Initial conformations of macrocyclic compounds with rotational symmetry generated from a molecular fragment

An algorithm is described for generating atomic Cartesian coordinates of conformations of macrocyclic molecules possessing exact rotational or rotation-reflection symmetries. A fragment representing the asymmetric unit of the molecule is suitably oriented in space, and then a symmetry operator is applied to generate the initial coordinates of the molecule. An empirical force field of interatomic interactions is used to generate equilibrium conformations. Results of calculations performed on two cyclic polylactones and one crown ether using this approach are given. They reveal that symmetric conformations of these molecules are often preferred. Since the latter conformations are probably responsible for the specialized properties of these molecules, this method should facilitate doing theoretical studies on these kinds of compounds.     

Disilane Cleavage with Selected Alkali and Alkaline Earth Metal Salts

The industry-scale production of methylchloromonosilanes in the Müller–Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes Me_nSi₂Cl_6-n (n=1–6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono- and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride.     

On L ∞-Morphisms of Cyclic Chains

Recently the first two authors (Cattaneo and Felder in 2008) constructed an L _∞-morphism using the S ¹-equivariant version of the Poisson Sigma Model. Its role in the deformation quantization was not entirely clear. We give here a “good” interpretation and show that the resulting formality statement is equivalent to formality on cyclic chains as conjectured by Tsygan and proved recently by several authors (Dolgushev et al. in 2008; Willwacher in 2008).     

Fock representations and BRST cohomology inSL(2) current algebra

We investigate the structure of the Fock modules overA ₁ ⁽¹⁾ introduced by Wakimoto. We show that irreducible highest weight modules arise as degree zero cohomology groups in a BRST-like complex of Fock modules. Chiral primary fields are constructed as BRST invariant operators acting on Fock modules. As a result, we obtain a free field representation of correlation functions of theSU(2) WZW model on the plane and on the torus. We also consider representations of fractional level arising in Polyakov's 2D quantum gravity. Finally, we give a geometrical, Borel-Weil-like interpretation of the Wakimoto construction.     

Molecular beam study of methyl nitrite photodissociation after excitation into the second absorption band at 248 and 193 nm

The photodissociation dynamics of methyl nitrite in the second UV absorption band (B band) has been studied by molecular beam photofragment spectroscopy at 248 and 193 nm. Angular distributions were measured at both wavelengths yielding anisotropy parameters β=1.36±0.10 at 248 nm and β=1.08±0.07 at 193 nm which indicates a predominantly in plane transition. Measurements performed atT _rot<10 K and atT _rot≧70 K show that within the experimental error β is independent of the rotational temperature of the molecular beam implying that the dissociation process is considerably faster than a typical rotational period. The anglex between the electronic transition moment and the dissociation direction is estimated to be 28° at 248 nm and 34° at 193 nm. The difference ofx at these two wavelengths is proposed to be the effect of an admixture of a perpendicular transition at 193 nm. No contribution attributable to the electronically excited methoxy fragment was found in the TOF spectrum after excitation at 193 nm. It is concluded that the production quantum yield for electronically excited methoxy is smaller than 7%.     

On the structure of unitary conformal field theory II: Representation theoretic approach

Two-dimensional, unitary rational conformal field theory is studied from the point of view of the representation theory of chiral algebras. Chiral algebras are equipped with a family of co-multiplications which serve to define tensor product representations. Chiral vertices arise as Clebsch-Gordan operators from tensor product representations to irreducible subrepresentations of a chiral algebra. The algebra of chiral vertices is studied and shown to give rise to representations of the braid groups determined by Yang-Baxter (braid) matrices. Chiral fusion is analyzed. It is shown that the braid- and fusion matrices determine invariants of knots and links. Connections between the representation theories of chiral algebras and of quantum groups are sketched. Finally, it is shown how the local fields of a conformal field theory can be reconstructed from the chiral vertices of two chiral algebras.     

Bidentate ligation of heme analogues; novel biomimetics of peroxidase active site

The multifunctional nature of proteins that have iron-heme cofactors with noncovalent histidine linkage to the protein is controlled by the heme environment. Previous studies of these active-site structures show that the primary difference is the length of the iron-proximal histidine bond, which can be controlled by the degree of H-bonding to this histidine. Great efforts to mimic these functions with synthetic analogues have been made for more than two decades. The peroxidase models resulted in several catalytic systems capable of a large range of oxidative transformations. Most of these model systems modified the porphyrin ring covalently by directly binding auxiliary elements that control and facilitate reactivity; for example, electron-donating or -withdrawing substituents. A biomimetic approach to enzyme mimicking would have taken a different route, by attempting to keep the porphyrin ring system unaltered, as close as possible to its native form, and introducing all modifications at or close to the axial coordination sites. Such a model system would be less demanding synthetically, would make it easy to study the effect of a single structural modification, and might even provide a way to probe effects resulting from porphyrin exchange. We introduce here an alternative model system based on these principles. It consists of a two component system: a bis-imidazolyl ligand and an iron-porphyrin (readily substituted by a hemin). All modifications were introduced only to the ligand that engulfs the porphyrin and binds to the iron's fifth and sixth coordination sites. We describe the design, synthesis, and characterization of nine different model compounds with increased complexity. The primary tool for characterizing the environment of each complex Fe(III) center was the Extended X-ray Absorption Fine Structure (EXAFS) measurements, supported by UV/Vis, IR, and NMR spectroscopy and by molecular modeling. Introduction of asymmetry, by attaching different imidazoles as head groups, led to the formation of two axial bonds of different length. Addition of H-bonds to one of the imidazoles in an advanced model increased this differentiation and expanded the porphyrin ring. These complexes were found to be almost identical in structure to peroxidase active sites. Similarly to the peroxidases and other synthetic models, these compounds stabilize the green, compound I-like intermediate, and catalyze the oxidation of organic substrates.     

Reverse Saturable Optical Absorption of C60, Soluble Methanofullerenes, and Fullerodendrimers in Sol-Gel Mesoporous Silica Host Matrices

Porous sol-gel glasses with various pore size distributions are prepared and either impregnated with pure C₆₀ or soaked with methanofullerenes or fullerodendrimers derivative solution. Induced absorption or reverse saturable absorption (RSA) has been studied in both types of solid materials. The samples impregnated by pure C₆₀ mainly contain well-dispersed fullerene molecules. Unlike crystalline films of C₆₀, their absorption dynamics can be well described by a 5-level model, developed for non-interacting C₆₀-molecules in solutions. Methanofullerene samples, on the other hand, show signs of micellar aggregation and therefore RSA dynamics, which are influenced by solid state effects. Fullerodendrimers derivatives lead to the highest quantum yield.