Planarization of B7- and B12- clusters by isoelectronic substitution: AlB6- and AlB11-

Small boron clusters have been shown to be planar from a series of combined photoelectron spectroscopy and theoretical studies. However, a number of boron clusters are quasiplanar, such as B(7)(-) and B(12)(-). To elucidate the nature of the nonplanarity in these clusters, we have investigated the electronic structure and chemical bonding of two isoelectronic Al-doped boron clusters, AlB(6)(-) and AlB(11)(-). Vibrationally resolved photoelectron spectra were obtained for AlB(6)(-), resulting in an accurate electron affinity (EA) for AlB(6) of 2.49 ± 0.03 eV. The photoelectron spectra of AlB(11)(-) revealed the presence of two isomers with EAs of 2.16 ± 0.03 and 2.33 ± 0.03 eV, respectively. Global minimum structures of both AlB(6)(-) and AlB(11)(-) were established from unbiased searches and comparison with the experimental data. The global minimum of AlB(6)(-) is nearly planar with a central B atom and an AlB(5) six membered ring, in contrast to that of B(7)(-), which possesses a C(2v) structure with a large distortion from planarity. Two nearly degenerate structures were found for AlB(11)(-) competing for the global minimum, in agreement with the experimental observation. One of these isomers with the lower EA can be viewed as substituting a peripheral B atom by Al in B(12)(-), which has a bowl shape with a B(9) outer ring and an out-of-plane inner B(3) triangle. The second isomer of AlB(11)(-) can be viewed as an Al atom interacting with a B(11)(-) cluster. Both isomers of AlB(11)(-) are perfectly planar. It is shown that Al substitution of a peripheral B atom in B(7)(-) and B(12)(-) induces planarization by slightly expanding the outer ring due to the larger size of Al.     

Aluminum avoids the central position in AlB9- and AlB10-: photoelectron spectroscopy and ab initio study

The structures and the electronic properties of two Al-doped boron clusters, AlB(9)(-) and AlB(10)(-), were investigated via joint photoelectron spectroscopy and high-level ab initio study. The photoelectron spectra of both anions are relatively broad and have no vibrational structure. The geometrical structures were established by unbiased global minimum searches using the Coalescence Kick method and comparison between the experimental and calculated vertical electron detachment energies. The results show that both clusters have quasi-planar structures and that the Al atom is located at the periphery. Chemical bonding analysis revealed that the global minimum structures of both anions can be described as doubly (σ- and π-) aromatic systems. The nona-coordinated wheel-type structure of AlB(9)(-) was found to be a relatively high-lying isomer, while a similar structure for the neutral AlB(9) cluster was previously shown to be either a global minimum or a low-lying isomer.     

All-boron analogues of aromatic hydrocarbons: B17- and B18-

We have investigated the structural and electronic properties of the B(17)(-) and B(18)(-) clusters using photoelectron spectroscopy (PES) and ab initio calculations. The adiabatic electron detachment energies of B(17)(-) and B(18)(-) are measured to be 4.23 ± 0.02 and 3.53 ± 0.05 eV, respectively. Calculated electron detachment energies are compared with experimental data, confirming the presence of one planar C(2v) ((1)A(1)) isomer for B(17)(-) and two nearly isoenergetic quasi-planar C(3v) ((2)A(1)) and C(s) ((2)A') isomers for B(18)(-). The stability and planarity/quasi-planarity of B(17)(-) and B(18)(-) are ascribed to σ- and π-aromaticity. Chemical bonding analyses reveal that the nature of π-bonding in B(17)(-) and B(18)(-) is similar to that in the recently elucidated B(16)(2-) and B(19)(-) clusters, respectively. The planar B(17)(-) cluster can be considered as an all-boron analogue of naphthalene, whereas the π-bonding in the quasi-planar B(18)(-) is reminiscent of that in coronene.     

