Boris B. Averkiev

A concentric planar doubly π-aromatic B19− cluster

Atomic clusters often show unique, size-dependent properties and have become a fertile ground for the discovery of novel molecular structures and chemical bonding. Here we report an investigation of the B₁₉⁻ cluster, which shows chemical bonding reminiscent of that in [10]annulene (C₁₀H₁₀) and [6]circulene (C₂₄H₁₂). Photoelectron spectroscopy reveals a relatively simple spectrum for B₁₉⁻, with a high electron-binding energy. Theoretical calculations show that the global minimum of B₁₉⁻ is a nearly circular planar structure with a central B₆ pentagonal unit bonded to an outer B₁₃ ring. Chemical bonding analyses reveal that the B₁₉⁻ cluster possesses a unique double π-aromaticity in two concentric π-systems, with two π-electrons delocalized over the central pentagonal B₆ unit and another ten π-electrons responsible for the π-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compounds; it can only be found in atomic clusters.

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.

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.

Theoretical design of planar molecules with a nona- and decacoordinate central atom

The maximum coordination number of the central atom in planar molecules generated by now in mole-cular beams was 8. We made use of the chemical bond model developed for planar boron clusters to check the possibility of existence of planar molecules with the coordination numbers 9 and 10. The objects of or study were the AlB9 and AlB10+ clusters which have local minima corresponding to highly symmetrical D9h and D10h structures, respectively. According to our calculations, the highly symmetrical structure of AlB9 is a global minimum or a low-lying isomer, and, therefore, it holds promise as a new ligand for coordination chemistry. The energy of the highly symmetrical structure of AlB10+ with the coordination number 10 is too high, and this structure is hardly synthetically feasible. Thus, 9 is presently the maximum coordination number of an atom in a planar molecule.

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)(-).

Planar nitrogen-doped aluminum clusters AlxN− (x=3–5)

The electronic and geometrical structures of three nitrogen-doped aluminum clusters, Al(x)N(-) (x=3-5), are investigated using photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra have been obtained for the nitrogen-doped aluminum clusters at four photon energies (532, 355, 266, and 193 nm). Global minimum structure searches for Al(x)N(-) (x=3-5) and their corresponding neutrals are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structures and low-lying isomers are compared with the experimental observations. Planar structures have been established for all the three Al(x)N(-) (x=3-5) anions from the joint experimental and theoretical studies. For Al(5)N(-), a low-lying nonplanar isomer is also found to contribute to the experimental spectra, signifying the onset of two-dimensional to three-dimensional transition in nitrogen-doped aluminum clusters. The chemical bonding in all the planar clusters has been elucidated on the basis of molecular orbital and natural bond analyses.

All-Transition Metal Aromaticity and Antiaromaticity

Though aromaticity in compounds containing a transition-metal atom has already been discussed for quite a long time, aromaticity in all-transition metal systems have been recognized only recently. There are examples of σ-, π-, and δ-aromaticity based on s-, p-, and d-AOs. We derived the counting rules for σ −, π-, δ-, and ϕ-aromaticity/antiaromaticity for both singlet/triplet coupled model triatomic and tetratomic systems so that one could use those to rationalize aromaticity and antiaromaticity in all-transition metal systems. These rules can be easily extended for any cyclic systems composed out of odd or even number of atoms. We elucidated the application of these rules to the all-transition metal cyclic systems: Au3 +/Au3 −, Na2Zn3, Hg4 6 −, Mo3O9 2 −, Sc3 −, Hf3, and Ta3 − clusters. We believe that the use of concepts of aromaticity, antiaromaticity and conflicting aromaticity can be an important theoretical tool for deciphering chemical bonding in various known and novel chemical compounds containing transition metal atoms.

Synthesis and some heterocyclisation reactions of CF2H- and CF2Cl-substituted 1,1-dicyanoethylenes

Abstract New CF 2 X-analogues of 1,1-dicyano-2,2-bis(trifluoromethyl)ethylene ( 1 ) (X=H, Cl) were synthesised by the condensation of polyfluoroketones with malononitrile followed by dehydration using thionyl chloride (or phosphorus pentoxide). The heterocyclisation reactions of new CF 2 X-analogues of 1,1-dicyano-2,2-bis(trifluoromethyl)ethylene with amidines, 5-aminopyrazoles and 3-methyl-2-pyrazolin-5-ones were systematically investigated.

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.