Constantin Romanescu

B22- and B23-: all-boron analogues of anthracene and phenanthrene.

Clusters of boron atoms exhibit intriguing size-dependent structures and chemical bonding that are different from bulk boron and may lead to new boron-based nanostructures. We report a combined photoelectron spectroscopic and ab initio study of the 22- and 23-atom boron clusters. The joint experimental and theoretical investigation shows that B(22)(-) and B(23)(-) possess quasi-planar and planar structures, respectively. The quasi-planar B(22)(-) consists of fourteen peripheral atoms and eight interior atoms in a slightly buckled triangular lattice. Chemical bonding analyses of the closed-shell B(22)(2-) species reveal seven delocalized π orbitals, which are similar to those in anthracene. B(23)(-) is a perfectly planar and heart-shaped cluster with a pentagonal cavity and a π-bonding pattern similar to that in phenanthrene. Thus, B(22)(-) and B(23)(-), the largest negatively charged boron clusters that have been characterized experimentally to date, can be viewed as all-boron analogues of anthracene and phenanthrene, respectively. The current work shows not only that boron clusters are planar at very large sizes but also that they continue to yield surprises and novel chemical bonding analogous to specific polycyclic aromatic hydrocarbons.

Transition-Metal-Centered Monocyclic Boron Wheel Clusters (M©Bn): A New Class of Aromatic Borometallic Compounds

Atomic clusters have intermediate properties between that of individual atoms and bulk solids, which provide fertile ground for the discovery of new molecules and novel chemical bonding. In addition, the study of small clusters can help researchers design better nanosystems with specific physical and chemical properties. From recent experimental and computational studies, we know that small boron clusters possess planar structures stabilized by electron delocalization both in the σ and π frameworks. An interesting boron cluster is B(9)(-), which has a D(8h) molecular wheel structure with a single boron atom in the center of a B(8) ring. This ring in the D(8h)-B(9)(-) cluster is connected by eight classical two-center, two-electron bonds. In contrast, the clusters central boron atom is bonded to the peripheral ring through three delocalized σ and three delocalized π bonds. This bonding structure gives the molecular wheel double aromaticity and high electronic stability. The unprecedented structure and bonding pattern in B(9)(-) and other planar boron clusters have inspired the designs of similar molecular wheel-type structures. But these mimics instead substitute a heteroatom for the central boron. Through recent experiments in cluster beams, chemists have demonstrated that transition metals can be doped into the center of the planar boron clusters. These new metal-centered monocyclic boron rings have variable ring sizes, M©B(n) and M©B(n)(-) with n = 8-10. Using size-selected anion photoelectron spectroscopy and ab initio calculations, researchers have characterized these novel borometallic molecules. Chemists have proposed a design principle based on σ and π double aromaticity for electronically stable borometallic cluster compounds, featuring a highly coordinated transition metal atom centered inside monocyclic boron rings. The central metal atom is coordinatively unsaturated in the direction perpendicular to the molecular plane. Thus, chemists may design appropriate ligands to synthesize the molecular wheels in the bulk. In this Account, we discuss these recent experimental and theoretical advances of this new class of aromatic borometallic compounds, which contain a highly coordinated central transition metal atom inside a monocyclic boron ring. Through these examples, we show that atomic clusters can facilitate the discovery of new structures, new chemical bonding, and possibly new nanostructures with specific, advantageous properties.

A photoelectron spectroscopy and ab initio study of B21−: Negatively charged boron clusters continue to be planar at 21

The structures and chemical bonding of the B(21)(-) cluster have been investigated by a combined photoelectron spectroscopy and ab initio study. The photoelectron spectrum at 193 nm revealed a very high adiabatic electron binding energy of 4.38 eV for B(21)(-) and a congested spectral pattern. Extensive global minimum searches were conducted using two different methods, followed by high-level calculations of the low-lying isomers. The global minimum of B(21)(-) was found to be a quasiplanar structure with the next low-lying planar isomer only 1.9 kcal/mol higher in energy at the CCSD(T)/6-311-G* level of theory. The calculated vertical detachment energies for the two isomers were found to be in good agreement with the experimental spectrum, suggesting that they were both present experimentally and contributed to the observed spectrum. Chemical bonding analyses showed that both isomers consist of a 14-atom periphery, which is bonded by classical two-center two-electron bonds, and seven interior atoms in the planar structures. A localized two-center two-electron bond is found in the interior of the two planar isomers, in addition to delocalized multi-center σ and π bonds. The structures and the delocalized bonding of the two lowest lying isomers of B(21)(-) were found to be similar to those in the two lowest energy isomers in B(19)(-).

