Mixed saturated-unsaturated alkyl-chain assemblies: solid solutions of zinc stearate and zinc oleate

The linear saturated stearic acid and the bent mono-unsaturated oleic acid do not mix and form solid solutions. However, the zinc salts of these acids can. From X-ray diffraction and DSC measurements we show that the layered zinc stearate and zinc oleate salts form a homogeneous solid solution at all composition ratios. The solid solutions exhibit a single melting endotherm, with the melting temperature varying linearly with composition but with the enthalpy change showing a minimum. By monitoring features in the infrared spectra that are characteristic of the global conformation of the hydrocarbon chain, and hence can distinguish between stearate and oleate chains, it is shown that solid solution formation is realized by the introduction of gauche defects in a fraction of the stearate chains that are then no longer linear. This fraction increases with oleate concentration. It has also been possible from the spectroscopic measurements to establish a quantitative relation between molecular conformational order and the thermodynamic enthalpy of melting of the solid solutions.     

Room temperature molecular and lattice structures of a homologous series of anhydrous zinc(II) n-alkanoate

The room temperature structures and lattice arrangements of a homologous series of zinc(II) n-alkanoates from chain length, n(C) = 4-20, inclusive, have been studied using infrared spectroscopy, X-ray diffraction and polarizing light microscopy. Lattice parameters from single crystal and powder diffraction data, for zinc(II) hexanoate, are compared to validate the use of the powder method. Since they are in excellent agreement, the powder data are analyzed by a software programme to determine lattice parameters for all the homologues. These are used, in conjunction with infrared, X-ray, density and molecular model calculations to determine molecular and lattice structures. The compounds are isostructural, in that, each zinc atom is tetrahedrally coordinated to oxygen atoms from four different carboxylate groups and each ligand forms a Z,E-type bidentate bridge with two tetrahedral zinc atoms resulting in a syn-anti arrangement. The hydrocarbon chains are in the fully extended all-trans configuration and are tilted at an average angle of 60 degrees to the zinc basal plane. For the short chain length compounds with n(C) < or =8, a double bilayer in-plane-perpendicular-perpendicular-in-plane arrangement of hydrocarbon chains, with two molecules per unit cell, is indicated. For the others, an interdigitating in-plane-in-plane bilayer with head-to-tail interactions, with one molecule per unit cell, is proposed. A geometric model is presented to account for odd-even chain effects and to explain the differences in melting points and densities between these adducts. All the compounds crystallize in the monoclinic space group with P symmetry and are arranged in a two-dimensional network along the ac plane within the unit cell.     

Evaluation of molecular structure in Langmuir monolayers of zinc stearate and zinc 12-hydroxystearate by IR external reflection spectroscopy

 In situ polarized Fourier transform IR external reflection spectra of Langmuir monolayers of zinc stearate and zinc 12-hydroxystearate on a water surface were recorded for various surface areas, and their molecular structures were estimated. In the zinc stearate monolayer, the wavenumbers and the absorbances of the antisymmetric and symmetric methylene stretching bands did not change during monolayer compression, which means that orientational and conformational changes of the hydrocarbon chain did not occur. However, wavenumber changes of the antisymmetric and symmetric carboxylate stretching bands were observed during surface compression. The change in the binding nature of the zinc cation to the carboxylate group was speculated. Moreover, it was elucidated that the structure of the hydrocarbon chain in the zinc 12-hydroxystearate monolayer was different from that in the zinc stearate monolayer. Received: 21 March 2001 Accepted: 6 July 2001     

Melting of saturated fatty acid zinc soaps

The melting of alkyl chains in the saturated fatty acid zinc soaps of different chain lengths, Zn(C(n)H(2n+1)COO)(2); n = 11, 13, 15, and 17, have been investigated by powder X-ray diffraction, differential scanning calorimetry, and vibrational spectroscopy. These compounds have a layer structure with the alkyl chains arranged as tilted bilayers and with all methylene chains adopting a planar, all-trans conformation at room temperature. The saturated fatty acid zinc soaps exhibit a single reversible melting transition with the associated enthalpy change varying linearly with alkyl chain length, but surprisingly, the melting temperature remaining constant. Melting is associated with changes in the conformation of the alkyl chains and in the nature of coordination of the fatty acid to zinc. By monitoring features in the infrared spectra that are characteristic of the global conformation of the alkyl chains, a quantitative relation between conformational disorder and melting is established. It is found that, irrespective of the alkyl chain length, melting occurs when 30% of the chains in the soap are disordered. These results highlight the universal nature of the melting of saturated fatty acid zinc soaps and provide a simple explanation for the observed phenomena.     

