Ciliation and lamellae in crystalline polymers

Molecular cilia, the uncrystallized portions of chains already partly attached to polymer crystals, exert a profound influence on the course of polymeric crystallization with ultimate responsibility for the divergence of adjacent lamellae which leads to spherulitic growth. Their effective size and pressure have been measured, in α-polypropylene, from electron microscopic measurements of the separation and maximum curvature of lamellae in row structures. That cilia exist and extend the effective region occupied by a lamella beyond the geometrical confines of its fold surfaces is important for crystallization theory and may well have implications for the connection of lamellae into networks and for gelation.     

The Shape of Orthogonal Cycles in Three Dimensions

Let σ be a directed cycle whose edges have each been assigned a desired direction in 3D (East, West, North, South, Up, or Down) but no length. We say that σ is a shape cycle. We consider the following problem. Does there exist an orthogonal representation Γ of σ in 3D space such that no two edges of Γ intersect except at common endpoints and such that each edge of Γ has the direction specified in σ? If the answer is positive, we say that σ is simple. This problem arises in the context of extending orthogonal graph drawing techniques from 2D to 3D. We give a combinatorial characterization of simple shape cycles that yields linear time recognition and drawing algorithms.     

Recent Reviews. 41

Reviews are listed in order of appearance in the sources indicated. In multidisciplinary review journals, only those reviews which fall within the scope of this Journal are included. Sources are listed alphabetically in three categories: regularly issued review journals and series volumes, contributed volumes, and other monographs. Titles are numbered serially, and these numbers are used for reference in the index. Major English-language sources of critical reviews are covered. Encyclopedic treatises, annual surveys such as Specialist Periodical Reports, and compilations of symposia proceedings are omitted. This installment of Recent Reviews covers principally the early part of the 1996 literature. Previous installment: J. Org. Chem. 1996, 61(7), 2572-80. These Recent Review articles are now also available in a database form on the ACS Organic Division WWW site http://www.organic.emory.edu/acsorg/index.html.     

Regioselective Alkylation of Lithium Enolates Derived from 2-Heptanone

The benzylation and $\text{u}$-butylation of the kinetic enolate mixture derived from 2-heptanone has been studied in DME in the absence and in the presence of lithium ion complexing agents (triglyme, benzo-14-crown-4, DMF (neat), DMF, and HMPA).     

Ceria-Zirconia Nanostructured Materials for Catalytic Applications: Textural Characteristics and Redox Properties

Highly dispersed solid solutions of ceria-zirconia with nominal composition Ce_0.6Zr_0.4O₂ were prepared by sol-gel technique and the influence of the gel aging time (1 day–3 weeks) on the morphological and textural properties of the materials was investigated. The samples were calcined at 923 K for 8 h, then characterized by XRD, BET surface area and pore size measurements by gas adsorption and by mercury porosimetry. DTA, TEM and SEM analyses were also carried out. The aging of the gel seems to have a remarkable influence on the morphology of the materials, whose surface area and porosity increase by increasing aging time. The phase composition of the solid solutions of ceria-zirconia, as determined by XRD analysis, seems to be independent from the synthetic procedure; in any case a cubic phase (space group Fm-3m) was identified as the main component. XRD and TEM characterizations show the presence of highly dispersed crystallites, with diameters in the range 60–80 Å. The redox properties and hydrogen chemisorption capacity have been studied by TPR/TPD/TPO experiments. Oxygen storage capacity measurements were carried out by a pulse technique, to determine the overall reducibility of the solid, OSCC (Oxygen storage capacity complete). OBC (Oxygen buffering capacity) measurement were also performed to determine the ability of the ceria-zirconia oxides to attenuate fast oscillations (0.1 Hz) of oxygen partial pressure. The correlation between textural, structural and redox properties of the oxides is discussed.     

Synthesis of 1-deoxysphingosine derivatives with conformationally restricted pyrrolidinediol head groups

[structure: see text] A family of cyclic 1-deoxysphingolipid derivatives of structure 4 has been designed and synthesized, which may serve as tumorigenesis suppressors for various cancers. Compound 4 is a second-generation analogue developed from sphingosine (1), in which a hydroxyl substituent is moved from C1 to C5 and a methylene is added for conformational rigidity between the C2-nitrogen substituent and C4. The synthetic chemistry for pyrrolidine ring closure at C3-C4 features ring-closing metathesis followed by hydroboration-oxidation.     

Systematics of proton emission

A very simple formula is presented that relates the logarithm of the half-life, corrected by the centrifugal barrier, with the Coulomb parameter in proton decay processes. The corresponding experimental data lie on two straight lines which appear as a result of a sudden change in the nuclear shape marking two regions of deformation independently of the angular momentum of the outgoing proton. This feature provides a powerful tool to assign experimentally quantum numbers in proton emitters.     

Benign coupling of reactions and separations with reversible ionic liquids

Reversible ionic liquids are a novel class of solvents that combine an effective medium where reactions occur with a ‘built-in’ separation ability for facile recovery of the products and catalysts, making the solvent available for recycle. We report the utility of these solvents in a number of reactions (Claisen-Schmidt condensation, Heck C-C coupling, and CO₂ capture) and discuss the effectiveness of the separation. We also provide insight into the challenges and limitations of using these unique solvent systems to couple reactions and separations.