Molecular wheel B8(2-) as a new inorganic ligand. photoelectron spectroscopy and ab initio characterization of LiB8(-)

The bare B(8) cluster was previously reported to be a D(7h) molecular wheel with a triplet group state. The B(8)(2-) dianion was predicted to be a closed-shell singlet and double aromatic D(7h) molecular wheel. Here we report the experimental observation of B(8)(2-) stabilized by a Li(+) cation in LiB(8)(-) and its experimental characterization using photoelectron spectroscopy. Theoretical searches lead to a C(7v) LiB(8)(-) global minimum structure, and its calculated photodetachment transitions are in good agreement with the experimental values. Except for a small out-of-plane distortion due to the asymmetric Li(+) capping, the B(8)(2-) unit in LiB(8)(-) is nearly identical to the bare B(8)(2-), suggesting it is a robust and stable structural unit and may be used as a new ligand and building block in chemistry.     

Probing the structures and chemical bonding of boron-boronyl clusters using photoelectron spectroscopy and computational chemistry: B(4)(BO)(n) (-) (n = 1-3)

The electronic and structural properties of a series of boron oxide clusters, B(5)O(-), B(6)O(2) (-), and B(7)O(3) (-), are studied using photoelectron spectroscopy and density functional calculations. Vibrationally resolved photoelectron spectra are obtained, yielding electron affinities of 3.45, 3.54, and 4.94 eV for the corresponding neutrals, B(5)O, B(6)O(2), and B(7)O(3), respectively. Structural optimizations show that these oxide clusters can be formulated as B(4)(BO)(n) (-) (n = 1-3), which involve boronyls coordinated to a planar rhombic B(4) cluster. Chemical bonding analyses indicate that the B(4)(BO)(n) (-) clusters are all aromatic species with two π electrons.     

Al(6)(2-) - fusion of two aromatic Al(3)(-) units. A combined photoelectron spectroscopy and ab initio study of M(+)[Al(6)(2-)] (M = Li,Na, K,Cu, and Au)

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl(6)(-) (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl(6)(-) (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl(6)(-) clusters can be viewed as an M(+) cation interacting with an Al(6)(2-) dianion. Al(6)(2-) was found to possess an O(h) ground-state structure, and all of the MAl(6)(-) clusters possess a C(3v) ground-state structure derived from the O(h) Al(6)(2-). Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C(3v)ground-state structures for the MAl(6)(-) clusters. A detailed molecular orbital (MO) analysis is conducted for Al(6)(2-) and compared with Al(3)(-). It was shown that Al(6)(2-) can be considered as the fusion of two Al(3)(-) units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e' MOs of Al(3)(-), rather than those derived from the differences between the occupied 2a(1)' and 2a(2)' ' MOs of Al(3)(-), provides the key bonding interactions for the fusion of the two Al(3)(-) into Al(6)(2-). Because there are only four bonding MOs (one pi and three sigma MOs), an analysis of resonance structures was performed for the O(h)Al(6)(2-). It is shown that every face of the Al(6)(2-) octahedron still possesses both pi- and sigma-aromaticity, analogous to Al(3)(-), and that in fact Al(6)(2-) can be viewed to possess three-dimensional pi- and sigma-aromaticity with a large resonance stabilization.     

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.     

Density functional study of 8- and 11-vertex polyhedral borane structures: comparison with bare germanium clusters