Effect of double bond position on lipid bilayer properties: insight through atomistic simulations

Phosphatidylcholines (PCs) are among the most common phospholipids in plasma membranes. Their structural and dynamic properties are known to be strongly affected by unsaturation of lipid hydrocarbon chains, yet the role of the exact positions of the double bonds is poorly understood. In this work, we shed light on this matter through atomic-scale molecular dynamics simulations of eight different one-component lipid bilayers comprised of PCs with 18 carbons in their acyl chains. By introducing a single double bond in each acyl chain and varying its position in a systematic manner, we elucidate the effects of a double bond on various membrane properties. Studies in the fluid phase show that a number of membrane properties depend on the double bond position. In particular, when the double bond in an acyl chain is located close to the membrane-water interface, the area per lipid is considerably larger than that found for a saturated lipid. Further, when the double bond is shifted from the interfacial region toward membrane center, the area per lipid is observed to increase and have a maximum when the double bond is in the middle of the chain. Beyond this point, the surface area decreases systematically as the double bond approaches membrane center. These changes in area per lipid are accompanied by corresponding changes in membrane thickness and ordering of the chains. Further changes are observed in the tilt angles of the chains, membrane hydration together with changes in the number of gauche conformations, and direct head group interactions. All of these effects can be associated with changes in acyl chain conformations and local effects of the double bond on the packing of the surrounding atoms.     

Computational studies of carbon nanotube-hydrocarbon bond strengths at nanotube ends: effect of link heteroatom and hydrocarbon structure

Semiempirical and density functional electronic structure theory methods were used to study SWNT-X--R bond strengths, where the single-walled carbon nanotube (SWNT) had an armchair or zigzag structure, the link heteroatom X was O, N(H), or S and the hydrocarbon chain R was CH(2)CH(3), CH(OH)CH(3), CHCH(2), or CH(CF(3))CH(3). In all systems the hydrocarbon was bonded to the end of the nanotube. The SWNT-X--R bond (that is, the bond joining the link atom to the hydrocarbon) is more than 0.4 eV stronger for armchair than for zigzag nanotubes with the same diameters, irrespective of whether O, N, or S are used as link atoms or whether OH, C==C, or CF(3) groups are present in the hydrocarbon chain. This raises the possibility for selective manipulation of armchair/zigzag nanotubes using a variety of link atoms and hydrocarbon structures. The SWNT-O--CH(CF(3))CH(3) bond is weaker than the SWNT-O--CH(2)CH(3) bond (for both armchair and zigzag nanotubes), while inclusion of a double bond in the ethyl chain increases the bond strengths. Also, SWNT-S--CH(2)CH(3) and SWNT-N(H)--CH(2)CH(3) bonds are stronger than SWNT-O--CH(2)CH(3) bonds.     

Iron catalyzed polyethylene chain growth on zinc: a study of the factors delineating chain transfer versus catalyzed chain growth in zinc and related metal alkyl systems

