Gold as hydrogen. An experimental and theoretical study of the structures and bonding in disilicon gold clusters Si2Au(n)- and Si2Au(n) (n = 2 and 4) and comparisons to Si2H2 and Si2H4

In a previous communication, we showed that a single Au atom behaves like H in its bonding to Si in a series of Si-Au clusters, SiAu(n) (n = 2-4) (Kiran et al. Angew. Chem., Int. Ed. 2004, 43, 2125). In this article, we show that the H analogy of Au is more general. We find that the chemical bonding and potential energy surfaces of two disilicon Au clusters, Si(2)Au(2) and Si(2)Au(4), are analogous to Si(2)H(2) and Si(2)H(4), respectively. Photoelectron spectroscopy and ab initio calculations are used to investigate the geometrical and electronic structures of Si(2)Au(2)(-), Si(2)Au(4)(-), and their neutral species. The most stable structures for both Si(2)Au(2) and Si(2)Au(2)(-) are found to be C(2)(v), in which each Au bridges the two Si atoms. For Si(2)Au(4)(-), two nearly degenerate dibridged structures in a cis (C(2)(h)) and a trans (C(2)(v)) configuration are found to be the most stable isomers. However, in the neural potential energy surface of Si(2)Au(4), a monobridged isomer is the global minimum. The ground-state structures of Si(2)Au(2)(-) and Si(2)Au(4)(-) are confirmed by comparing the computed vertical detachment energies with the experimental data. The various stable isomers found for Si(2)Au(2) and Si(2)Au(4) are similar to those known for Si(2)H(2) and Si(2)H(4), respectively. Geometrical and electronic structure comparisons with the corresponding silicon hydrides are made to further establish the isolobal analogy between a gold atom and a hydrogen atom.     

On the chemical bonding of gold in auro-boron oxide clusters AunBO- (n = 1-3)

During experiment on Au-B alloy clusters, an auro-boron oxide cluster Au2BO- was observed to be an intense peak dominating the Au-B mass spectra, along with weaker signals for AuBO- and Au3BO-. Well-resolved photoelectron spectra have been obtained for the three new oxide clusters, which exhibit an odd-even effect in electron affinities. Au2BO- is shown to be a closed shell molecule with a very high electron detachment energy, whereas AuBO and Au3BO neutrals are shown to be closed shell species with large HOMO-LUMO gaps, resulting in relatively low electron affinities. Density functional calculations were performed for both AunBO- (n = 1-3) and the corresponding HnBO- species to evaluate the analogy between bonding of gold and hydrogen in these clusters. The combination of experiment and theory allowed us to establish the structures and chemical bonding of these tertiary clusters. We find that the first gold atom does mimic hydrogen and interacts with the BO unit to produce a linear AuBO structure. This unit preserves its identity when interacting with additional gold atoms: a linear Au-[AuBO] complex is formed when adding one extra Au atom and two isomeric Au2-[AuBO] complexes are formed when adding two extra Au atoms. Since BO- is isoelectronic to CO, the AunBO- species can be alternatively viewed as Aun interacting with a BO- unit. The structures and chemical bonding in AunBO- are compared to those in the corresponding AunCO complexes.     

Electronic structures and thermochemical properties of the small silicon-doped boron clusters B(n)Si (n=1-7) and their anions

We perform a systematic investigation on small silicon-doped boron clusters B(n)Si (n=1-7) in both neutral and anionic states using density functional (DFT) and coupled-cluster (CCSD(T)) theories. The global minima of these B(n)Si(0/-) clusters are characterized together with their growth mechanisms. The planar structures are dominant for small B(n)Si clusters with n≤5. The B(6)Si molecule represents a geometrical transition with a quasi-planar geometry, and the first 3D global minimum is found for the B(7)Si cluster. The small neutral B(n)Si clusters can be formed by substituting the single boron atom of B(n+1) by silicon. The Si atom prefers the external position of the skeleton and tends to form bonds with its two neighboring B atoms. The larger B(7)Si cluster is constructed by doping Si-atoms on the symmetry axis of the B(n) host, which leads to the bonding of the silicon to the ring boron atoms through a number of hyper-coordination. Calculations of the thermochemical properties of B(n)Si(0/-) clusters, such as binding energies (BE), heats of formation at 0 K (ΔH(f)(0)) and 298 K (ΔH(f)([298])), adiabatic (ADE) and vertical (VDE) detachment energies, and dissociation energies (D(e)), are performed using the high accuracy G4 and complete basis-set extrapolation (CCSD(T)/CBS) approaches. The differences of heats of formation (at 0 K) between the G4 and CBS approaches for the B(n)Si clusters vary in the range of 0.0-4.6 kcal mol(-1). The largest difference between two approaches for ADE values is 0.15 eV. Our theoretical predictions also indicate that the species B(2)Si, B(4)Si, B(3)Si(-) and B(7)Si(-) are systems with enhanced stability, exhibiting each a double (σ and π) aromaticity. B(5)Si(-) and B(6)Si are doubly antiaromatic (σ and π) with lower stability.     