Density functional theory (DFT) at the hybrid B3LYP level has been applied to the polyhedral boranes B(n)H(n)(z) (n = 8 and 11, z = -2, -4, and -6) for comparison with isoelectronic germanium clusters Ge(n)(z). The energy differences between the global minima and other higher energy borane structures are much larger relative to the case of the corresponding bare germanium clusters. Furthermore, for both B(8)H(8)(2-) and B(11)H(11)(2-), the lowest energy computed structures are the corresponding experimentally observed most spherical deltahedra predicted by the Wade-Mingos rules, namely the D(2)(d) bisdisphenoid and the C(2)(v) edge-coalesced icosahedron, respectively. Only in the case of B(8)H(8)(2-) is there a second structure close (+2.6 kcal/mol) to the D(2)(d) bisdisphenoid global minimum, namely the C(2)(v) bicapped trigonal prism corresponding to the "square" intermediate in a single diamond-square-diamond process that can lead to the experimentally observed room temperature fluxionality of B(8)H(8)(2-). Stable borane structures with 3-fold symmetry (e.g., D(3)(h), C(3)(v), etc.) are not found for boranes with 8- and 11-vertices, in contrast to the corresponding germanium clusters where stable structures derived from the D(3)(d) bicapped octahedron and D(3)(h) pentacapped trigonal prism are found for the 8- and 11-vertex systems, respectively. The lowest energy structures found for the electron-rich boranes B(8)H(8)(4-) and B(11)H(11)(4-) are nido polyhedra derived from a closo deltahedron by removal of a relatively high degree vertex, as predicted by the Wade-Mingos rules. They relate to isoelectronic species found experimentally, e.g., B(8)H(12) and R(4)C(4)B(4)H(4) for B(8)H(8)(4-) and C(2)B(9)H(11)(2-) for B(11)H(11)(4-). Three structures were found for B(11)H(11)(6-) with arachno type geometry having two open faces in accord with the Wade-Mingos rules.     

Carbon avoids hypercoordination in CB6(-), CB6(2-), and C2B5(-) planar carbon-boron clusters

The structures and bonding of CB6-, C2B5-, and CB62- are investigated by photoelectron spectroscopy and ab initio calculations. It is shown that the global minimum structures for these systems are distorted heptacyclic structures. The previously reported hexacyclic structures with a hypercoordinate central carbon atom are found to be significantly higher in energy and were not populated under current experimental conditions. The reasons why carbon avoids hypercoordination in these planar carbon-boron clusters are explained through detailed chemical-bonding analyses.     

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.     

Anion photoelectron spectroscopy and density functional study of small aluminum-vanadium oxide clusters

Small aluminum-vanadium oxide clusters, AlVO(y)(-) (y = 1-3) and Al(x)VO(2)(-) (x = 2, 3), were investigated with anion photoelectron spectroscopy and density functional calculations. The adiabatic detachment energies of AlVO(y)(-) were estimated to be 1.06 ± 0.05, 1.50 ± 0.08, and 2.83 ± 0.08 eV for y = 1, 2, and 3. Those of Al(2)VO(2)(-) and Al(3)VO(2)(-) were estimated to be 1.22 ± 0.08 and 1.25 ± 0.08 eV. Comparison of theoretical calculations with experimental measurement suggests that the most probable structure of AlVO(-) cluster is quasilinear with O atom in the middle. AlVO(2)(-) has an irregular chain structure of Al-O-V-O and a C(2v) cyclic structure very close in energy. The structure of AlVO(3)(-) cluster is evolved from the C(2v) cyclic AlVO(2)(-) structure by adding the third O atom to the V atom. Al(2)VO(2)(-) has a pair of nearly degenerate Al-O-V-O-Al chain structures that can be considered as cis and trans forms. Al(3)VO(2)(-) probably has two low-lying isomers each containing a four-membered ring. The structures of the corresponding neutral clusters are discussed.     

