Calculation of effective material parameters by field averaging over lattices with non-negligible unit cell size

The Pendry et?al. field averaging method for calculation of effective material parameters is reviewed and its limits explored. The method is then extended so that it can accurately calculate the effective material parameters of lattices where the unit cell size is appreciable but still quasistatic (d??0.1?? ₀). The new algorithm is verified by calculating the effective material properties of periodically placed particles suspended in free space, as the unit cell size becomes appreciable. Results of our proposed formulation are then compared with the Pendry et?al. and conventional volumetric averaging algorithms.     

X-ray absorption spectroscopy and density functional theory studies of [(H3buea)FeIII-X]n- (X = S2-, O2-, OH-): comparison of bonding and hydrogen bonding in oxo and sulfido complexes

Iron L-edge, iron K-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [Fe(III)H(3)buea(X)](n-) (X = S(2-), O(2-), OH(-)). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metal-ligand bonding and to evaluate the effects of H-bonding in Fe(III)(-)O vs Fe(III)(-)S complexes. It was found that the Fe(III)(-)O bond, while less covalent, is stronger than the Fe(III)(-)S bond. This dominantly reflects the larger ionic contribution to the Fe(III)(-)O bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the Fe-O bond. These results were extended to consider an Fe(IV)(-)O complex with the same ligand environment. It was found that hydrogen bonding to Fe(IV)(-)O is less energetically favorable than that to Fe(III)(-)O, which reflects the highly covalent nature of the Fe(IV)(-)O bond.     

Asymmetric induction during electron transfer mediated photoreduction of carbonyl compounds: role of zeolites

Photochemistry of 17 aryl alkyl ketones included within cation exchanged zeolites has been examined. In solution five of the 17 ketones undergo intramolecular hydrogen abstraction reaction even in the presence of a chiral amine and the rest are photoreduced to the corresponding alcohol. Within zeolites all 17 ketones yielded in presence of a chiral amine, the corresponding alcohol as the major product. When a chiral amine was used as the coadsorbent within alkali ion exchanged zeolites, enantiomerically enriched alcohol was formed in all cases. The best chiral induction was obtained with phenyl cyclohexyl ketone (enantiomeric excess: 68%). 1H-13C Cross Polarization Magic Angle Spinning (CP-MAS) experiments, with a model ketone (perdeuterated acetophenone) and chiral amine (pseudoephedrine) included within MY zeolites, suggested that the cation brings the reactant and the chiral amine closer. The role of the cation in such a process is also revealed by the computation results. The results presented here highlight the importance of a supramolecular structure in forcing a closer interaction between a reactant and a chiral inductor that could be used to achieve asymmetric induction in photoproducts.     

Triangles and tetrahedra: metal directed self-assembly of metallo-supramolecular structures incorporating bis-beta-diketonato ligands

The interaction of six aryl-linked bis-beta-diketones, including a new naphthylene linked species, with copper(II), iron(III) and, in one instance gallium(III), has been investigated with the aim of obtaining metallo-supramolecular assemblies exhibiting different geometries. New examples of two assembly types incorporating the above bis-beta-diketones (L) were generated. The first type is represented by a range of molecular triangles of formula [Cu(3)(L-H(2))(3)](solvent)(n) while the second is given by a corresponding selection of less-common neutral molecular tetrahedra of formula [Fe(4)(L-H(2))(6)](solvent)(n) as well as [Ga(4)(L-H(2))(6)].8.5THF.0.5H(2)O; an example of each type has been characterised by X-ray crystallography. A magnetochemical investigation of [Fe(4)(-H(2))(6)].6THF is reported. The susceptibility is Curie like and consistent with very weak coupling occurring between the iron(III) d(5)(high spin) centres. The X-ray structures of two trinuclear copper(II) as well as a tetranuclear iron(III) and a tetranuclear gallium(III) assembly confirm their discrete triangular and tetrahedral geometries, respectively. The structure of the gallium(III) species is closely related to that of the corresponding iron(III) species. The tetrahedral structures provide rare examples of such assemblies encapsulating guest solvent molecules--in each case tetrahydrofuran is incorporated in the central cavity.     

Functionalized Alkynylplatinum(II) Polypyridyl Complexes for Use as Sensitizers in Dye‐Sensitized Solar Cells

A series of platinum(II) alkynyl‐based sensitizers has been synthesized and found to show light‐to‐electricity conversion properties. These dyes were developed as sensitizers for the application in nanocrystalline TiO₂ dye‐sensitized solar cells (DSSCs). Their photophysical and electrochemical properties were studied. The excited‐state property was probed using nanosecond transient absorption spectroscopy, which showed the formation of a charge‐separated state that arises from the intramolecular photoinduced charge transfer from the platinum(II) alkynylbithienylbenzothiadiazole moiety (donor) to the polypyridyl ligand (acceptor). A lifetime of 3.4 μs was observed for the charge‐separated state. A dye‐sensitized solar cell based on one of the complexes showed a short‐circuit photocurrent of 7.12 mA cm^?2, an open circuit voltage of 780 mV, and a fill factor of 0.65, thus giving an overall power conversion efficiency of 3.6 %.     

