The geometrics of tetracoordinate carbon

Walsh diagram for the distortion of methane (D2d→ Td→ D2d→ D4h and D4h→ C4v) shows qualitatively the reasons for the instability of planar or pyramidal tetracoordinate carbon compounds. Various methods available for stabilization of these unusual geometries are reviewed briefly

Hypercarbons in polyhedral structures

Though carbon is mostly tetravalent and tetracoordinated, there are several examples where the coordination number exceeds four. Structural varieties that exhibit hypercarbons in polyhedral structures such as polyhedral carboranes, sandwich complexes, encapsulated polyhedral structures and novel planar aromatic systems with atoms embedded in the middle are reviewed here. The structural variety anticipated with hypercoordinate carbon among carboranes is large as there are many modes of condensation that could lead to large number of new patterns. The relative stabilities of positional isomers of polyhedral carboranes, sandwich structures, and endohedral carboranes are briefly described. The mno rule accounts for the variety of structural patterns. Wheel-shaped and planar hypercoordinated molecules are recent theoretical developments in this area

A covalent way to stuff fullerenes

A novel direction in endohedral chemistry of fullerenes is proposed where empty space may be stuffed by covalently bound units

[n]peristylanes and [n]oxa[n]peristylanes (n=3-6): a theoretical study

Theoretical studies at the HF and Becke3LYP levels using 6-31G∗ basis sets were carried out on a series of [n]peristylanes and [n]oxa[n]peristylanes (n = 3-6) to understand their structure and energetics. The structures of the [3]- and [4]peristylanes (1, 2) and their oxa-derivatives (5, 6) were calculated to have the anticipated high symmetry, Cnv. In contrast, a Cs structure (9) at HF/6-31G∗ and another (25) at the Becke3LYP/6-31G∗ level were calculated for the [5]oxa[5]peristylane. The energy difference between them is extremely small even though there are major differences in the structures indicating a very soft potential energy surface. On the other hand, the potential energy surface of [6]oxa[6]peristylane is not as soft. Similar structures were also calculated for the top rings. Calculations on the seco-compounds 11-14 and 15-19 (Table 4) indicate that there is no unusual strain involved in the formation of 27 from 19. The Li+ interaction energies of the [n]oxa[n]peristylanes are 61.7 (n = 3), 72.8 (n = 4), 84.2 (n = 5) and 91.7 (n = 6) kcal mol-1 at the Becke3LYP/6-31G∗ level. Dramatic differences between the C-C bond lengths obtained from the solid state X-ray diffraction studies and those from the calculations for the [n]oxa[n]peristylanes were also observed

Unknowns in the Chemistry of boron

Structural links between benzenoid aromatics and graphite as well as saturated hydrocarbons and diamond are seen in high school text books. A similar understanding is only beginning to emerge in the chemistry of boron. Unlike benzenoid aromatics where the condensation is usually by edge-sharing, there are several ways of condensing polyhedral boranes. These include edge-sharing, triangular face sharing, four atom sharing and a single atom sharing. An electron counting mno rule similar to the Huckel 4n + 2 π electron counting rule will be presented for mono and condensed polyhedral boranes. Application of this rule shows that the structural details of β-rhombohedral boron, such as the vacancies and extra occupancies in the unit cell are a consequence of the electronic requirements rather than defects in the structure. While this is the beginning of a general understanding, there is a long way to go before the quantitative details emerge in this area, along with the synthesis and chemistry of a variety of condensed boranes. These ideas also provide a relationship between boron and fullerenes. Further, designing stable boron-rich candidates for physiological applications and materials for use in extreme conditions are areas that are waiting to be explored

Electronic structure study of the reactivity centres in Ti<SUB>8</SUB>C<SUB>12</SUB> clusters

The reactivity centres of Ti8C12, for the three structures suggested in conformity with experimental observations, are studied by extended Huckel theory, The C2 unit can complex with transition metal fragments such as Pt(PH3)2 with the unusual net result of transferring two electrons to Ti8C12. The metal centre, Ti can accommodate extra two-electron donors like CO. Model systems are used to explain the carbon and metal environment in Ti8C12

