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

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

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

Stability of polyhedral borane anions and carboranes

Polyhedral borane anions and carboranes that can be constructed formally from the interaction of rings and caps will be stable with six interstitial electrons. Interstitial electron count is obtained by summing the number of &#960; electrons of the ring and the electrons of the caps involved in ring cap binding. Thus B7H7-2 (D5h) has 6 interstitial electrons (none from the B5H5 ring, two each from the twobh caps and two negative charge),mndo calculations on isoelectronic pyramidal molecules B6H6-4 (C5v), B5H5CH-3 (C5v), B5H5-4 (C4v), B4H4CH-3 (C4v), B4H4 -4 (Td) and B3H3CH-3 (C3v) suggests a criterion based on the out-of-plane bendings of the ring B-H bonds to select the best combination of borocycles and BH or CH caps. Three-membered borocycle prefers CH cap, five-membered borocycle prefers BH cap. The preference of four-membered ring for BH or CH cap is not as pronounced. The extra stability of B12H12-2 arises from the geometry of the icosahedron. The relative stabilities ofnido andcloso carboranes follow from these rules

A non-least motion pathway for 1,2-shift in cyclic vinyl cations

Qualitative molecular orbital arguments are presented to support a novel non-least motion pathway for 1,2-migrations in cyclic vinyl cations. Ab initio calculations using a split valence 3-21G basis set on the bridged vinyl cation with varying HCC angle &#952; (5) show that the bridging hydrogen moves out of the C2H2 plane at &#952; less than 170&#176;. This is due to the decreased interaction of the bridging group or atom with the distorted &#960; bond. Semi-empirical calculations using the MINDO/3 method were carried out on the classical and two types of bridged structures for the cyclopentenyl cation. The transition state corresponding to the non-least motion pathway is found to be of lower energy than that for the least motion pathway