The bis(imino)pyridine iron complex, [[2,6-(MeC=N-2,6-iPr2C6H3)2C5H)N]FeCl2] (1), in combination with MAO and ZnEt2 (> 500 equiv.), is shown to catalyze polyethylene chain growth on zinc. The catalyzed chain growth process is characterized by an exceptionally fast and reversible exchange of the growing polymer chains between the iron and zinc centers. Upon hydrolysis of the resultant ZnR2 product, a Poisson distribution of linear alkanes is obtained; linear alpha-olefins with a Poisson distribution can be generated via a nickel-catalyzed displacement reaction. Other dialkylzinc reagents such as ZnMe2 and ZniPr2 also show catalyzed chain growth; in the case of ZnMe2 a slight broadening of the product distribution is observed. The products obtained from Zn(CH2Ph)2 show evidence for chain transfer but not catalyzed chain growth, whereas ZnPh2 shows no evidence for chain transfer. The Group 13 metal alkyl reagents AlR3 (R = Me, Et, octyl, IBu) and GaR3 (R = Et, nBu) act as highly efficient chain transfer agents, whereas GaMe3 exhibits behavior close to catalyzed chain growth. LinBu, MgnBu2 and BEt3 result in very low activity catalyst systems. SnMe4 and PbEt4 give active catalysts, but with very little chain transfer to Sn or Pb. The remarkably efficient iron catalyzed chain growth reaction for ZnEt2 compared to other metal alkyls can be rationalized on the basis of: (1) relatively low steric hindrance around the zinc center, (2) their monomeric nature in solution, (3) the relatively weak Zn-C bond, and (4) a reasonably close match in Zn-C and Fe-C bond strengths.     

Effects of end groups on phase transition and segmental mobility of poly(N‐isopropylacrylamide) chains in D2O

Phase transition and mobility of poly(N‐isopropylacrylamide) (PNIPA) chains with three different types of end groups (hydroxyl, carbon–carbon double bond, and camphoric sulfonic groups) have been studied by measurements of the normal ¹H NMR spectrum, spin–spin relaxation time, and 2D NOESY spectrum. It is found that at room temperature not only the end group parts but also the part of the PNIPA chain with hydroxyl end group have higher mobility than corresponding parts of PNIPA with double bond and camphoric sulfonic end groups. The lower critical solution temperatures (LCST) of PNIPAs modified with hydrophilic hydroxyl and hydrophobic double bond end groups are inversely dependent and directly dependent on the molecular weight of polymer respectively, whereas the LCST of PNIPA with the camphoric sulfonic end group bearing both hydrophobic and hydrophilic structures is independent of the molecular weight. The double bond end groups collapse simultaneously with inner segments of the PNIPA chain, whereas the hydroxyl and camphoric sulfonic end groups still exhibit higher mobility and do not shrink tightly after heating‐induced collapsing of inner segments. It is suggested that the hydroxyl and camphoric sulfonic end groups locate on the surface of globules, but the double bond end groups are probably buried inside the globules. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Carbocyclisation of zinc enolates onto unactivated double bonds: a mechanistic point of view

The recent results concerning the carbocyclisation reaction of zinc enolates onto an unactivated double bond are summarized, particularly on the stereochemical point of view. The implications of the observed stereochemistries on the mechanism of the reaction (O-centred zinc enolate vs C-centred zinc enolate) are discussed. Some new results leading stereoselectively to 1-amino-1-carbomethoxy-2-methyl cyclopentanes are reported, as well as the consequences of the observed stereochemistry as a strong evidence for a C-centred zinc enolate intermediate. To cite this article: É. Sliwinski et al., C. R. Chimie 6 (2003) 000–000.     

Preparation of propargyl hydroperoxides by regioselective oxidation of allenic zinc reagents with molecular oxygen

Treatment of allenic zinc reagents (R(1)R(2)C[double bond]C[double bond]C(R(3))ZnL), generated by the reaction of propargyl derivative (R(1)R(2)C(X)[triple bond](CH) with triorganozincates (R(3)(3)ZnLi), under oxygen atmosphere in the presence of ZnCl(2) and chlorotrimethylsilane afforded propargyl hydroperoxides (R(1)R(2)C(OOH)C[triple bond];CR(3)) regioselectively. In this reaction, the use of ZnCl(2) and chlorotrimethylsilane as additives is essential for the transformation of the initially generated allenic reagents to more reactive chlorozinc species.     

杯芳烃-卟啉化合物及其金属配合物的Langmuir-Blodgett膜研究

通过对杯芳烃-卟啉化合物及其锌、钯配合物LB膜的表面压-表面积等温线、紫外可见光谱和傅里叶变换红外光谱的检测,研究了它们在气/液界面和Langmuir-Blodgett膜中的性质,并用量子力学方法优化了杯芳烃-卟啉化合物的构型.结果表明,成膜时3种化合物分子中卟啉环都倾斜地排列在亚相表面,且卟啉环间存在π-π相互作用.连接杯芳烃和卟啉之间的碳氢链在膜性质中起着重要的作用,杯芳烃-卟啉化合物分子中这种碳氢链的有序性小于其锌、钯配合物分子中的这种碳氢链有序性.     

