 are the face-centered and two-face-centered doped structures based upon the tetrahedral structure Au−20. In contrast, the ground states of Na-doped gold clusters (NaAu−19 and Na2Au−18) exhibit flat-cage configurations. The PES of these ground states are depicted that may be helpful to identify their configurations in the future experiments. The face-centered and two-face-centered doped tetrahedral structures of CuAu−19 and Cu2Au−18 have a large HOMO–LUMO gap, indicating that they are chemically stable.

Architectures, electronic structures, and stabilities of Cu-doped Ge n clusters: density functional modeling

The present study reports the geometries, electronic structures, growth behavior, and stabilities of neutral and ionized copper-doped germanium clusters containing 1-20 Ge atoms within the framework of linear combination of atomic orbitals density functional theory (DFT) under the spin-polarized generalized gradient approximation. It was found that Cu-capped Ge(n) (or Cu-substituted Ge( n+1)) and Cu-encapsulated Ge(n) clusters mostly occur in the ground state at a particular cluster size (n). In order to explain the relative stabilities of the ground-state clusters, parameters such as the average binding energy per atom (BE), the embedding energy (EE), and the fragmentation energy (FE) of the clusters were calculated, and the resulting values are discussed. To explain the chemical stabilities of the clusters, parameters such as the energy gap between the highest occupied and the lowest unoccupied molecular orbitals (the HOMO-LUMO gap), the ionization energy (IP), the electron affinity (EA), the chemical potential (μ), the chemical hardness (η), and the polarizability were calculated, and the resulting values are also discussed. Natural atomic orbital (NAO) and natural bond orbital (NBO) analyses were also used to determine the electron-counting rule that should be applied to the most stable Ge(10)Cu cluster. Finally, the relevance of the calculated results to the design of Ge-based superatoms is discussed.

Spectroscopic Studies of the Iron and Manganese Reconstituted Tyrosyl Radical in Bacillus Cereus Ribonucleotide Reductase R2 Protein

Ribonucleotide reductase (RNR) catalyzes the rate limiting step in DNA synthesis where ribonucleotides are reduced to the corresponding deoxyribonucleotides. Class Ib RNRs consist of two homodimeric subunits: R1E, which houses the active site; and R2F, which contains a metallo cofactor and a tyrosyl radical that initiates the ribonucleotide reduction reaction. We studied the R2F subunit of B. cereus reconstituted with iron or alternatively with manganese ions, then subsequently reacted with molecular oxygen to generate two tyrosyl-radicals. The two similar X-band EPR spectra did not change significantly over 4 to 50 K. From the 285 GHz EPR spectrum of the iron form, a g 1-value of 2.0090 for the tyrosyl radical was extracted. This g 1-value is similar to that observed in class Ia E. coli R2 and class Ib R2Fs with iron-oxygen cluster, suggesting the absence of hydrogen bond to the phenoxyl group. This was confirmed by resonance Raman spectroscopy, where the stretching vibration associated to the radical (C-O, ν7a = 1500 cm−1) was found to be insensitive to deuterium-oxide exchange. Additionally, the 18O-sensitive Fe-O-Fe symmetric stretching (483 cm−1) of the metallo-cofactor was also insensitive to deuterium-oxide exchange indicating no hydrogen bonding to the di-iron-oxygen cluster, and thus, different from mouse R2 with a hydrogen bonded cluster. The HF-EPR spectrum of the manganese reconstituted RNR R2F gave a g 1-value of ∼2.0094. The tyrosyl radical microwave power saturation behavior of the iron-oxygen cluster form was as observed in class Ia R2, with diamagnetic di-ferric cluster ground state, while the properties of the manganese reconstituted form indicated a magnetic ground state of the manganese-cluster. The recent activity measurements (Crona et al., (2011) J Biol Chem 286: 33053–33060) indicates that both the manganese and iron reconstituted RNR R2F could be functional. The manganese form might be very important, as it has 8 times higher activity.

On the triplet ground state of tetrahedral X4 clusters (X = Li, Na, K, Cu)

The lowest electronic state of distorted tetrahedral X(4) clusters (with X = Li, Na, K, Cu) is studied at coupled-cluster level using high-quality atomic basis sets. The ground state is found to have a triplet spin symmetry for this kind of geometry and for all the considered atomic species. The equilibrium geometries correspond to Jahn-Teller-distorted oblate tetrahedra having D(2d) symmetry, and tetrahedric structures are local minima on the potential-energy surfaces for the triplet states. Their energies lie between 0.2 eV (for the K(4) cluster) and 0.9 eV (for Cu(4)) above the absolute minimum of the corresponding systems, which is a spin singlet having a rhombus geometry.

