Fragile networks and rheology of concentrated suspensions

Suspensions consisting of particles of colloidal dimensions have been reported to form connected structures. When attractive forces act between particles in suspension they may flocculate and, depending on particle concentration, shear history and other parameters, flocs may build-up in a three-dimensional network which spans the suspension sample. In this paper a floc network model is introduced to interpret the elastic behavior of flocculated suspensions at small deformations. Elastic percolation concepts are used to explain the variation of the elastic modulus with concentration. Data taken from the suspension rheology literature, and new results with suspensions of magnetic -Fe₂O₃ and non-magnetic -Fe₂O₃ particles in mineral oil are interpreted with the model proposed.Non-zero elastic modulus appeared at threshold particle concentrations of about 0.7 vol.% and 0.4 vol.% of the magnetic and non-magnetic suspensions, respectively. The difference is attributed to the denser flocs formed by magnetic suspensions. The volume fraction of particles in the flocs was estimated from the threshold particle concentration by transforming this concentration into a critical volume concentration of flocs, and identifying this critical concentration with the theoretical percolation threshold of three-dimensional networks of different coordination numbers. The results obtained indicate that the flocs are low-density structures, in agreement with cryo-scanning electron micrographs. Above the critical concentration the dynamic elastic modulus G was found to follow a scaling law of the type G ( _f - _f ^c ) ^f , where _f is the volume fraction of flocs in suspension, and _f ^c is its threshold value. For magnetic suspensions the exponent f was found to rise from a low value of about 1.0 to a value of 2.26 as particle concentration was increased. For the non-magnetic a similar change in f was observed; f changed from 0.95 to 3.6. Two other flocculated suspension systems taken from the literature showed a similar change in exponent. This suggests the possibility of a change in the mechanism of stress transport in the suspension as concentration increases, i.e., from a floc-floc bond-bending force mechanism to a rigidity percolation mechanism.     

Exploring the Potential of Fulvalene Dimetals as Platforms for Molecular Solar Thermal Energy Storage: Computations,Syntheses, Structures,Kinetics, and Catalysis

A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu₂ in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on ΔH_storage. Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1′,3,3′‐tetra‐tert‐butyl ( 4 ), 1,2,2′,3′‐tetraphenyl ( 9 ), diiron ( 28 ), diosmium ( 24 ), mixed iron‐ruthenium ( 27 ), dimolybdenum ( 29 ), and ditungsten ( 30 ) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO₃–SiO₂ as a good candidate, although catalyst decomposition remains a challenge.     

Diffraction Pattern Observed in Shadow Region of Thin Wedge

Diffraction patterns of wedges are studied with special attention to the finish of the samples. We assume that the sharply finished vertex should generate two hemispherical waves by the end effect of the cylindrical waves issued from two sides of the wedge under illumination of the coherent light. It is found that the assumption is valuable to the investigation of the diffraction pattern appearing in the shadow region of the wedge.     

para-Bridged symmetrical pillar[5]arenes: their Lewis acid catalyzed synthesis and host-guest property

Condensation of 1,4-dimethoxybenzene (DMB) with paraformaldehyde in the presence of BF3.O(C2H5)2 gave novel para-bridged pentacyclic pillar DMB (DMpillar[5]arene). Moreover, para-bridged pentacyclic hydroquinone (pillar[5]arene) was prepared. Pillar[5]arene formed 1:1 host-guest complexes with dialkyl viologen and alkyl pyridinium derivatives. However, pillar[5]arene did not form complexes with the diadamantyl viologen derivative since a bulky adamantyl group was unable to thread the cavity of pillar[5]arene.     

Chiral anion-mediated asymmetric ring opening of meso-aziridinium and episulfonium ions

Reactions proceeding through cationic intermediates that lack a Lewis or Br?nsted basic site present a challenge for traditional asymmetric catalysis based on chiral metals or organocatalysts. We present an enantioselective ring opening of tetrasubstituted meso-aziridinium ions with alcohol nucleophiles proceeding through a chiral ion pair with a binaphthol-phosphate anion. The reaction is initiated by silver-induced ring closure of beta-chloroamines using the Ag salt of the chiral anion as in situ generated catalyst. Use of insoluble Ag2CO3 as silver source is essential to obtain high enantioselectivity; we believe the chiral phosphate acts as a "chiral anion phase transfer catalyst" to bring silver ion into the organic phase. The chiral anion concept can also be extended to the related asymmetric opening of meso-episulfonium ions generated by protonation of trichloroacetimidates vicinal to sulfides.     