Aromatic Metal-Centered Monocyclic Boron Rings: Co©B8− and Ru©B9−†

Bulk boron, which is characterized by 3D cage-like structural features, is a refractory material. 2] However, 3D cage structures were suggested to be unstable for small boron clusters, and planar or quasi-planar structures have been proposed instead. Experimental studies combined with high-level calculations have shown that small boron cluster ions are planar up to at least B20 , whereas Bn + ions have been found to be planar up to n = 16. The chemical bonding in the planar boron clusters has been found to be quite remarkable; in addition to the strong and localized bonding in the circumferences, there are two types of delocalized bonding—the in-plane s and the out-of-plane p bonding, each of which follows the (4N + 2) H ckel rule for aromaticity. In particular, systems with six s and six p electrons (N = 1) are doubly aromatic, and give rise to highly symmetric planar clusters, such as B8 2 and B9 , which each contain a central B atom and a B7 and B8 monocyclic ring, respectively. In the D7h B8 2 and D8h B9 molecular wheels, each B atom in the circumference contributes two electrons to the B–B peripheral covalent bonds and one electron to the delocalized bonds, whereas the central B atom contributes all its valence electrons to the delocalized bonds. These novel bonding situations suggest that other atoms with appropriate numbers of valence electrons and sizes may be able to replace the central boron atom to produce M Bn-type clusters. [12]

Observation of the Highest Coordination Number in Planar Species: Decacoordinated Ta©B10− and Nb©B10− Anions

Coordination number is one of the most fundamental characteristics of molecular structures. Molecules with high coordination numbers often violate the octet and the 18 electron rules and push the boundary of our understanding of chemical bonding and structures. We have been searching for the highest possible coordination number in a planar species with equal distances between the central atom and all peripheral atoms. To successfully design planar chemical species with such high coordination one must take into account both mechanical and electronic factors. The mechanical factor requires the right size of the central atom to fit into the cavity of a monocyclic ring. The electronic factor requires the right number of valence electrons to achieve electronic stability of the high-symmetry structure. Boron is known to form highly symmetric planar structures owing to its ability to participate simultaneously in localized and delocalized bonding. The planar boron clusters consist of a peripheral ring featuring strong two-center-two-electron (2c-2e) B–B s bonds and one or more central atoms bonded to the outer ring through delocalized s and p bonds. The starting point for the present work is that the bare eight-atom and nine-atom planar boron clusters were found to reach coordination number seven in the D7h B8 neutral or B8 2 as a part of the LiB8 cluster 3] or eight in the D8h B9 molecular wheel. The CB6 2 , C3B4, and CB7 wheel-type structures with hexaand heptacoordinated carbon atom were first considered computationally by Schleyer and co-workers. The high symmetry hypercoordinated structures were found to be local minima because they “fulfill both the electronic and geometrical requirements for good bonding”. 9] In particular, Schleyer and co-workers pointed out that the wheel structures are p aromatic with 6 p electrons. In joint photoelectron spectroscopy (PES) and theoretical studies it was shown that carbon occupies the peripheral position in such clusters rather than the center, because C is more electronegative than B and thus prefers to participate in localized 2c-2e s bonding, which is possible only at the circumference of the wheel structures. 11] A series of planar wheel-type boron rings with a main group atom in the center and coordination numbers 6– 10 have been probed theoretically. So far the joint PES and ab initio studies of aluminum-doped boron clusters showed that the aluminum atom avoids the central position in the AlB6 , AlB7 , AlB8 , AlB9 , AlB10 , and AlB11 systems. Recently, a transition-metal-doped boron cluster, Ru B9 , with the highest coordination number known to date was reported. We developed a chemical bonding model, which allows the design of planar molecules with high coordination numbers. According to the model, 2n electrons in the M Bn species form n 2c-2e peripheral B–B s bonds. The remaining valence electrons form two types of delocalized bonding, in-plane s and out-of-plane p bonding, and therefore, should satisfy the (4N + 2) H ckel rule separately for s and p aromaticity to attain highly symmetric structures with high electronic stability. In pure wheel-type boron clusters each B atom in the circumference contributes two electrons to the B–B peripheral covalent bonds and one electron to the delocalized bonds, whereas the central B atom contributes all its valence electrons to the delocalized bonds. Thus, out of 26 valence electrons in B8 2 or 28 in B9 , 14 or 16 valence electrons form peripheral covalent 2c-2e s bonds, leaving six s and six p electrons (N = 1 for the 4N + 2 rule) for double (s and p) aromaticity. However, pure planar boron clusters cannot go beyond coordination number eight because of the mechanical factor (the small size of the central boron atom). For example, the B10 cluster does not contain a ninecoordinated boron atom, because the boron atom is too small to fit in the central position of a B9 ring. [2] Since the central atom participates only in delocalized bonding, atoms more electronegative than boron such as carbon avoid the central position. 11,19] Transition-metal atoms, on the other hand, are well-suited for the central position in M Bn species. To satisfy the peripheral B B bonding and the s and p H ckel aromaticity for N = 1, the electronic requirement for the central atom in high-symmetry species, such as M Bn k , is x = 12 n k, where x is the valence of the transition-metal atom M. Ru B9 satisfies the formula and is the first example of an [*] T. R. Galeev, Prof. Dr. A. I. Boldyrev Department of Chemistry and Biochemistry Utah State University, Logan, UT 84322 (USA) E-mail: a.i.boldyrev@usu.edu Homepage: http://www.chem.usu.edu/~boldyrev/