Ultraviolet and infrared photodissociation of Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar clusters

Ion-molecule complexes of the form Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar are produced by laser vaporization in a pulsed nozzle cluster source. These clusters are mass-selected and studied with ultraviolet (355 nm) photodissociation and resonance-enhanced infrared photodissociation spectroscopy in the C-H stretch region of benzene. In the UV, Si(+)(C6H6)n clusters (n = 1-5) fragment to produce the Si(+)(C6H6)n mono-ligand species, suggesting that this ion has enhanced relative stability. IR photodissociation of Si(+)(C6H6)n complexes occurs by the elimination of benzene, while Si(+)(C6H6)(n)Ar complexes lose Ar. Resonances reveal C-H vibrational bands in the 2900-3300 cm(-1) region characteristic of the benzene ligand with shifts caused by the silicon cation bonding. The IR spectra confirm that the major component of the Si(+)(C6H6)n ions studied have the pi-complex structure rather than the isomeric insertion products suggested previously.     

The gold-ammonia bonding patterns of neutral and charged complexes Au m 0+/-1-(NH3)n. I. Bonding and charge alternation

The gold-ammonia bonding patterns of the complexes which are formed between the ammonia clusters (NH(3))(1< or =n< or =3) and gold clusters of different sizes that range from one gold atom to the tri-, tetra-, and 20-nanogold clusters are governed by two basic and fundamentally different ingredients: the anchoring Au-N bond and the nonconventional N-H...Au hydrogen bond. The latter resembles, by all features, a conventional hydrogen bond and is formed between a typical conventional proton donor N-H group and the gold cluster that behaves as a nonconventional proton acceptor. We provide strong computational evidence that the gold-ammonia bonding patterns exhibit distinct characteristics as the Z charge state of the gold cluster varies within Z=0,+/-1. The analysis of these bonding patterns and their effects on the N-H...N H-bonded ammonia clusters are the subject of this paper.     

Photoelectron spectroscopy of silicon doped gold and silver cluster anions

Photoelectron spectra of low temperature silicon doped gold cluster anions Au(n)Si(-) with n = 2-56 and silver cluster anions Ag(n)Si(-) with n = 5-82 have been measured. Comparing the spectra as well as the general size dependence of the electron detachment energies to the results on undoped clusters shows that the silicon atom changes the apparent free electron count in the clusters. In the case of larger gold clusters (with more than about 30 gold atoms) the silicon atom seems to consistently delocalize all of its four valence electrons, while in the case of the silver clusters a less uniform behavior is observed. Here the silicon atoms act partly as electron donors, partly as electron acceptors, without following an obvious simple principle. Additionally some structural information can be obtained from the measured spectra: while Ag(54)Si(-) seems to adopt an icosahedral structural motif, Au(54)Si(-) seems to take on a low symmetry structure, much like the corresponding pure 55 atom clusters. This indicates that for such larger clusters the incorporation of a single silicon atom does not change the ground state geometry significantly.     

Ab-initio SCF study of the nature of bonding in neutral and cationic silicon hydrides and analogous carbon compounds

The nature of bonding in a number of neutral and cationic silicon hydrides has been studied on the basis of atomic charge (q_A), valency (V_A), molecular valency (V_M) and localised molecular orbitals (LMO). Several structural isomers of each species have been considered. It is found that the most stable isomer is generally associated with maximum q_si and minimum V_si and V_M. Protonation of the neutral hydrides leads to an increase in the positive charge of Si, and in molecular valency. The bonding characteristics of the most stable isomers of the silicon hydrides have been compared with that of analogous hydrocarbons. It has been observed that the difference in bonding stems mainly from the highly electro-positive nature of the Si atom which prefers to form three-centre rather than multiple bonds. The molecular hardness parameters (η) have also been calculated. The most stable isomers of silicon hydrides are generally associated with maximum η. The HSAB principle has been tested by considering some hydrogenation and proton transfer reactions. It is qualitatively satisfied in both cases.     

