Nonlinear rheology of concentrated spherical silica suspensions: 3. Concentration dependence

Nonlinear rheology was examined for concentrated suspensions of spherical silica particles (with radius of 40 nm) in viscous media, 2.27/1 (wt/wt) ethylene glycol/glycerol mixture and pure ethylene glycol. The particles were randomly and isotropically dispersed in the media in the quiescent state, and their effective volume fraction φ_eff ranged from 0.36 to 0.59. For small strains, the particles exhibited linear relaxation of the Brownian stress σ_B due to their diffusion. For large step strains γ, the nonlinear relaxation modulus G(t,γ) exhibited strong damping and obeyed the time-strain separability. This damping was related to γ-insensitivity of strain-induced anisotropy in the particle distribution that resulted in decreases of σ_B/γ. The damping became stronger for larger φ_eff. This φ_eff dependence was related to a hard-core volume effect, i.e., strain-induced collision of the particles that is enhanced for larger φ_eff. Under steady/transient shear flow, the particles exhibited thinning and thickening at low and high γ˙, respectively. The thinning behavior was well described by a BKZ constitutive equation using the G(t,γ) data and attributable to decreases of a Brownian contribution, σ_B/γ˙. The thickening behavior, not described by this equation, was related to dynamic clustering of the particles and corresponding enhancement of the hydrodynamic stress at high γ˙. In this thickening regime, the viscosity growth η₊ after start-up of flow was scaled with a strain γ˙t. Specifically, critical strains γ_d and γ_s for the onset of thickening and achievement of the steadily thickened state were independent of γ˙ but decreased with increasing φ_eff. This φ_eff dependence was again related to the hard-core volume effect, flow-induced collision of the particles enhanced for larger φ_eff. Received: 26 June 1998 Accepted: 9 December 1998     

Generation of “Sumanenylidene”: A Ground‐State Triplet Carbene on a Curved π‐Conjugated Periphery

We have observed the generation of sumanenylidene ( 2 ), a divalent, neutral‐carbon species at the benzylic position of sumanene ( 1 ). We also clarified both experimentally and theoretically that the ground state of compound 2 was a triplet state and that its singlet–triplet energy gap (ΔE_ST) was similar to that in fluorenylidene. The curved structure of compound 2 led to slightly better spin delocalization over the two adjacent aromatic rings than in planar systems, because of the unpaired spins on the σ and π orbitals. Synthetic application of the carbene precursor, diazosumanene ( 5 ), with a variety of thiocarbonyl compounds revealed its utility for the preparation of tetrasubstituted alkene compounds (e.g., that contain a strongly electron‐donating unit) that are directly conjugated to the sumanene ( 1 ) moiety.     

Specific Induced Circular Dichroism and Enhanced B to Z Transitions of Duplexes Stabilized by Chromophore‐Linked Alkynylnucleoside Residues

We have developed an induced circular dichroism (ICD) probe with a chromophore‐linked alkynyldeoxyribose skeleton for analyzing higher‐order structures of DNA duplexes in the visible‐light region. When CG‐repeated oligonucleotides (ODNs) with the probe at their 5′ ends adopted Z‐form duplexes at a high NaCl concentration, strong ICD signals were observed at the absorptive region of the chromophore. On the other hand, their B‐form duplexes, formed at a low NaCl concentration, produced a faint ICD signal. The specific ICD for the Z‐form duplexes was found to appear only when the chromophores were attached at the 5′ ends of each of the ODNs. Furthermore, the chromophoric alkynylnucleoside residues effectively promoted the B to Z transition of the ODN.     

Convergent assembly of the spiroacetal subunit of didemnaketal B

A highly convergent synthesis of the C9-C28 spiroacetal subunit of didemnaketal B has been accomplished. Assembly of the C9-C15 alkylborate and C16-C21 enol phosphate by means of Suzuki-Miyaura coupling and acid-catalyzed cyclization of the derived dihydroxy enol ether enabled a rapid and efficient construction of the spiroacetal subunit. The C22-C28 side chain was incorporated via Nozaki-Hiyama-Kishi coupling to complete the synthesis.     

pH and concentration dependence of luminescent characteristics in glass-encapsulated Eu-complex

In order to improve the stability of red phosphor with efficient and high color-purity characteristics under ultraviolet excitation, we encapsulated Eu(TTA)₃phen(tris(2-thenoyltrifluoroacetonato)(1,10-phenanthroline)europium(III)) by sol–gel derived silica glasses and studied their photoluminescence (PL) characteristics. The dependence of pH values in the starting solution revealed that the process condition should be within the modest region with the optimum pH value around 6–7. The PL intensity increased with increasing the concentration of Eu(TTA)₃phen initially, then it showed saturating tendency above 0.6 mol%. An improvement of reliability by the glass encapsulation was exemplified by monitoring the long-term PL intensity under constant UV irradiation. We consider that the glass network carefully prepared by sol–gel process is effective for preventing free oxygen and water from attacking Eu(TTA)₃phen molecule without loss of PL output.     

Stereospecific Synthesis of a Dihydroxyethylene Isostere of Cyclohexylalanine Amide, (2S,3R,4S)-4-Amino-5-Cyclohexyl-1-Morpholino-2,3-Pentanediol(ACMP) from a Protected Sugar

ABSTRACT

A transition-state analogue of a renin inhibitor at the scissile site, a dihydroxyethylene isostere of cyclohexylalanine amide, (2S,3R,4S)-4-amino-5-cyclohexyl-1-morpholino-2,3-pentanediol(ACMP), was synthesized from 1,2:5,6-di-O-isopropylidene-α-D-allofuranose stereospecifically.