Probing the structural and electronic properties of small vanadium monoxide clusters

The structural evolution and bonding of a series of early transition-metal oxide clusters, V(n)O(q) (n = 3-9, q = 0,-1), have been investigated with the aid of previous photoelectron spectroscopy (PES) and theoretical calculations. For each vanadium monoxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium monoxide clusters obtained from the DFT calculations. The results demonstrate that the combination of photoelectron spectroscopy experiments and DFT calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers. The second difference energies and adsorption energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The adsorption energies of one O atom on the anionic (6.64 → 8.16 eV) and neutral (6.41 → 8.13 eV) host vanadium clusters are shown to be surprisingly high, suggesting strong capabilities to activate O by structural defects in vanadium oxides.

Quantum-classical determination of stable isomers of the Li*Arn clusters

We present a quantum-classical determination of stable isomers of LiArn, clusters with both an electronic ground state and excited states of the lithium atom. The Li atom is perturbed by argon atoms in LiArn clusters. Its electronic structure obtained as the eigenfunctions of a single-electron operator describing the electron in the field of a Li+Arn core, the Li+ and Ar atoms are replaced by pseudo-potentials. These pseudo-potentials include core-polarisation operators to account for the polarisation and correlation of the inert core with the valence lithium electron (Rhouma et al., 2002). The geometry optimisation of the ground and excited states of LiArn (n = 1–12) clusters is carried out via the Basin-Hopping method of Wales et al. (Wales and Doyle, 1997; Wales and Scheraga, 1999). The geometries of the ground and ionic states of LiArn clusters were used to determine the energy of the high excited states of the neutral LiArn clusters. The variation of the excited state energies of LiArn clusters as a function of the number of argon atoms shows an approximate Rydberg character, corresponding to the picture of an excited electron surrounding an ionic cluster core, is already reached for the 3s state.

Microsolvation of DMSO: Density functional study on the structure and polaraizabilities

AbstractThe structure and stability for the association of water with dimethyl sulfoxide (DMSO) are investigated using the density functional M06‐2X level theory. Stable complexes are formed by the formation of hydrogen bonding between water and oxygen atom of DMSO molecule, while the electrostatic force between water and DMSO plays a vital role in deciding the structure. The water‐DMSO interactions are stronger than the interwater hydrogen bonds, which can be inferred from the shorter DMSO‐water bond distance compared with the water–water bond distance. The calculated solvent association energy does not saturate, and it remains favorable to attach additional water molecules to the existing water network. The calculated IR spectra shifts supports the formation stronger hydrogen bonding, while the electrostatic potential (ESP) plot supports the existence of weaker electrostatic interaction in the studied clusters. The polarizabilities for the ground state clusters were found to increase monotonically with the cluster size. The presence of additional electrostatic bonding between water and DMSO, devastates the linear hydrogen‐bonding network. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Structural and electronic properties of ScnOm(n=1−3,m=1−2n) clusters: Theoretical study using screened hybrid density functional theory

The structural and electronic properties of small scandium oxide clusters ScnOm (n = 1 - 3, m = 1 - 2n) are systematically studied within the screened hybrid density functional theory. It is found that the ground states of these scandium oxide clusters can be obtained by the sequential oxidation of small "core" scandium clusters. The fragmentation analysis demonstrates that the ScO, Sc2O2, Sc2O3, Sc3O3, and Sc3O4 clusters are especially stable. Strong hybridizations between O-2p and Sc-3d orbitals are found to be the most significant character around the Fermi level. In comparison with standard density functional theory calculations, we find that the screened hybrid density functional theory can correct the wrong symmetries and yield more precise description for the localized 3d electronic states of scandium.

Assignment of the Photoelectron Spectra of FeS3– by Density Functional Theory, CASPT2, and RCCSD(T) Calculations

