Lei-Ming Wang

Carbon Avoids Hypercoordination in CB6-, CB62-, 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.

Relativistic effects and the unique low-symmetry structures of gold nanoclusters.

The atomic structures of bare gold clusters provide the foundation to understand the enhanced catalytic properties of supported gold nanoparticles. However, the richness of diverse structures and the strong relativistic effects have posed considerable challenges for a systematic understanding of gold clusters with more than 20 atoms. We use photoelectron spectroscopy of size-selected anions, in combination with first principles calculations, to elucidate the structures of gold nanoclusters in a critical size regime from 55 to 64 atoms (1.1-1.3 nm in diameter). Au(55)(-) is found to be a nonicosahedral disordered cluster as a result of relativistic effects that induce strong surface contractions analogous to bulk surface reconstructions, whereas low-symmetry core-shell-type structures are found for Au(56)(-) to Au(64)(-). Au(58) exhibits a major electron-shell closing and is shown to possess a low-symmetry, but nearly spherical structure with a large energy gap. Clear spectroscopic and computational evidence has been observed, showing that Au(58)(-) is a highly robust cluster and additional atoms are simply added to its surface from Au(59)(-) to Au(64)(-) without inducing significant structural changes. The unique low-symmetry structures characteristic of gold nanoclusters due to the strong relativistic effects allow abundant surface defects sites, providing a key structure-function relationship to understand the catalytic capabilities of gold nanoparticles.

CB7−: Experimental and Theoretical Evidence against Hypercoordinate Planar Carbon†

In organic chemistry, saturated carbon is known to bond to four ligands tetrahedrally, as first recognized independently by J. H. van t Hoff and J. A. Le Bel in 1874. However, after the proposal by Hoffmann and co-workers of tetracoordinate planar carbon in 1970, extensive experimental and theoretical efforts were made to search for so-called anti-van t Hoff/ anti-Le Bel molecules (for recent reviews, see references [2– 4]). In particular, the first experimental and theoretical realization of pentaatomic planar-coordinated carbon species in 1999 and 2000, which confirmed earlier theoretical predictions, 10] has stimulated renewed interest in designing new tetracoordinate and even hypercoordinate planar carbon molecules. Notably, a series of hypercoordinate planar carbon species with boron ligands have been proposed. Although none of these species is the global minimum on the potentialenergy surfaces, it has been suggested that they may be viable experimentally. The two proposed hexaand heptacoordinate carbon species are D6h CB6 2 [13a,b,d,14c,15] and D7h CB7 , respectively. The CB7 species is isoelectronic to B8 2 , which we have shown previously to have a global-minimum D7h structure with a heptacoordinate boron atom. The D7h CB7 can be viewed as replacing the central B ion in B8 2 by a C atom. Herein we report serendipitous experimental observation of CB7 . It was investigated by photoelectron spectroscopy (PES) and ab initio calculations, which showed that the observed species is a C2v CB7 ion in which the C atom replaces a B ion at the rim of the D7h B8 2 molecular wheel. The experiment was performed with a laser-vaporization cluster source and a magnetic-bottle photoelectron spectrometer (see Experimental Section). We recently modified our cluster source by adding a 10-cm-long and 0.3-cm-diameter stainless steel tube to enhance cluster cooling. We were using boron clusters, which we have previously investigated extensively, to test the new cluster-source conditions. A B-enriched disk target containing a small amount of Au was used as the laser-vaporization target. Under certain conditions, when the vaporization laser was not perfectly aligned, we noted that in addition to the pure boron clusters we were also able to produce clusters containing one or two carbon atoms, as shown in Figure 1. The carbon most likely

Experimental and theoretical investigations of CB8−: towards rational design of hypercoordinated planar chemical species

We demonstrated in our joint photoelectron spectroscopic and ab initio study that wheel-type structures with a boron ring are not appropriate for designing planar molecules with a hypercoordinate central carbon based on the example of CB(8), and CB(8)(-) clusters. We presented a chemical bonding model, derived from the adaptive natural density partitioning analysis, capable of rationalizing and predicting planar structures either with a boron ring or with a carbon atom occupying the central hypercoordinate position. According to our chemical bonding model, in the wheel-type structures the central atom is involved in delocalized bonding, while peripheral atoms are involved in both delocalized bonding and two-center two-electron (2c-2e) sigma-bonding. Since carbon is more electronegative than boron it favors peripheral positions where it can participate in 2c-2e sigma-bonding. To design a chemical species with a central hypercoordinate carbon atom, one should consider electropositive ligands, which would have lone pairs instead of 2c-2e peripheral bonds. Using our extensive chemical bonding model that considers both sigma- and pi-bonding we also discuss why the AlB(9) and FeB(9)(-) species with octacoordinate Al and Fe are the global minima or low-lying isomers, as well as why carbon atom fits well into the central cavity of CAl(4)(2-) and CAl(5)(+). This represents the first step toward rational design of nano- and subnano-structures with tailored properties.

