Aromaticity multifold

Multifold Aromaticity, Multifold Antiaromaticity, and Conflicting Aromaticity Implications for Stability and Reactivity of Clusters... 

Anderson and coworkers [59-66] produced boron cluster cations Bj-B in molecular beams using laser vaporization and studied their chemical reactivity and fragmentation properties. The structures of B3 —IBI3 cations have been established computationally (see review [7] for details) represented in Figure 29.1. In this chapter, we are discussing stability and reactivity of Bj — B 3 cations on the basis of their multifold aromaticity, multifold antiaromaticity, and conflicting aromaticity. 

The presented consideration of the family of cationic boron clusters exemplifies that the assessment of stability and reactivity of clusters can be performed at the qualitative level using multifold aromaticity, multifold antiaromaticity, and variety of conflicting aromaticities. 

Boldyrev and co-workers have shown that aromaticity /antiaromaticity profiles of these metal clusters have become interesting owing to their variety like multifold aromaticity, multifold antiaromaticity and conflicting aromaticity. They can be a-aromatic, a-antiaromatic, Ti-aro-matic, TT-antiaromatic or having, multiple (a- and n-) (anti) aromaticity and conflicting aromaticity (a-aromatic and Ti-antiaromatic at the same time or vice versa). In case d and f electrons are involved 5- and (j)- (anti) aromaticities and other combinations to give all possible multiple - and conflicting - type varieties are also possible. 

The electronic criteria of aromaticity/antiaromaticity are based on the electronic structure of the molecules. Molecular orbitals (MOs) that are highly delocalized over the entire cyclic nuclear framework support cyclic electron delocalization which characterizes aromaticity. Based on the symmetry of the highly delocalized MOs we can distinguish between the orbital-types of aromaticity, e.g., a-, it-, 8-, and (p-aromaticity. Many planar cyclic clusters formulated as c-E and c-E L (E = element, L = H, C, O, NH, n = 3-6) exhibit the so-called multifold aromaticity, which arises from various combinations of the aforementioned orbital types. Molecular orbitals are multicenter monoelectronic wave functions constructed from the overlap of atomic orbitals (AOs) belonging to the nuclear centers. The o-aromaticity is supported by delocalized ct-MOs, which can result from the linear combination of S-, p-, d-, or f-AOs. Similarly jr-aromaticity is supported by delocalized tt-MOs... 

Keywords Adaptive natural density partitioning All transition metal aromaticity Chemical bonding Cluster Multifold aromaticity... 

These results indicate that all the clusters (Si ", Si3Al+, AljSij, BejNj, SijBe, AI3P) studied in this work have highly negative NICS (0.0), NICS (0.5), NICS (1.0) values indicating the evidence of multifold aromaticity (n and a aromaticity) in... 

Aromaticity/antiaromaticity in cluster systems has certain peculiarities when compared with organic compounds. The striking feature of chemical bonding in cluster systems is the multifold nature of aromaticity, antiaromaticity, and conflicting aromaticity [3-10]. Double aromaticity (the simultaneous presence of [Pg.439]

In view of the multifold important biological activities of isoquinoline and P-carboline alkaloids, increasing efforts have been made to perform Pictet-Spengler reactions stereoselectively. The most obvious chiral auxiliary to induce asymmetric control is the stereogenic center of the corresponding aromatic amino acids, such as DOPA or tryptophan. 

The most comprehensive theoretical insight into the problem of aromaticity of cyclic transition metal systems was discussed in detail in a very recent review article published in Phys Chem Chem Phys by Boldyrev and coworkers [17], In this article the multifold nature of aromaticity, antiaromaticity, and even conflicting aromaticity (a-, Tt- and 8-aromaticity) has been analyzed in terms of the chemical bonding and electron counting rules. Therefore, we will discuss and comment herein on the latest developments in the field of stable aromatic three-membered rings of transition metal atoms. 

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