Structures and relative stability of medium-sized silicon clusters. V. Low-lying endohedral fullerenelike clusters Si31-Si40 and Si45

We have performed unconstrained search for low-lying structures of medium-sized silicon clusters Si(31)-Si(40) and Si(45), by means of the minimum-hopping global optimization method coupled with a density-functional based tight-binding model of silicon. Subsequent geometric optimization by using density-functional theory with the PBE, BLYP, and B3LYP functionals was carried out to determine the relative stability of various candidate low-lying silicon clusters obtained from the unconstrained search. The low-lying characteristics of these clusters can be affirmed by comparing the binding energies per atom of these clusters with previously determined lowest-energy clusters(Si(n)) in the size range of 21相似文献   

Ab Initio Theoretical Investigation on the Geometrical and Electronic Structures of AlAun-/0 (n = 2-4) Clusters

A systematic density functional theory and wave function theory investigation on the geometrical and electronic properties of AlAu n 0/-(n=2-4) clusters has been performed in this work. AlAu n-anions prove to possess ground states of the V-shaped C2v AlAu2 - , umbrella-shaped C3v AlAu3- , and perfect tetrahedral T d AlAu4- , while their neutrals favor the V-shaped C2v AlAu2 , perfect planar triangular D3h AlAu3 , and severely distorted C s AlAu4 , respectively. Aluminum aurides appear to be analogous to the corresponding aluminum hydrides, expect C s AlAu4 . Molecular orbitals (MOs) analyses also support this conclusion. Detailed orbital analyses indicate that Au 6s makes 94-96% and Au 5d makes 6-4% contribution to the Au-based orbitals in Al-Au bonds, which is smaller than the BAu n0/- series, partially reflecting the relativistic effect of gold. The one-electron detachment energies of the anions and characteristic stretching vibrational frequencies of Al-Au bonds between 100-400 cm -1 have been calculated to facilitate future experimental characterization of these clusters.     

Structure evolution of gold cluster anions between the planar and cage structures by isoelectronic substitution: Au(n)- (n = 13-15) and MAu(n)- (n = 12-14; M = Ag, Cu)

The structural and electronic effects of isoelectronic substitution by Ag and Cu atoms on gold cluster anions in the size range between 13 and 15 atoms are studied using a combination of photoelectron spectroscopy and first-principles density functional calculations. The most stable structures of the doped clusters are compared with those of the undoped Au clusters in the same size range. The joint experimental and theoretical study reveals a new C(3v) symmetric isomer for Au(13)(-), which is present in the experiment, but has hitherto not been recognized. The global minima of Au(14)(-) and Au(15)(-) are resolved on the basis of comparison between experiment and newly computed photoelectron spectra that include spin-orbit effects. The coexistence of two isomers for Au(15)(-) is firmly established with convincing experimental evidence and theoretical calculations. The overall effect of the isoelectronic substitution is minor on the structures relative to those of the undoped clusters, except that the dopant atoms tend to lower the symmetries of the doped clusters.     

Modeling competitive interactions in proteins: vibrational spectroscopy of M+(n-methylacetamide)1(H2O)n=0-3, M=Na and K, in the 3 microm region

To properly understand the preferred structures and biological properties of proteins, it is important to understand how they are influenced by their immediate environment. Competitive intrapeptide, peptide...water, ion...water, and ion...peptide interactions, such as hydrogen bonding, play a key role in determining the structures, properties, and functionality of proteins. The primary types of hydrogen bonding involving proteins are intramolecular amide...amide (N-H...O=C) and intermolecular amide...water (O-H...O=C and H-O...H-N). n-Methylacetamide (NMA) is a convenient model for investigating these competitive interactions. An analysis of the IR photodissociation (IRPD) spectra of M+(n-methylacetamide)1(H2O)n=0-3 (M=Na and K) in the O-H and N-H spectral regions is presented. Ab initio calculations (MP2/cc-pVDZ) are used as a guide in identifying both the type and location of hydrogen bonds present. In larger clusters, where several structural isomers may be present in the molecular beam, ab initio calculations are also used to suggest assignments for the observed spectral features. The results presented offer insight to the nature of ion...NMA interactions in an aqueous environment and reveal how different ion...ligand pairwise interactions direct the extent of water...water and water...NMA hydrogen bonding observed.     

Density-functional study of structural and electronic properties of Si(n)C(n) (n=1-10) clusters

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the structural and electronic structure of Si(n)C(n) (n=1-10) clusters. The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster size n equals 4. Cagelike structures are favored as the cluster size increases. A distinct segregation between the silicon and carbon atoms is observed for these clusters. It is found that the C atoms favor to form five-membered rings as the cluster size n increases. However, the growth motif for Si atoms is not observed. The Si(n)C(n) clusters at n=2, 6, and 9 are found to possess relatively higher stability. On the basis of the lowest-energy geometries obtained, the size dependence of cluster properties such as binding energy, HOMO-LUMO gap, Mulliken charge, vibrational spectrum, and ionization potential has been computed and analyzed. The bonding characteristics of the clusters are discussed.     