Photoelectron spectroscopy and ab initio study of the doubly antiaromatic B(6) (2-) dianion in the LiB(6) (-) cluster

A metal-boron mixed cluster LiB(6) (-) was produced and characterized by photoelectron spectroscopy and ab initio calculations. A number of electronic transitions were observed and used to compare with theoretical calculations. An extensive search for the global minimum of LiB(6) (-) was carried out via an ab initio genetic algorithm technique. The pyramidal C(2v) ((1)A(1)) molecule was found to be the most stable at all levels of theory. The nearest low-lying isomer was found to be a triplet C(2) ((3)B) structure, 9.2 kcal/mol higher in energy. Comparison of calculated detachment transitions from LiB(6) (-) and the experimental photoelectron spectra confirmed the C(2v) pyramidal global minimum structure. Natural population calculation revealed that LiB(6) (-) is a charge-transfer complex, Li(+)B(6) (2-), in which Li(+) and B(6) (2-) interact in a primarily ionic manner. Analyses of the molecular orbitals and chemical bonding of B(6) (2-) showed that the planar cluster is twofold (pi- and sigma-) antiaromatic, which can be viewed as the fusion of two aromatic B(3) (-) units.     

CnAl (n=2-11)团簇的结构特征与稳定性

采用密度泛函理论(DFT)的B3LYP方法, 在6-311++G**水平上对CnAl(n=2-11)团簇的几何构型和电子结构进行了结构优化和振动频率计算. 结果表明, n=2的CnAl团簇基态结构为Al原子与两个C原子相连形成的环状结构, n=3-11均为Al原子端基配位的线状结构. 通过对基态结构的能量分析, 得到了n为偶数的CnAl团簇比n为奇数团族稳定的结论.     

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.     

InnNa和InnNa+(n=2-8)的团簇结构和电子性质

在密度泛函理论B3LYP水平上, 对InnNa和InnNa+(n=2-8)团簇进行了结构优化和振动频率计算. 计算结果表明, InnNa(n=2、3、4、6)最稳定结构中的对称性分别为C2v、C3v、C4v和C2v, 而InnNa(n=5、7、8)的最稳定结构的对称性为C1点群. 从InnNa(n=4-8)的最稳定结构可以看出, Na原子均位于四个In原子形成的四边形面上. 对于InnNa+(n=2-8), 除了In2Na+、In4Na+和In7Na+, 其它结构都与其中性结构相似. 进一步计算InnNa(n=2-8)团簇的平均结合能、能量的二阶差分以及绝热电离能表明, InnNa(n=2-8)团簇能量的二阶差分呈现奇偶交替特征, In4Na和In6Na较其它团簇更为稳定, 而且理论计算得到的绝热电离能和实验结果吻合得很好.     

A photoelectron spectroscopic and theoretical study of B16- and B16(2-): an all-boron naphthalene

The structure and chemical bonding of B16- were studied using ab initio calculations and photoelectron spectroscopy. Its global minimum is found to be a quasi-planar and elongated structure (C2h). Addition of an electron to B16- resulted in a perfectly planar and closed shell B16(2-) (D2h), which is shown to possess 10 pi electrons with a pi-bonding pattern similar to that of naphthalene and can thus be considered as an "all-boron naphthalene", a new member in the growing family of hydrocarbon analogues of boron clusters.     

Structures and stability of B-doped Al clusters: AlnB and AlnB2 (n=1-7)

Various structural possibilities for Al(n)B(m) (n=1-7, m=1-2) neutral isomers were investigated using B3LYP6-311G(d) and CCSD(T)6-311G(d) methods. Our calculations predicted the existence of a number of previously unknown isomers. The B atom favors to locate over/inside of all clusters in this series. All structures of the Al(n)B (n=2-7) may be derived from capping/putting a B atom over/inside the Al(n) cluster. All Al(n)B(2) (n=1-5) may be understood as two substitutions of Al atoms by B atoms in the Al(n+2) molecule. The strong B-B bond is a dominant factor in the building-up principle of mixed Al(n)B(2) neutral clusters. The second difference in energy showed that the Al(n)B(m) clusters with even n+m are more stable than those with odd n+m. Our results and analyses revealed that the mixed Al-B clusters exhibit aromatic behaviors.     