Ordering in asymmetric block copolymer films by a compressible fluid

We examine the morphological structures of asymmetric poly(ethylene oxide)-b-poly(1,1'-dihydroperflurooctyl methacrylate) (PEO-b-PFOMA) thin films upon annealing in a compressible fluid, supercritical CO2 (Sc-CO2). The strong affinity between PFOMA and CO2 is found to induce phase segregation when annealing PEO-b-PFOMA films at the same temperature as compared with vacuum. In vacuum, PEO-b-PFOMA films remain disordered from 80 to 180 degrees C, whereas, in Sc-CO2 at 13.9 MPa, an upper order-disorder transition (UODT) between 116 and 145 degrees C is found. In Sc-CO2, the observed ordered structure is layers of PEO spheres embedded in the matrix of PFOMA, followed by a brush layer, in which PEO wets the substrate. The swelling isotherms of PFOMA and PEO in CO2 are correlated with the Sanchez-Lacombe equation of state (SLEOS) to estimate the interaction parameters, XPFOMA-CO2 and XPEO-CO2. The phase segregation (order) induced by CO2 relative to vacuum at a given temperature is explained in terms of two factors: (1) copolymer volume fraction upon dilution with CO2, phi, and (2) the relative interaction parameter, DeltaX= XPEO-CO2 - XPFOMA-CO2. The latter factor favors order and is dominant at low temperatures over the phi factor, which always favors disorder. At high temperatures (above the T(ODT)), the preferential swelling of PFOMA by CO2 is less pronounced ( DeltaX decreases), and the copolymer is disordered.     

The chemistry and complexes of small cyano anions

The chemistry of the anions dicyanamide and tricyanomethanide (dca and tcm, respectively) has produced a plethora of discoveries over the past few decades, particularly in relation to transition-metal coordination polymers with magnetic coupling. Over recent years there have been an increasing number of reports of heterofunctionalised cyano-containing anions, typically derivatives of dicyanomethanide. Our own group has been particularly concerned with the amide- and nitroso-functionalised anions carbamoyldicyanomethanide (cdm) and dicyanonitrosomethanide (dcnm), respectively. This feature article examines the fascinating diversity of materials and complexes that can be obtained using small cyano anions, ranging from coordination polymers to heterometallic clusters and hydrogen bonding networks. In particular, we focus on results from our own laboratories in the past few years. The magnetic properties of these materials are briefly discussed.     

Primary Studies on Variation in Position of Trifluoromethyl Groups in Several Aromatic Group‐14 Derivatives by 19F‐NMR Spectroscopy

Variation in the position of CF₃ groups in several aromatic Group‐14 compounds was studied by ¹⁹F‐NMR spectroscopy. In these compounds R_nECl_4?n (n=1 or 2; E=Si, Ge, or Sn; R=2,4,6‐(CF₃)₃C₆H₂ (=Ar), 2,6‐(CF₃)₂C₆H₃ (=Ar′), or 2,4‐(CF₃)₂C₆H₃ (=Ar″)), Ar, Ar′, and Ar″ are all bulky, strongly electron‐withdrawing ligands. The ¹⁹F‐NMR studies of the variation in position of the CF₃ substituents in these compounds as revealed by chemical shifts could be correlated with the electronegativities of the central elements E, and with intramolecular E–F interactions derived from single‐crystal X‐ray diffraction data. These interactions are considered to play an important role in the stabilization of these compounds.     

Monomeric Copper(II) Sites Supported on Alumina Selectively Convert Methane to Methanol

Monomeric Cu^II sites supported on alumina, prepared using surface organometallic chemistry, convert CH₄ to CH₃OH selectively. This reaction takes place by formation of CH₃O surface species with the concomitant reduction of two monomeric Cu^II sites to Cu^I, according to mass balance analysis, infrared, solid‐state nuclear magnetic resonance, X‐ray absorption, and electron paramagnetic resonance spectroscopy studies. This material contains a significant fraction of Cu active sites (22 %) and displays a selectivity for CH₃OH exceeding 83 %, based on the number of electrons involved in the transformation. These alumina‐supported Cu^II sites reveal that C?H bond activation, along with the formation of CH₃O‐ surface species, can occur on pairs of proximal monomeric Cu^II sites in a short reaction time.