Transition-metal-centered nine-membered boron rings: MⓒB9 and MⓒB9(-) (M = Rh, Ir).

We report the observation of two transition-metal-centered nine-atom boron rings, RhⓒB(9)(-) and IrⓒB(9)(-). These two doped-boron clusters are produced in a laser-vaporization supersonic molecular beam and characterized by photoelectron spectroscopy and ab initio calculations. Large HOMO-LUMO gaps are observed in the anion photoelectron spectra, suggesting that neutral RhⓒB(9) and IrⓒB(9) are highly stable, closed shell species. Theoretical calculations show that RhⓒB(9) and IrⓒB(9) are of D(9h) symmetry. Chemical bonding analyses reveal that these complexes are doubly aromatic, each with six completely delocalized π and σ electrons, which describe the bonding between the central metal atom and the boron ring. This work establishes firmly the metal-doped B rings as a new class of novel aromatic molecular wheels.

Probing the structures of neutral boron clusters using infrared/vacuum ultraviolet two color ionization: B11, B16, and B17.

The structures of neutral boron clusters, B(11), B(16), and B(17), have been investigated using vibrational spectroscopy and ab initio calculations. Infrared absorption spectra in the wavelength range of 650 to 1550 cm(-1) are obtained for the three neutral boron clusters from the enhancement of their near-threshold ionization efficiency at a fixed UV wavelength of 157 nm (7.87 eV) after resonant absorption of the tunable infrared photons. All three clusters, B(11), B(16), and B(17), are found to possess planar or quasi-planar structures, similar to their corresponding anionic counterparts (B(n) (-)), whose global minima were found previously to be planar, using photoelectron spectroscopy and theoretical calculations. Only minor structural changes are observed between the neutral and the anionic species for these three boron clusters.

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.

Valence isoelectronic substitution in the B8 − and B9 − molecular wheels by an Al dopant atom: Umbrella-like structures of AlB7 − and AlB8 −

The structures and the electronic properties of two aluminum-doped boron clusters, AlB(7)(-) and AlB(8)(-), were investigated using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of AlB(7)(-) and AlB(8)(-) are both broad, suggesting significant geometry changes between the ground states of the anions and the neutrals. Unbiased global minimum searches were carried out and the calculated vertical electron detachment energies were used to compare with the experimental data. We found that the Al atom does not simply replace a B atom in the parent B(8)(-) and B(9)(-) planar clusters in AlB(7)(-) and AlB(8)(-). Instead, the global minima of the two doped-clusters are of umbrella shapes, featuring an Al atom interacting ionically with a hexagonal and heptagonal pyramidal B(7) (C(6v)) and B(8) (C(7v)) fragment, respectively. These unique umbrella-type structures are understood on the basis of the special stability of the quasi-planar B(7)(3-) and planar B(8)(2-) molecular wheels derived from double aromaticity.

Superexcited state reconstruction of HCl using photoelectron and photoion imaging

The velocity-map imaging technique was used to record photoelectron and photofragment ion images of HCl following two-photon excitation of the E Sigma(+)(0+), V 1Sigma(+)(0+) (nu=9,10,11) states and subsequent ionization. The images allowed us to determine the branching ratios between autoionization and dissociation channels for the different intermediate states. These branching ratios can be explained on the basis of intermediate state electron configurations, since the configuration largely prohibits direct ionization in a one-electron process, and competition between autoionization and dissociation into H* (n=2)+Cl and H+Cl*(4s,4p,3d) is observed. From a fit to the vibrationally resolved photoelectron spectrum of HCl+ it is apparent that a single superexcited state acts as a gateway to autoionization and dissociation into H+Cl*(4s). Potential reconstruction of the superexcited state to autoionization was undertaken and from a comparison of different autoionization models it appears most likely that the gateway state is a purely repulsive and low-n Rydberg state with a (4Pi) ion core.