On incidence structures of nonsingular points and hyperbolic lines of ovoids in finite orthogonal spaces

We study the point-line incidence structures of nonsingular points and hyperbolic secant lines associated with ovoids in finite orthogonal spaces. We show that these incidence structures frequently produce partial linear spaces and the parameters of the bipartite graphs (called ovoidal graphs) associated with these structures produce simple and effective isomorphism invariants to distinguish non-isomorphic ovoids. We prove explicit formulas for these isomorphism invariants for a number of infinite families of 2-transitive ovoids.     

Unweaving the rainbow: a review of the relationship between single-walled carbon nanotube molecular structures and their chemical reactivity

Single-walled carbon nanotubes (SWNTs) are a fundamental family of distinct molecules, each bearing the possibility of different reactivities due to their intrinsically distinct chemical properties. SWNT syntheses generate a heterogeneous mixture of species with varying electronic character, lengths, diameters and helicities, (n,m), as well as other amorphous, graphitic and metal catalyst impurities. In recent years, selective syntheses and post-synthetic separation strategies have advanced, driven by the requirement for pure SWNTs displaying particular features. Covalent surface modifications are widely-used to adapt SWNTs for specific applications with modified solubility, compatibility and specific functionalities. In many cases, such reactions have been found to be selective, illuminating the fundamentally distinct chemistry of each (n,m) species. This differential reactivity has found immediate utility in facilitating the sorting of nanotubes according to specific diameter, electronic properties and, most importantly, helicity. In this tutorial review, we discuss a wide range of selective reactions, the mechanisms that are thought to govern selectivity, and the challenges of separating, characterising and regenerating the modified SWNTs.     

Characterization and dioxygen reactivity of a new series of coordinatively unsaturated thiolate-ligated manganese(II) complexes

The synthesis, structural, and spectroscopic characterization of four new coordinatively unsaturated mononuclear thiolate-ligated manganese(II) complexes ([Mn(II)(S(Me2)N(4)(6-Me-DPEN))](BF(4)) (1), [Mn(II)(S(Me2)N(4)(6-Me-DPPN))](BPh(4))·MeCN (3), [Mn(II)(S(Me2)N(4)(2-QuinoPN))](PF(6))·MeCN·Et(2)O (4), and [Mn(II)(S(Me2)N(4)(6-H-DPEN)(MeOH)](BPh(4)) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported [Mn(II)(S(Me2)N(4)(2-QuinoEN))](PF(6)) (2) (Coggins, M. K.; Kovacs, J. A. J. Am. Chem. Soc.2011, 133, 12470). Dioxygen addition to complexes 1-5 is shown to result in the formation of five new rare examples of Mn(III) dimers containing a single, unsupported oxo bridge: [Mn(III)(S(Me2)N(4)(6-Me-DPEN)](2)-(μ-O)(BF(4))(2)·2MeOH (6), [Mn(III)(S(Me2)N(4)(QuinoEN)](2)-(μ-O)(PF(6))(2)·Et(2)O (7), [Mn(III)(S(Me2)N(4)(6-Me-DPPN)](2)-(μ-O)(BPh(4))(2) (8), [Mn(III)(S(Me2)N(4)(QuinoPN)](2)-(μ-O)(BPh(4))(2) (9), and [Mn(III)(S(Me2)N(4)(6-H-DPEN)](2)-(μ-O)(PF(6))(2)·2MeCN (10). Labeling studies show that the oxo atom is derived from (18)O(2). Ligand modifications, involving either the insertion of a methylene into the backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of μ-oxo dimers 6-10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to μ-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2 and dioxygen. Complexes 3-5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively minor ligand modifications have upon the stability of metastable dioxygen-derived species.     

Contamination of holey/lacey carbon films in STEM

In many cases, the key to obtaining good TEM results is in the sample preparation itself. Even once a thin specimen is achieved, other factors determine how well the sample will behave in the microscope. One of the main hindrances to TEM and STEM-EELS analysis is the build up of carbon contamination on the sample under the electron beam. This process may occur due to the nature of the sample itself or the support grids or films on which the sample sits. Here, we investigate contamination on holey and lacey carbon films from three different suppliers. We find that all grids have a large amount of mobile hydrocarbon contamination on them, as well as other larger contaminant species on the surface. Even after a variety of cleaning routines, none of the films are clean enough for STEM-EELS experiments requiring long acquisition times.     

Functional polymers and sequential copolymers by phase transfer catalysis. XIX. Thermotropic polythioethers and copolythioethers based on 4,4′-dithiolbiphenyl

Thermotropic liquid crystalline (LC) polythioethers and copolythioethers have been synthesized from 4,4′-dithiolbiphenyl (TB) and α,ω-dibromoalkanes having between two and twelve methylene units. Copolythioethers were prepared from TB and a 1:1 mol/mol ratio of 1,5-dibromopentane and α,ω-dibromoalkanes having nine to twelve methylene units. Peculiarities of the phase-transfer catalyzed (PTC) polyetherification reaction in the preparation of these polymers have led to aliphatic-ether structural units. This side reaction has been explained by a possible neighboring group effect with sulfur, through sulfonium formation, and is consistent with a mechanism previously proposed in PTC polyetherifications. While not significantly enhancing solubility, the copolymeric nature of these polymers have led to lower melting and isotropization temperatures with broad mesophase thermal stability ranges. Lastly, unique spherulitic textures resembling higher-ordered smectic mesophases have been observed in all the prepared polymers.     

Mesoscale modeling of electrical percolation in fiber-filled systems

The research described in this paper primarily involves mesoscale simulations: dissipative particle dynamics (DPD) of packed assemblies of oriented fibers suspended in a viscous medium. Computer simulations have been performed in order to explore how the aspect ratio and degree of fiber alignment affect the critical volume fraction (percolation threshold) required to achieve electrical conductivity. The fiber network impedance was assessed using Monte Carlo simulations after establishing the structural arrangement with DPD. The predictions are compared with the predictions of classical percolation theory and found to be in close agreement. The approach is thus validated and can be extended to systems that cannot be tackled analytically; in particular, the work is motivated by long-standing interest in materials which display a complex percolation behavior.     

Comparison of computational methods applied to oxazole,thiazole, and other heterocyclic compounds

A variety of computational methods, including the semiempirical techniques AM1, PM3, and MNDO, and the thermochemical basis sets of Benson and Stine, was used to calculate and compare heats of formation (ΔH_f°) data for optimized geometries of a variety of aromatic and nonaromatic heterocycles. Detailed analyses, including 6-31G* and MP2/6-31G* ab initio calculations, were performed for the oxazole and thiazole heterocycles. The results indicate a scatter among the methods sensitive to the nature of the heterocycle. This was in particular evident in the oxazole molecule, where AM1 gave a singularly high value of ΔH_f° consistent with longer calculated bond lengths, particularly about the oxygen atom. Aromatic stabilization energy appears to be addressed differently among the employed methods. Implications of this contrast applied to calculation of macromolecular systems containing heterocyclic units are discussed.