Face-selectivity in [4+2]-cycloadditions to novel polycyclic benzoquinones. Remarkable stereodirecting effects of a remote cyclopropane ring and an olefinic bond

π-Face selectivity in Diels-Alder reactions between specially crafted bicyclo[2.2.2]octane-fused benzoquinones, where the dienophilic moiety is imbedded in an isosteric environment, can be modulated by a remote olefinic bond and a cyclopropane ring. Quantum mechanical calculations while reproducing the observed diastereoselectivities at the TS level indicate the involvement of ground state orbital effects

Reduction of 1,4-dichlorobut-2-yne by titanocene to a 1,2,3-butatriene. Formation of a 1-titanacyclopent-3-yne and a 2,5-dititanabicyclo[2.2.0]hex-1(4)-ene

The 2,5-dititanabicyclo[2.2.0]hex-1(4)-ene (bis-titanocene-&#956;-(Z)-1,2,3-butatriene complex) (3) is formed starting from [Cp2Ti(&#951;2-Me3SiC2SiMe3)] by in situ generated titanocene and 1,4-dichlorobut-2-yne via the 1-titanacyclobut-3-yne (2)

Mechanism of gallic acid biosynthesis in bacteria (Escherichia coli) and walnut (Juglans regia)

Gallic acid (GA), a key intermediate in the synthesis of plant hydrolysable tannins, is also a primary anti-inflammatory, cardio-protective agent found in wine, tea, and cocoa. In this publication, we reveal the identity of a gene and encoded protein essential for GA synthesis. Although it has long been recognized that plants, bacteria, and fungi synthesize and accumulate GA, the pathway leading to its synthesis was largely unknown. Here we provide evidence that shikimate dehydrogenase (SDH), a shikimate pathway enzyme essential for aromatic amino acid synthesis, is also required for GA production. Escherichia coli (E. coli) aroE mutants lacking a functional SDH can be complemented with the plant enzyme such that they grew on media lacking aromatic amino acids and produced GA in vitro. Transgenic Nicotianatabacum lines expressing a Juglans regia SDH exhibited a 500% increase in GA accumulation. The J. regia and E. coli SDH was purified via overexpression in E. coli and used to measure substrate and cofactor kinetics, following reduction of NADP+ to NADPH. Reversed-phase liquid chromatography coupled to electrospray mass spectrometry (RP-LC/ESI–MS) was used to quantify and validate GA production through dehydrogenation of 3-dehydroshikimate (3-DHS) by purified E. coli and J. regia SDH when shikimic acid (SA) or 3-DHS were used as substrates and NADP+ as cofactor. Finally, we show that purified E. coli and J. regia SDH produced GA in vitro

Closo versus Hypercloso Metallaboranes: A DFT Study

The structures and electronic relationship of 9-, 10-, 11-, and 12-vertex closo and hypercloso (isocloso) etallaboranes are explored using OFT calculations. The role of the transition metal in stabilizing the hypercloso borane structures is explained using the concept of orbital compatibility. The hypercloso structures, C6H6MBn-1Hn-1 (n = 9-12; M = Fe, Ru, and Os) are taken as model complexes. Calculations on metal free polyhedral borane BnHn suggest that n vertex hypercloso structures need only n skeleton electron pairs (SEPs), but the structure will have one or more six-degree vertices, whereas the corresponding closo structures with n + 1 SEPs have only four- and five-degree vertices. This high-degree vertex of hypercloso structures can be effectively occupied by transition metal fragments with their highly diffused orbitals. Calculations further show that a heavy transition metal with more diffused orbitals prefers over a light transition metal to form hypercloso geometry, This is in accordance with the fact that there are more experimentally characterized hypercloso structures with the heavy transition metals. The size of the exohedral ligands attached to the metal atom also plays a role in deciding the stability of the hypercloso structure. The interaction between the borane and the metal fragments in the hypercloso geometry is analyzed using the fragment molecular orbital approach. The interconversion of the closo and hypercloso structures by the addition and removal of the electrons is also discussed in terms of the correlation diagrams