Investigating bonding in small silicon-carbon clusters: exploration of the potential energy surfaces of Si3C4, Si4C3, and Si4C4 using ab initio molecular dynamics

A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si-C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms.     

Structural and electronic properties of Si(n), Si(n)-, and PSi(n-1) clusters (2 < or = n < or = 13): Theoretical investigation based on ab initio molecular orbital theory

The geometric and electronic structures of Si(n), Si(n)-, and PSi(n-1) clusters (2 < or = n < or = 13) have been investigated using the ab initio molecular orbital theory formalism. The hybrid exchange-correlation energy functional (B3LYP) and a standard split-valence basis set with polarization functions (6-31+G(d)) were employed to optimize geometrical configurations. The total energies of the lowest energy isomers thus obtained were recalculated at the MP2/aug-cc-pVTZ level of theory. Unlike positively charged clusters, which showed similar structural behavior as that of neutral clusters [Nigam et al., J. Chem. Phys. 121, 7756 (2004)], significant geometrical changes were observed between Si(n) and Si(n)- clusters for n = 6, 8, 11, and 13. However, the geometries of P substituted silicon clusters show similar growth as that of negatively charged Si(n) clusters with small local distortions. The relative stability as a function of cluster size has been verified based on their binding energies, second difference in energy (Delta2 E), and fragmentation behavior. In general, the average binding energy of Si(n)- clusters is found to be higher than that of Si(n) clusters. For isoelectronic PSi(n-1) clusters, it is found that although for small clusters (n < 4) substitution of P atom improves the binding energy of Si(n) clusters, for larger clusters (n > or = 4) the effect is opposite. The fragmentation behavior of these clusters reveals that while small clusters prefer to evaporate monomer, the larger ones dissociate into two stable clusters of smaller size. The adiabatic electron affinities of Si(n) clusters and vertical detachment energies of Si(n)- clusters were calculated and compared with available experimental results. Finally, a good agreement between experimental and our theoretical results suggests good prediction of the lowest energy isomeric structures for all clusters calculated in the present study.     

Naked deltahedral silicon clusters in solution: synthesis and characterization of Si9(3-) and Si5(2-)

Elusive for over 100 years, deltahedral Zintl ions of silicon are finally synthesized and structurally characterized. The two clusters reported here, Si93- and Si52-, are analogous to known clusters of the heavier members of this group: Ge, Sn, and Pb.     

A new class of silicon-carbon clusters: a full study of the hydrogenated SinC2H2, n=3,4,5, clusters in comparison with their isoelectronic carboranes C2BnHn+2

The structural and electronic characteristics of the Si(n)C(2)H(2), n=3,4,5, clusters are studied by ab initio calculations based on coupled cluster and density functional theory using the hybrid B3LYP functional. It is demonstrated that all three clusters are structurally and electronically homologous to the corresponding isoelectronic organometallic carboranes C(2)B(n)H(n+2). This homology, which is in full agreement with the analogy of Si(6) (2-) and B(6)H(6) (2-) demonstrated recently by the author [J. Chem. Phys. 127, 014314 (2007)], includes not only the ground states but also the lower-lying isomers as well. These lowest lying isomers can be obtained by ortho, para, and meta substitutions from the corresponding Si(n) (2-), n=3,4,5, dianions. The energetic ordering of the low-lying isomers is in full agreement with the known valence and topological charge stability rules developed for carboranes. The hydrogenated clusters are much more stable than their nonhydrogenated counterparts. It is suggested that Si(3)C(2)H(2), Si(4)C(2)H(2), and Si(5)C(2)H(2), which can be probably found in interstellar space, are special examples of a general class of silicon-carbon clusters of the form Si(n)C(2)H(2), with analogous properties and similarities to the corresponding carboranes C(2)B(n)H(n+2). It is furthermore illustrated that the lowest energy structures of the Si(n)C(2) clusters can be obtained through a systematic and straightforward procedure from the Si(n)C(2)H(2) clusters. The present results could hopefully make possible the exploitation of the rich borane and carborane chemistry for the design and development of novel silicon and silicon-carbon composite nanomaterials.     