Planar to Linear Structural Transition in Small Boron-Carbon Mixed Clusters: CxB5-x - (x ) 1-5)

Bulk carbon and boron form very different materials, which are also reflected in their clusters. Small carbon clusters form linear structures, whereas boron clusters are planar. For example, it is known that the B(5)(-) cluster possesses a C(2v) planar structure and C(5)(-) is a linear chain. Here we study B/C mixed clusters containing five atoms, C(x)B(5-x)(-) (x = 1-5), which are expected to exhibit a planar to linear structural transition as a function of the C content. The C(x)B(5-x)(-) (x = 1-5) clusters were produced and studied by photoelectron spectroscopy; their geometric and electronic structures were investigated using a variety of theoretical methods. We found that the planar-to-linear transition occurs between x = 2 and 3: the global minimum structures of the B-rich clusters, CB(4)(-) and C(2)B(3)(-), are planar, similar to B(5)(-), and those of the C-rich clusters, C(3)B(2)(-) and C(4)B(-), are linear, similar to C(5)(-).

Evolution of the electronic properties of Snn− clusters (n=4–45) and the semiconductor-to-metal transition

The electronic structure of Sn(n) (-) clusters (n=4-45) was examined using photoelectron spectroscopy at photon energies of 6.424 eV (193 nm) and 4.661 eV (266 nm) to probe the semiconductor-to-metal transition. Well resolved photoelectron spectra were obtained for small Sn(n) (-) clusters (n< or =25), whereas more congested spectra were observed with increasing cluster size. A distinct energy gap was observed in the photoelectron spectra of Sn(n) (-) clusters with n< or =41, suggesting the semiconductor nature of small neutral tin clusters. For Sn(n) (-) clusters with n> or =42, the photoelectron spectra became continuous and no well-defined energy gap was observed, indicating the onset of metallic behavior for the large Sn(n) clusters. The photoelectron spectra thus revealed a distinct semiconductor-to-metal transition for Sn(n) clusters at n=42. The spectra of small Sn(n) (-) clusters (n< or =13) were also compared with those of the corresponding Si(n) (-) and Ge(n) (-) clusters, and similarities were found between the spectra of Sn(n) (-) and those of Ge(n) (-) in this size range, except for Sn(12) (-), which led to the discovery of stannaspherene (the icosahedral Sn(12) (2-)) previously [L. F. Cui et al., J. Am. Chem. Soc. 128, 8391 (2006)].

Photoelectron spectroscopy and Ab initio study of the structure and bonding of Al7N- and Al7N.

The electronic and geometrical structures of Al7N- are investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of Al7N- have been obtained at three photon energies with six resolved spectral features at 193 nm. The spectral features of Al7N- are relatively broad, in particular for the ground state transition, indicating a large geometrical change from the ground state of Al7N- to that of Al7N. The ground state vertical detachment energy is measured to be 2.71 eV, whereas only an upper limit of approximately 1.9 eV can be estimated for the ground state adiabatic detachment energy due to the broad detachment band. Global minimum searches for A7N- and Al7N are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structure and compared with the experimental data. Calculated results are in excellent agreement with the experimental data. The global minimum structure of Al7N- is found to possess C3v symmetry, which can be viewed as an Al atom capping a face of a N-centered Al6N octahedron. In the ground state of Al7N, however, the capping Al atom is pushed inward with the three adjacent Al-Al distances being stretched outward. Thus, even though Al7N still possesses C3v symmetry, it is better viewed as a N-coordinated by seven Al atoms in a cage-like structure. The chemical bonding in Al7N- is discussed on the basis of molecular orbital and natural bond analysis.

Experimental and theoretical investigation of three-dimensional nitrogen-doped aluminum clusters Al8N− and Al8N

The structure and electronic properties of the Al(8)N(-) and Al(8)N clusters were investigated by combined photoelectron spectroscopy and ab initio studies. Congested photoelectron spectra were observed and experimental evidence was obtained for the presence of multiple isomers for Al(8)N(-). Global minimum searches revealed several structures for Al(8)N(-) with close energies. The calculated vertical detachment energies of the two lowest-lying isomers, which are of C(2v) and C(s) symmetry, respectively, were shown to agree well with the experimental data. Unlike the three-dimensional structures of Al(6)N(-) and Al(7)N(-), in which the dopant N atom has a high coordination number of 6, the dopant N atom in the two low-lying isomers of Al(8)N(-) has a lower coordination number of 4 and 5, respectively. The competition between the Al-Al and Al-N interactions are shown to determine the global minimum structures of the doped aluminum clusters and results in the structural diversity for both Al(8)N(-) and Al(8)N.