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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

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

Pressure-induced metallization in solid boron

Different phases of solid boron under high pressure are studied by first principles calculations. The $\alpha$-B$_{12}$ structure is found to be stable up to 270 GPa. Its semiconductor band gap (1.72 eV) decreases continuously to zero around 160 GPa, where the material transforms to a weak metal. The metallicity, as measured by the density of states at the Fermi level, enhances as the pressure is further increased. The pressure-induced metallization can be attributed to the enhanced boron-boron interactions that cause bands overlap. These results are consist with the recently observed metallization and the associated superconductivity of bulk boron under high pressure (M.I.Eremets et al, Science{\bf 293}, 272(2001)).Comment: 14 pages, 5 figure

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

Strategies to stabilize exohedral η<SUP>5</SUP>- and η<SUP>6</SUP>-fullerene transition metal organometallic complexes: a molecular orbital treatment

Transition metal fragments are designed to overcome the unfavourable interaction arising from the splayed-out π-orbitals of the five-and six-membered rings of C60 in complex formation. Semiemprical studies at the PM3(tm) level on a series of C60MCnHn complexes suggest that, with the appropriate transition metal fragment, it is possible to stabilize η6-complexes of C60. Isodesmic equations of the type CmHmMCnHn + C60→ C60MCnHn + CmHm indicate that C3H3Co and C3H3Rh are ideal fragments in stabilizing η6-C60 complexes. In comparison, η5-complexes are less favourable; structural modifications such as those in the recently synthesized C60Ph5- should readily help η5-bonding

The fragment molecular orbital approach in organometallic reactivity. Reactions of the binuclear complexes

The fragment molecular orbital approach has emerged as a framework model in theoretical organometallic chemistry. This provides an understanding of the electronic structure and bonding in polynuclear complexes in terms of their constituent fragments that are well understood. Symmetry properties of the frontier molecular orbitals of organometallic complexes help in explaining their reactivity towards various reagents

The ubiquitous icosahedral B<SUB>12</SUB> in boron chemistry

Though boranes exhibit a wide variety of polyhedral structures, all the three polymorphs of elemental boron essentially contain icosahedral B12 units as the predominant building block in their unit cell. Theoretical and experimental studies on boranes show that the icosahedral arrangement leads to most stable boranes and borane anions. This paper attempts to explain the phenomenal stability associated with the icosahedral B12 structure. Using fragment molecular orbital theory, the remarkable stability of B12H2-12 amongcloso boranes are explained. The preferential selection icosahedral B12 unit by elemental boron is explained by improvising a contrived B84 sub-unit of the &#946;-rhombohedron, the most stable polymorph. This also leads to a novel covalent way of stuffing fullerenes with icosahedral symmetry