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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

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

[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

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

On the &#960;<SUP>2</SUP>S + &#960;<SUP>2</SUP>S pathways toward [n]-prismanes

Empirical force field calculations indicate that1c,2c and3c rather than1a,2a and3a are more favourable precursors for photocycloadditions to give [5]-, [6]- and [7]-prismane respectively

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

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)

An ab initio and matrix isolation infrared study of the 1:1 C<SUB>2</SUB>H<SUB>2</SUB>-CHCl<SUB>3</SUB> adduct

The details of weak C-H&#183;&#183;&#183;&#960; interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C2H2-CHCl3 adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C2H2 and CHCl3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C2H2-CHCl3 complex has a weak hydrogen bond involving a C-H&#183;&#183;&#183;&#960; interaction, where the C2H2 acts as a proton acceptor and the CHCl3 as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl3 and a hydrogen in C2H2. The combination of the C-H&#183;&#183;&#183;&#960; interaction and the secondary Cl&#183;&#183;&#183;H interaction determines the structure and the energetics of the C2H2-CHCl3 complex. In addition to the vibrational assignments for the C2H2-CHCl3 complex we have also observed and assigned features owing to the proton accepting C2H2 submolecule in the acetylene dimer

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