Isotactic-b-syndiotactic stereoblock poly(methyl methacrylate) by chiral metallocene/Lewis acid hybrid catalysts

Stereoblock polymerization with chiral ansa-metallocene/strong Lewis acid hybrid catalysts capable of switching stereospecificity of the methyl methacrylate polymerization produces the highly stereoregular isotactic-b-syndiotactic stereoblock poly(methyl methacrylate).     

Contribution à l'étude du comportement de la catéchine en milieu alcalin

Contribution to the study of the behaviour of catechin in alkaline medium When catechin ( 1 ) is warmed with aqueous alkali it first undergoes rapid epimerisation. After some time a novel bridged carbocyclic enol, catechinic acid ( 3 ) separates out and may be isolated in excellent yield. Eventually catechinic acid isomerises to 2-(3′,4′-dihydroxyphenyl)-3,9-epoxy-6-oxo-bicyclo[3,3,1]-non-7-en-8,9-diol ( 4 ) via an open chain anion ( 5 ) which is also involved in the epimerisation of catechin. Assignments of structure are given and the equilibria observed are discussed.     

PRODEN: A new program for calculating integrated projected populations

Two new computational methods for the evaluation and partitioning of projected, planar, and averaged slab electron densities, implemented in the program PRODEN, are presented. The new algorithms for the projection, demarcation, and integration of electron densities are described and evaluated in terms of speed and numerical accuracy. Integrated Projected Populations are analyzed and some of the advantages and limitations of the methods are discussed.     

The rearrangement of 3,4-disubstitued-Δ2-1,2,4-oxadiazolin-5-thiones

The light and metal catalysed rearrangement of 3,4-disubstituted-Δ²-1,2,4-oxadiazolin-5-thiones (3) to the corresponding Δ²-1,2,4-thiodiazolin-5-ones (4) is described. The thermal stability of (3) is in great great contrast to the ease of rearrangement of the acyclic analogues. The rearrangement of (3) and (4) under electron impact are described.     

Electronic structure of axially ligated rhodium carboxylates: π back-bonding revisited

Fenske-Hall molecular orbital calculations have been applied to a new, structurally isolable analog of the intermediate involved in catalytic rhodium carboxylate carbenoid transformations. Results from the structural characterization of axially ligated rhodium acetate phenylisonitrile complexes have been utilized in approximate molecular orbital calculations. The results from the calculations suggest that a significant degree of π back-bonding exists between the metal and isonitrile fragments which, by analogy, implies that π back-bonding should also exist in the rhodium carbenoid intermediate. Sensitivity of the Fenske-Hall method to the magnitude of π back-donation in these complexes was gauged through calculations involving different phenylisonitrile groups in which the π back-bonding ability was modulated through derivativization. The reliability of the Fenske-Hall method was evaluated through a comparison to a high-level calculation. Received: 13 December 1996 / Accepted: 18 March 1997     

Zinc solid-state NMR spectroscopy of human carbonic anhydrase: implications for the enzymatic mechanism

The pH dependence of the (67)Zn solid-state nuclear magnetic resonance spectroscopy of human carbonic anhydrase (CAII) has been investigated to characterize the nature of the fourth ligand. CAII, through the Zn(2+)-bound hydroxide, catalyzes the deceptively simple reaction: CO(2) + H(2)O <==> HCO(3)(-) + H(+). The accepted mechanism for CAII would predict that water would be bound to the Zn(2+) at pH 5 and hydroxide would be bound at pH 8.5. The measured values for the electric field gradient (EFG) or quadrupole coupling constant (Cq) for CAII are independent of pH within the limits of the experimental error, i.e., 9.8 +/- 0.2 MHz. The EFG interaction has been predicted by ab initio electronic structure calculations for water and hydroxide bound to the zinc, including various levels of hydrogen bonding. After comparing the predicted Cq's with the experimental values, we conclude that the species present from pH 5-8.5 is the hydroxide form. The NMR data presented here is not consistent with the accepted mechanism for CAII. We show that the NMR data is consistent with an alternative mechanism of CAII.     

Bislactonization of unsaturated diacids using lead tetraacetate

The reaction of lead tetraacetate with unsaturated carboxylic acids (or salts) to from bislactones (γ or δ) can be controlled to produce efficiently $\text{cis}$ addition of two carboxylic oxygens to the double bond, in consonance with an initial plumbolactonization step followed by S_N2 displacement of lead.     

The first structural and spectroscopic characterization of a neptunyl polyoxometalate complex

The reaction between PW9O349- and NpO2+ has yielded the first structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analogue, and there is also spectroscopic evidence for its existence in solution. The complex is readily extracted into toluene, and this may have significance in the sequestering and/or separation of the neptunyl ion in terms of nuclear waste management.     

The first genuine observation of fluorescent mononuclear phthalocyanine aggregates

An initial photophysical study of a tetra-solketal-substituted zinc phthalocyanine is reported; at low temperature this compound exhibits very strong aggregation, and a new red shifted emission peak is observed, lambda max approximately 750 nm, attributed to a fluorescent phthalocyanine dimer.     

Intrinsic and extrinsic factors in anion electron-stimulated desorption: D- from deuterated hydrocarbons condensed on Kr and water ice films

The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process.