The geometric structures of FeS(3) and FeS(3)(-) with spin multiplicities ranging from singlet to octet were optimized at the B3LYP level, allowing two low-lying conformations for these clusters to be identified. The planar D(3h) conformation contains three S(2-) atomic ligands (S(3)Fe(0/-)), whereas the C(2v) structure contains, in addition to an atomic S(2-) ligand, also a S(2)(2-) ligand that is side-on-bound to the iron cation: an η(2)-S(2)FeS conformation. Subsequently, energy differences between the various states of these conformations were estimated by carrying out geometry optimizations at the multireference CASPT2 level. Several competing structures for the ground state of the anionic cluster were recognized at this level. Relative stabilities were also estimated by performing single-point RCSSD(T) calculations on the B3LYP geometries. The ground state of the neutral complex was unambiguously found to be (5)B(2). The ground state of the anion is considerably less certain. The 1(4)B(2), 2(4)B(2), (4)B(1), and (6)A(1) states were all found as low-lying η(2)-S(2)FeS(-) states. Also, (4)B(2) of S(3)Fe(-) has a comparable CASPT2 energy. In contrast, B3LYP and RCCSD(T) mutually agree that the S(3)Fe(-) state is at a much higher energy. Energetically, the bands of the photoelectron spectra of FeS(3)(-) are reproduced at the CASPT2 level as ionizations from either the (4)B(2) or (6)A(1) state of η(2)-S(2)FeS. However, the Franck-Condon factors obtained from a harmonic vibrational analysis at the B3LYP level show that only the (4)B(2)-to-(5)B(2) ionization, which preserves the η(2)-S(2)Fe-S conformation, provides the best vibrational progression match with the X band of the experimental photoelectron spectra.

Structural, electronic and magnetic properties of Co nRh ( n=1–8) clusters from density functional calculations

The geometries, stabilities, electronic and magnetic properties of Co n Rh ( n=1–8) clusters have been investigated systematically within the framework of the generalized gradient approximation density-functional theory. The results indicate that the most stable structures of Co n Rh ( n=1–8) clusters are all similar to those of corresponding Co n+1 clusters. Maximum peaks of second-order energy difference are found at n=2, 4 and 7, indicating that these clusters possess relatively higher stability than their respective neighbors. The magnetism of the ground state of alloy clusters all displays ferromagnetic coupling except for Co 3Rh. In addition, the doped Rh atom exhibits an important influence on the magnetism of alloy clusters, e.g., compared with corresponding pure Co n clusters, the local moment of Co atom is noticeably enhanced in Co n Rh alloy clusters at n=1, 2, 5, 6, 7 and 8, while reduced at n=3 and 4. Further analysis based on the average bond length, the charge transfer and the spin polarization has been made to clarify the different magnetic responses to Rh doping.

Correlations in nanoclusters of narrow-band metals: Numerical simulation of Auger-electron spectra

Calculations of energy and wave functions of ground and low-lying excited states of small clusters of transition metals are carried out using the method of the exact diagonalization of Hamilton matrix of the extended Hubbard model. The binding energy of two holes in the cluster is calculated, the possibility of their effective attraction is shown in the wide range of the interaction parameters. One-particle and two-particle densities of states are analyzed, and the Auger-electron spectra confirming the presence of the bound state are obtained.

Architecture, electronic structure and stability of TM@Ge(n) (TM = Ti, Zr and Hf; n = 1-20) clusters: a density functional modeling

The present study reports the geometry, electronic structure and properties of neutral and anionic transition metal (TM = Ti, Zr and Hf)) doped germanium clusters containing 1 to 20 germanium atoms within the framework of linear combination of atomic orbitals density functional theory under spin polarized generalized gradient approximation. Different parameters, like, binding energy (BE), embedding energy (EE), energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO), ionization energy (IP), electron affinity (EA), chemical potential etc. of the energetically stable clusters (ground state cluster) in each size are calculated. From the variation of these parameters with the size of the clusters the most stable cluster within the range of calculation is identified. It is found that the clusters having 20 valence electrons turn out to be relatively more stable in both the neutral and the anionic series. The sharp drop in IP as the valence electron count increases from 20 to 21 in neutral cluster is in agreement with predictions of shell models. To study the vibrational nature of the clusters, IR and Raman spectrum of some selected TM@Ge(n) (n = 15,16,17) clusters are also calculated and compared. In the end, relevance of calculated results to the design of Ge-based super-atoms is discussed.

Non-Gaussian noise in the in-plane transport of lightly doped La2−xSrxCuO4: Evidence for a collective state of charge clusters

A study of the in-plane resistance noise on a high-quality single crystal of La${}_{1.97}$Sr${}_{0.03}$CuO${}_{4}$ deep inside the spin-glass phase reveals the onset of non-Gaussianity and the insensitivity of the noise to the in-plane magnetic field. The results indicate that the charge dynamics become increasingly slow and correlated as temperature $T\ensuremath{\rightarrow}0$. The analysis of the higher-order noise statistics provides evidence for the existence of a collective ground state of charge clusters, which seem to coexist with charge-poor antiferromagnetic domains that are frozen at such low $T$.