Anion configuration at the air/liquid interface of ionic liquid [bmim]OTf studied by sum-frequency generation spectroscopy

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF 3-symmetric stretching (ss) mode and SO 3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF 3-ss peak of the OTf anion has the opposite sign with respect to that of the SO 3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF 3 group points away from the bulk into the air, whereas the SO 3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO 3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.     

Synthetic study on telomerase inhibitor, D8646-2-6: synthesis of the key intermediate using Sn(OTf)2 or Sc(OTf)3 mediated aldol-type reaction and Stille coupling

The synthesis of the key intermediate (4) in the proposed route to D8646-2-6 is described. The aldol reaction of the carbohydrate-containing pyrone 7 with the aldehyde 6 was accomplished by using LiHMDS and Sc(OTf)3 or Sn(OTf)2. The stepwise dehydration reaction of the aldol adduct 14, followed by Stille coupling with vinyl stannane 5 which contained phosphonate gave the desired 4.     

The vibrational structure of the X 1A1 - A 1B1 and A 1B1 - B 1A1 band systems of GeH2/GeD2 based on global potential energy surfaces

Transition probabilities were evaluated for the X (1)A(1)-A (1)B(1) and A (1)B(1)-B (1)A(1) systems of GeH(2) and GeD(2) to analyze the X-->A-->B photoexcitation. Franck-Condon factors (FCFs) and Einstein's B coefficients were computed by quantum vibrational calculations using the three-dimensional potential energy surfaces (PESs) of the X (1)A(1), A (1)B(1), and B (1)A(1) electronic states and the transition dipole moments for the X-A and A-B systems. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least squares method combined with the Shepard [Proceedings of the 1968 23rd ACM National Conference (ACM, New York, 1968)] interpolation. The barriers to linearity correcting the spin-orbit interaction are evaluated to be 22,000 cm(-1) for the X state, 6300 cm(-1) for the A state, and 560 cm(-1) for the B state. The obtained FCFs for the X-A and A-B systems indicate that the bending mode is strongly enhanced in the excitation since the equilibrium bond angle greatly varies within the three states. The photoexcitation and fluorescence spectra calculated for the X-A system agree well with the observed spectra. The theoretical lifetimes for lower vibrational levels of the A and B states were calculated from the fluorescence decay rates for the A-X, B-A, and B-X emissions, and the lifetimes for the A state are in good agreement with the observed values except those affected by predissociation.     

Copper(I) alkoxide-catalyzed alkynylation of trifluoromethyl ketones

A general method for direct alkynylation of trifluoromethyl ketones was developed by using CuO(t)Bu-xantphos or phenanthroline complexes as catalysts. The ligands significantly enhanced the catalyst activity. In addition, KOTf, generated in the catalyst preparation step, exhibited some acceleration effects. A preliminary extension to a catalytic enantioselective CF3-substituted tertiary propargyl alcohol synthesis (up to 52% ee) is also described.     

Catalytic enantioselective construction of tetrasubstituted carbons by self-assembled poly rare earth metal complexes

Rare earth metal-based enantioselective catalysts that can promote practical cyanation reactions of ketones and ketoimines were developed. These catalytic enantioselective tetrasubstituted carbon-forming reactions are useful platforms for the synthesis of biologically active compounds. ESI-MS and crystallographic studies of the asymmetric catalysts revealed that the active catalysts are polymetallic complexes produced through the assembly of modules. The higher-order structure of the polymetallic complexes has strong effects on catalyst activity and enantioselectivity. Controlling the higher-order structure of artificial polymetallic asymmetric catalysts is a new strategy for optimizing asymmetric catalysts. Recent progress in this approach is also described.