Electronic structure and stability of AlnPn (n = 2-4) clusters

The geometry, electronic configurations, harmonic vibrational frequencies, and stability of the structural isomers of aluminum phosphide clusters have been investigated using the density functional theory. For dimers and trimers, the lowest energy structures are cyclic (IIs, IIIs) with D(nh) symmetry. The caged structure with Td symmetry (Xs) lie lowest in energy among the tetramers. The Al--P bond dominates the structures for many isomers so that one preferred dissociation channel is loss of the AlP monomer. The hybridization and chemical bonding in the different structures are also discussed. Comparisons with silicon and boron nitride clusters, the ground state structures of Al(n)P(n) clusters are analogous to those of their corresponding Si(2n) counterparts. This similarity follows the isoelectronic principle.     

Low-energy isomer identification, structural evolution, and magnetic properties in manganese-doped gold clusters MnAu(n) (n = 1-16)

The size-dependent electronic, structural, and magnetic properties of Mn-doped gold clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. A number of new isomers are obtained for neutral MnAu(n) (n = 1-16) clusters to probe the structural evolution. The two-dimensional (2D) to three-dimensional (3D) transition occurs in the size range n = 7-10 with manifest structure competitions. From size n = 13 to n = 16, the MnAu(n) prefers a gold cage structure with Mn atom locating at the center. The relative stabilities of the ground-state MnAu(n) clusters show a pronounced odd-even oscillation with the number of Au atoms. The magnetic moments of MnAu(n) clusters vary from 3 μ(B) to 6 μ(B) with the different cluster size, suggesting that nonmagnetic Au(n) clusters can serve as a flexible host to tailor the dopant's magnetism, which has potential applications in new nanomaterials with tunable magnetic properties.     

The boron connection: a parallel description of aromatic, bonding, and structural characteristics of hydrogenated silicon-carbon clusters and isovalent carboranes

The aromatic, bonding, and structural characteristics of the Si 4C 2H 2-C 2B 4H 6, Si 2C 4H 4-C 4B 2H 6, and other Si n C 2H 2-C 2B n H n+2 ( n = 1, 2, 3, 5) isovalent pairs are studied using density functional theory (DFT) and coupled cluster methods to fully illustrate the homology of the two species. This homology, which is based on the replacement of the carborane B-H units by isovalent Si atoms, is extended to all three characteristics (structural, electronic, and aromatic) and includes all three lowest-energy structures of the isovalent pairs. This type of "boron connection", which has been tested for silicon clusters recently, seems to be a valid and extremely useful concept. For the aromatic properties of the Si n C 2H 2-C 2B n H n+2 species, expressed through the nucleus independent chemical shifts (NICS), a strange odd-even effect with respect to the number of Si atoms is observed which seems rather difficult to explain. To help possible future identification and characterization of the Si n C 2H 2 clusters, their infrared, Raman, and optical excitation spectra are calculated within the framework of DFT, using the 6-311+G(2d, p) basis set. It is expected that the present results would facilitate the exploitation of the well-known carborane and metallacarborane chemical properties and applications for the design and development of novel silicon-carbon-based composite materials.     

Geometries and electronic properties of the neutral and charged rare earth Yb-doped Si(n) (n = 1-6) clusters: a relativistic density functional investigation

The neutral and charged YbSi(n) (n = 1-6) clusters considering different spin configurations have been systematically investigated by using the relativistic density functional theory with generalized gradient approximation. The total bonding energies, equilibrium geometries, Mulliken populations (MP), Hirshfeld charges (HC), fragmentation energies, and highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps are calculated and discussed. The optimized geometries indicate that the most stable YbSi(n) (n = 1-6) clusters keep basically the analogous frameworks as the low-lying Si(n)(+1) clusters, while the charged species deviate from their neutral counterparts, and that the doped Yb tends to occupy the substitutional site of the neutral and charged YbSi(n) isomers. The relative stabilities are investigated in terms of the calculated fragmentation energies, exhibiting enhanced stabilities for the remarkably stable neutral and charged YbSi2 and YbSi5 clusters. Furthermore, the calculated MP and HC values show that the charges of the neutral and charged YbSi(n) clusters transfer from the Yb atom to Si(n) atoms and the Yb atom acts as an electron donor, and that the f orbitals of the Yb atom in the neutral and charged YbSi(n) clusters behave as core without involvement in chemical bonding. The calculated HOMO-LUMO gaps indicate that the YbSi2 and YbSi4+ clusters have stronger chemical stabilities. Comparisons of the Yb-doped Si(n) (n = 1-6) with available theoretical results of transition-metal-doped silicon clusters are made. The growth pattern is investigated also.