NMR investigation of concentrated alumina and silica slurries

The longitudinal relaxation times (T₁) of water in concentrated silica and alumina slurries were measured as a function of solids content. It was shown that the results could be fit very well with a two-phase fast-exchange model between free and surface-bound water. As expected, values of T₁ for bound water were in the order of 20–2000 times lower than that for free water, indicating a higher effective viscosity of the surface-bound water. The strength of the interaction depended on the particular surface, and all of the aluminas examined interacted more strongly with water than the two silicas studied, which themselves differed considerably. The chemical mechanical polishing (CMP) removal rate of tantalum by silica slurries was shown to be directly correlated with the interaction parameters, derived from the NMR relation times rather than with total surface hydroxyl group concentration.     

Influence of molecular architecture on the dewetting of thin polystyrene films

The control of dewetting for thin polymer films is a technical challenge and of significant academic interest. We have used polystyrene nanoparticles to inhibit dewetting of high molecular weight, linear polystyrene, demonstrating that molecular architecture has a unique effect on surface properties. Neutron reflectivity measurements were used to demonstrate that the nanoparticles were uniformly distributed in the thin (ca. 40 nm) film prior to high temperature annealing, yet after annealing, they were found to separate to the solid substrate, a silanized silicon wafer. Dewetting was eliminated when the nanoparticles separated to form a monolayer or above while below this surface coverage the dewetting dynamics was severely retarded. Blending linear polystyrene of similar molecular weight to the polystyrene nanoparticle with the high molecular weight polystyrene did not eliminate dewetting.     

Rapid imaging, using a cooled charge-coupled-device, of fluorescent two-dimensional polyacrylamide gels produced by labelling proteins in the first-dimensional isoelectric focusing gel with the fluorophore 2-methoxy-2,4-diphenyl-3(2H)furanone

A new method for visualising proteins in two-dimensional polyacrylamide gels was developed. Proteins were labelled with the fluorophore 2-methoxy-2,4-diphenyl-3(2H)furanone (MDPF) while present in the first-dimensional gel after isoelectric focusing and subsequently electrophoresed into the second-dimensional gel. High resolution spot patterns were produced and compared with other methods of visualisation. A new rapid imaging system based on a cooled charge-coupled-device was used to view the two-dimensional fluorescent protein spot patterns. The method allows the immediate and rapid imaging of two-dimensional gels at the end of electrophoresis with no further processing.     

On the regular heptagon

The geometry of rings with 5, 6, 7, 8 and 9 links, particularly those with seven links such as occur in cyclo‐heptane and analogous molecules* is re‐considered with the aid of distance geometry, now more practicable with modern mathematical software. Use is made of a quadric surface which may be fitted to nine points. *In admiration for the author I have ventured to appropriate the title of Archimedes’ work “On the regular heptagon”, which came to light only in 1926 [C. Schoy, Graeco‐Arabische Studien, Isis 8 (1926) 21–40]. “Regular”, as applied to rings in space admits of various definitions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photochemistry and photophysics of Cr(III) macrocyclic complexes

The phosphorescence and photochemical behavior of the macrocyclic complexes (1,4,7,10,13,16-hexaazacyclooctadecane)chromium(III) (Cr([18]-aneN(6))(3+); 1) and (4,4',4'-ethylidynetris(3-azabutan-1-amine)) chromium(III) (Cr(sen)(3+); 2) have been compared to each other and to the complex Cr(en)(3)(3+). For both macrocyclic complexes, phosphorescence from room temperature aqueous solutions is too weak to be observed, contrasting with Cr(en)(3)(3+), though both had somewhat longer 77 K lifetimes than Cr(en)(3)(3+). Phosphorescence lifetimes for these macrocyclics decreased with increasing temperature much faster than for Cr(en)(3)(3+) and a conventional extrapolation based on a fit of reciprocal lifetimes (corrected for the low-temperature contribution) to the Arrhenius equation gave estimated room temperature phosphorescence lifetimes of a few nanoseconds, consistent with the failure to observe room temperature emission. Fitting of the nonlinearity of the data seen in these plots suggested that two high-temperature processes were occurring with estimated activation parameters (E in kJ mol(-1) and A in s(-1)) for Cr([18]-aneN(6))(3+): E(1) = 40, A(1) = 1 x 10(16); E(2) = 24, A(2) = 1 x 10(14): Cr(sen)(3+); E(1) = 45, A(1) = 2 x 10(15); E(2) = 29, A(2) = 7 x 10(11). Cr([18]-aneN(6))(3+) was photochemically inert on irradiation. On irradiation into the lowest quartet ligand field absorption band, Cr(sen)(3+) photolyzes with a quantum yield of 0.098 +/- 0.001 at room temperature. Laser flash photolysis with conductivity detection showed that this photoreaction occurred faster than protonation of the liberated amine ligand at all practical proton concentrations. The quantum yield for irradiation directly into the doublet absorption band of Cr(sen)(3+) was 0.077 +/- 0.003. Photoaquation of Delta-Cr(sen)(3+) led to loss of optical activity and product analysis by capillary electrophoresis showed that both racemic and Delta-Cr(sen)(3+) photoaquate to a single main product, trans-Cr(sen-NH)(H(2)O)(4+). The product stereochemistry is shown to be consistent with predictions based on the angular overlap model for Cr(III) photochemistry, recognizing the additional constraints imposed by the ligand. The abnormally short room temperature solution lifetime of the doublet state is a result of a radiation-less process that competes with other processes depleting the doublet state. However, this doublet-state deactivation process does not lead to photoaquation but competes with BISC and photoaquation via the quartet state, resulting in an unprecedented reduction in photoaquation quantum yield on direct irradiation into the doublet state.     

Novel products from attempted michael addition to 1-methyl-1,2,5,6-tetrahydropyridine-4-carbonitrile

The reaction of some phenols with the title compound, in the presence of sodium, gives 2-(2-hydroxyaryl_piperidine derivatives. Geometrical isomers have been separated, which differ in having an equatorial (A) or axial (B) cyano group on the piperidine chair (the methyl and aryl groups are equatorial in both forms). The x-ray crystallographic structures of an example of each of A and B are reported and the proton NMR spectra are assigned.     

A new class of hybrid materials via salt inclusion: novel copper(II) arsenates Na(5)ACu(4)(AsO(4))(4)Cl(2) (A = Rb,Cs) composed of alternating covalent and ionic lattices

Two novel copper(II) arsenates Na5ACu4(AsO4)4Cl2 (A = Rb, Cs) were synthesized by conventional solid-state methods using reactive molten salt media. These compounds are isostructural and crystallize in an orthorhombic lattice (Fmmm, No. 69; Z = 8). The cell constants are a = 14.632(3) A, b = 18.872(2) A, c = 14.445(3) A, V = 3989(1) A3, for A = Rb; a = 14.638(3) A, b = 18.990(4) A, c = 14.418(3) A, V = 4008(1) A3, for A = Cs. Single-crystal structure studies reveal a new composite framework consisting of alternating covalent and ionic lattices. The covalent lattice contains highly oriented oligomeric mu-oxo [Cu4O12]16- tetrameric units with a cyclo-S8-like Cu4O4 magnetic core that resembles the building block of layered cuprates. The ionic slab consists of a novel framework of mixed alkali metal chloride lattice and rarely seen Na6O8 clusters. Similar to organic-inorganic hybrid materials, the title compounds present a new class of host-guest chemistry via salt inclusion reactions.