Cyclopentyl Rings Research Articles

Structure and Dynamics of Blends of Polyhedral Oligomeric Silsesquioxanes and Polyethylene by Atomistic Simulation

The influence of blending polyhedral oligomeric silsesquioxane substituted with cyclopentyl rings (CpPOSS) into a polyethylene (PE) matrix was probed using atomistic simulations. Composites of 5, 15, and 25 wt % CpPOSS were simulated. Interactions between POSS particles were found to promote organization of POSS within the polymer, where clear signs of aggregation were observed. Both particle and polymer dynamics were monitored. Particular attention was given to the structure and dynamics of the interface between particle and polymer. The interface was found to consist of a 3−5 A thick shell in which the structure of the matrix was altered from that of the bulk. In this region, the local polymer backbone orientation is biased toward a configuration parallel to the particle surface. Dynamically, the interface consists of a thick shell about 11 A thick in which polymer mobility is damped in the direction normal to and enhanced circumferentially to the surface of the CpPOSS particle. These results suggest th...

Structure and magnetism of a spin ladder system: (C5H9NH3)2CuBr4.

The crystal structure of bis(cyclopentylammonium)tetrabromocuprate(II) has been determined at room temperature and at -70 degrees C. The room temperature structure is orthorhombic, space group Pn2(1)a, with a = 12.092(6) A, b = 8.134(4) A, and c = 18.698(10) A. The low temperature structure is also orthorhombic, space group Pna2(1), with a = 24.111(5) A, b = 8.089(2) A, and c = 18.448(4) A. DSC studies reveal the presence of a weak endotherm at -13 degrees C. The structures of the two phases are very similar, differing only in the relative orientations of the cyclopentyl rings of the organic cations and slight displacements of the anionic tetrahedra. The CuBr(4)(2)(-) anions in the low temperature phase are arranged to define a spin ladder system through Cu-Br.Br-Cu two-halide exchange pathways. Magnetic susceptibility data have been analyzed and yield antiferromagnetic exchange strengths 2J(rail)/k = -11.6 K and 2J(rung)/k = -5.5 K with a singlet-triplet gap energy Delta/k(B) = 2.3 K. This is the first report of a spin ladder with a stronger interaction along the axis of the ladder than along the rungs.

Archaea in Black Sea water column particulate matter and sediments—evidence from ether lipid derivatives

Phytane and long-chain, C 40, head-to-head acyclic (C 40:0cy) and cyclic isoprenoids with 1–3 cyclopentyl rings (C 40:1cy, C 40:2cy, and C 40:3cy) derived from archaeal core lipids are present in Black Sea particulate matter from both the oxic and anoxic zones and in sediment cores from the central basin and the western continental margin. The occurrence of these ether-linked isoprenoids throughout the water column and sediments suggests that Archaea are widely distributed in the Black Sea. Methanogens are the probable source of ether-linked phytane and some of the acyclic C 40 compounds while psychrophilic archaea related to hyperthermophiles are the likely sources of the cyclic isoprenoids.

Determination of the Tacticity of Ring-opened Metathesis Polymers of Norbornene and Norbornadiene by13C NMR Spectroscopy of their Hydrogenated Derivatives

The 125 MHz 13 C nuclear magnetic resonance (NMR) spectra of the hydrogenated derivatives of ring-opened metathesis polymers (ROMP) of bicydo[2.2.1]hept-2-ene (norbornene) and bicydo[2.2.1]hepta-2,5-diene (norbornadiene) prepared using a range of initiators, have been analysed in detail. The signals due to the methylene carbon atoms in the enchained cyclopentyl rings of these polymers show fine structure which is assigned to m and r ring dyads and mm, mr/rm and rr ring triads, enabling the tacticity of the hydrogenated polymer, and thus of the polyolefin precursor, to be determined for high-cis or high-trans polymers. Novel methods for making high-cis polymers are described and solvent-dependent variations in the stereospecificity of the propagation reaction have been observed.

Significance of Ether Lipids Derived from Methanogens in Sediments from Modern and Ancient Basins: ABSTRACT

High amounts of biphytanyl carbon skeletons with 0-3 cyclopentyl rings and low amounts of 2, 6, 10, 14, 18-pentamethyleicosane (PME) were detected after treatment with hydrogen iodide of fractions of several sediment extracts. These compounds are biosynthesized by archaebacteria only. Since the sediments were deposited in normal marine environments, these lipids are derived from methanogenic bacteria and more specifically Methanosarcina barkeri. In recent sediments from in and below the extensive oxygen-minimum zone in the Arabian Sea high amounts were detected in surface sediment samples where oxygen is still present. Our data suggest that these ether-bound compounds may be indicators for methane-production in the upper part of the water column and possibly an extensive oxygen-minimum zone. The concentrations varied strongly (2 orders of magnitude) in the Miocene Monterey Formation suggesting significant variations in water column methanogenic activity. The stable carbon isotopic compositions of the acyclic and cyclic biphytane carbon skeletons are several per mil heavier than those of compounds derived from photoautotrophic organisms whereas the stable carbon isotopic compositions of PME are several per mil lighter. This indicates the presence of different species of methanogens utilizing different carbon substrates.