Relaxation of (CS2)2(-) to its global minimum mediated by water molecules: photoelectron imaging study

The coexistence of several isomers of (CS 2) 2 (-) is examined via photoelectron imaging at 355 and 266 nm. Assisted by theoretical calculations, the bands in the photoelectron spectra are assigned to the CS 2 (-).CS 2 ion-molecule complex ( C s symmetry, (2)A' electronic state) and two covalently bound dimer-anion structures: C 2 v ( (2)B 1) and D 2 h ( (2)B 3g). The isomer distribution depends sensitively on the ion source conditions, particularly the presence of water in the precursor gas mixture. The intensity variation of the photoelectron bands suggests that the presence of water enhances the formation of the global-minimum C 2 v ( (2)B 1) structure, particularly relative to the metastable (local-minimum) ion-molecule complex. This trend is rationalized with two assumptions. The first is that the presence of H 2O at the cluster formation stage facilitates the nonadiabatic transitions necessary for reaching the global-minimum dimer-anion equilibrium when starting from the CS 2 (-) + CS 2 asymptote. The second is that the initial clusters formed in the presence of water tend to have, on average, more internal energy, which is needed for overcoming the potential energy barriers separating the metastable equilibria from the global-minimum structure. As the covalent bonds are formed, excess solvent molecules are evaporated from the cluster, giving rise to stable (CS 2) 2 (-) dimer anions. In the (CS 2) n (-), n >or= 3, and (CS 2) 2 (-)(H 2O) m , m > 0, clusters, the population of the covalent-dimer core structures diminishes drastically due to more favorable solvent interactions with the monomer-anion (i.e., CS 2 (-)) core.     

Photoelectron spectroscopy and density functional calculations of CuSi(n)- (n = 4-18) clusters

We conducted a combined anion photoelectron spectroscopy and density functional theory study on the structural evolution of copper-doped silicon clusters, CuSi(n)(-) (n = 4-18). Based on the comparison between the experiments and theoretical calculations, CuSi(12)(-) is suggested to be the smallest fully endohedral cluster. The low-lying isomers of CuSi(n)(-) with n ≥ 12 are dominated by endohedral structures, those of CuSi(n)(-) with n < 12 are dominated by exohedral structures. The most stable structure of CuSi(12)(-) is a double-chair endohedral structure with the copper atom sandwiched between two chair-style Si(6) rings or, in another word, encapsulated in a distorted Si(12) hexagonal prism cage. CuSi(14)(-) has an interesting C(3h) symmetry structure, in which the Si(14) cage is composed by three four-membered rings and six five-membered rings.     

Low-lying isomers of the B9(-) boron cluster: the planar molecular wheel versus three-dimensional structures

The B(9)(-) cluster was found previously to be an unprecedented molecular wheel containing an octacoordinate planar boron with D(8h) symmetry in a combined photoelectron spectroscopy (PES) and theoretical study [H. J. Zhai et al., Angew. Chem., Int. Ed. 42, 6004 (2003)]. However, the PES spectra of B(9)(-) exhibit minor features that cannot be explained by the global minimum D(8h) structure, suggesting possible contributions from low-lying isomers at finite temperatures. Here we present Car-Parrinello molecular dynamics with simulated annealing simulations to fully explore the potential energy surface of B(9)(-) and search for low-lying isomers that may account for the minor PES features. We performed density functional theory (DFT) calculations with different exchange-correlation functionals and ab initio calculations at various levels of theory with different basis sets. Two three-dimensional low-lying isomers were found, both of C(s) symmetry, 6.29 (C(s)-2) and 10.23 (C(s)-1) kcal/mol higher in energy than the D(8h) structure at the highest CCSD(T) level of theory. Calculated detachment transitions from the C(s)-2 isomer are in excellent agreement with the minor features observed in the PES spectra of B(9)(-). The B(9)(-) cluster proves to be a challenge for most DFT methods and the calculated relative energies strongly depend on the exchange-correlation functionals, providing an excellent example for evaluating the accuracies of various DFT methods.