Electron-Spin-Resonance Study of Tetracyclopentanaphthalene Radical Ions

From an e.s.r. investigation of tetracyclopentanaphthalene cation and anion radicals it was found that the oscillation of the alkyl cyclopentyl rings is sufficiently rapid to average the splitting constants observed. The averaged splitting constants found were 0.5891, 0.1983 and 0.0207 mT in the anion, and 1.1637, 0.4387 and 0.0062 mT in the cation.

Lipids of Archaebacteria

Summary The archaebacteria currently consist of several distinct subgroups including methanogens, extreme halophiles and certain thermoacidophiles. The lipids of archaebacteria are distinguished from those of other prokaryotes and eukaryotes by the absence of fatty acid glycerol ester lipids and the predominance of nonsaponifiable lipids. The lipid composition of the archaebacteria consists of isoprenoid and hydroisoprenoid hydrocarbons and isopranyl glycerol ether lipids. The glycerol ethers of archaebacteria, which constitute the hydrophobic residues of the polar lipids and consequently the membrane interior are diphytanylglycerol diethers or dibiphytanyldiglycerol tetraethers. Either or both glycerol ether structures may be present, depending on genus. The tetraethers of the thermoacidophilic archaebacteria are more specialized in that the dibiphytanyl alkyl chains may contain 1 to 4 cyclopentyl rings. As a consequence of the presence of the tetraethers which can span the membrane, some archaebycterial membranes may exist as a lipid “monolayer” rather than the usual lipid bilayer. The structure of some diether-containing polar lipids of archaebacteria have been well established. The extent of the variety of tetraether containing polar lipid structures is still largely unknown, but both the symmetric and asymmetric substitution of polar head groups to the tetraether has been established in some instances. Among neutral lipids, squalenes and isoprenoid hydrocarbons appear to be universal. The exact pathways for the biosynthesis of the lipid components remain a challenge, but clearly the mevalonate pathway for isoprenoid biosynthesis is the major route of lipid synthesis in archaebacteria rather than the malonyl-CoA pathway for fatty acid biosynthesis in prokaryotes and eukaryotes. The isopranyl glycerol ethers are distinctive, providing a useful taxonomic tool and molecular marker for the identification of archaebacteria. The lipids can also serve as useful biochemical “fossil” evidence for tracing the earlier existence of the organisms. Overall, the discontinuity of archaebacterial lipids formulates a point for delineating early stages of biological evolution and supports the concept that archaebacteria represent a third line of evolutionary descent.

The dechlorination of poly(vinyl chloride) by zinc and tributyl tin hydride.: Characterization of the product microstructures by carbon-13 NMR

Poly(vinyl chloride) has been reacted with zinc in refluxing dioxane for times in the range 18–113 h. The structure of the polymeric product has been examined by 1H and 13C NMR spectroscopy. Contrary to earlier predictions, the reaction does not proceed only by 1,3-chlorine elimination to form cyclopropyl rings. Double bonds and methylene units are formed also, to give a polymer of considerable structural complexity. This complexity can be reduced by replacement of remaining chlorine with hydrogen after treatment with tributyl tin hydride. Analysis of the fully dechlorinated polymer shows that cyclopropyl rings are distributed randomly with a trans/cis ratio of about 2.6. A small number of cyclopentyl rings is observed also. A maximum of 9% of the original vinyl chloride units are dehydrochlorinated to give unconjugated vinylene units with a predominantly trans structure.

Dynamic mechanical properties of poly‐α‐olefins containing ring structures in side chains

AbstractDynamic loss modulus curves have been determined over a temperature range beginning at liquid nitrogen temperature for poly‐α‐olefin polymers containing various ring structures, i.e., phenyl, cyclohexyl, cyclopentyl, and naphthyl, in the side chain. Glass transition and appropriate secondary relaxation temperatures were observed for each polymer. Separation of each pendant ring structure from the main backbone chain by successive additions of methylene units results in lower glass‐transition temperatures. Comparison of polymers with similar side chains and different ring structures shows that the respective glass‐transition temperatures decrease in the order naphthyl > cyclohexyl > phenyl > cyclopentyl. Secondary relaxation peaks were obtained at about −150°C for polymers containing the cyclohexyl and cyclopentyl rings. A similar peak was observed for the polymer possessing a phenyl ring separated from the main chain backbone by two methylene units. The comparable polymer containing the naphthyl ring structure exhibited a broad secondary relaxation peak centered at −20°C. The polymers possessing cyclohexyl rings separated from the main chain backbone by one or two methylene units had an additional low temperature peak at −80°C. The molecular mechanism associated with this relaxation may be related to intramolecular transformations of the cyclohexyl ring between its “chair–chair” conformations.

Formation of Hydronaphthalenes from Cyclopentyl Rings in Acid-catalyzed Reactions1

Formation of Hydronaphthalenes from Cyclopentyl Rings in Acid-catalyzed Reactions¹