2-[4-Alkenyloxy)phenyl]-5-alkylpyrimidines The relationship between position and nature (E/Z) of the double bond and transition temperatures

Abstract

The 5-n-alkyl-2-[4-(n-alkoxy)phenyl]pyrimidmes are essential components of most commercial chiral smectic C mixtures for electrooptic display devices based on ferroelectric effects. This is due to their generally relatively low melting points, enantiotropic, relatively wide range smectic C mesophases, low viscosity and ease of preparation. An unsaturated carbon–carbon double bond has now been introduced into the terminal alkoxy chain of the 5-n-alkyl-2-[4-(alkoxy)phenyl]pyrimidines to produce the corresponding alkenyloxy substituted derivatives. The position and nature (E/Z) of the double bond has been varied systematically and the effect on the liquid crystal transition temperatures studied. A number of homologous series of the most interesting alkenyloxy substituted materials has been prepared and evaluated. The position and nature (E/Z) of the double bond changes the conformation of the alkenyloxy chain substantially. This can result in significantly higher smectic C transition temperatures for compounds with a trans double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) are observed for materials with a cis double bond (Z) at an odd number of carbon atoms from the molecular core. Comparisons with the corresponding alkoxy substituted materials (i.e. without a double bond) are made.     

Alpha-alkylation of ketones by addition of zinc enamides to unactivated olefins

A zinc enamide generated from the corresponding N-aryl imine undergoes addition to an unactivated olefin, such as ethylene, 1-octene, and isobutylene, to generate an alpha-alkylated gamma-zincioimine intermediate in good to excellent yield. Terminal and gem-disubstituted olefins react with >99% regioselectivity, allowing the C-C bond formation to take place at the more hindered carbon of the double bond. The organozinc intermediate undergoes further C-C bond formation with a carbon electrophile to give, upon hydrolysis of the imine, an alpha-alkylated ketone bearing a variety of functionalized primary, secondary, and tertiary alkyl groups.     

IN SITU COMPATIBILIZATION OF LDPE/NYLON-6 BLEND USING LOW MOLECULAR WEIGHT INTERFACIAL AGENT AS A CHEMICAL COMPATIBILIZER

In situ compatibilization of low density polyehylene (LDPE) (30%) and nylon-6(70%)blends through one-step reactive extrusion using t-BuOOH as an initiator and low molec-ular weight interfacial agents as compatibilizers was studied. The compatibilizer containeda long chain bydrocarbon, double bond and two polar functional groups which was capableof reacting with both LDPE and nylon-6 in the presence of initiator to form a copolymerat the interface of the two polymer phases. The extruded blends exhibited significant en-hancement in their compatibility based on morphological, thermal analysis and mechanicalstudies. The effect of the hydrocarbon chain length and structure of the functional groupof the compatibilizer was also examined. It was found that blends prepared by using thecompatibilizer containing longer hydrocarbon chain and amide group had better mechanicalproperties.     

2-(4-Alkylphenyl)-5-(alkenyloxy)pyrimidines: Synthesis, liquid crystal transition temperatures and some physical properties

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is the high melting points of the trans-substituted materials and the low melting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.     

endo-双环戊二烯阳离子聚合及聚合物链结构

使用不同阳离子引发剂和引发体系在甲苯、正己烷和二氯甲烷中引发endo-双环戊二烯(DCPD)聚合, 得到了低分子量的DCPD聚合物(PDCPD), 采用IR和1H NMR对PDCPD链结构进行了表征。结果表明, DCPD分子的降冰片烯(NB)双键和环戊烯(CP)双键均参与了聚合反应并根据不同链增长机理产生四种链结构单元: 通过NB双键的链增长反应可生成直接加成结构单元I和重排结构单元II, 通过CP双键的链增长反应生成直接加成结构单元III和跨环重排单元IV。正己烷和甲苯中所合成了PDCPD包含全部四种结构单元, 并呈现I相似文献   

Influence of the structure of the mesogenic core on the thermotropic properties of ω-unsaturated fluorinated liquid crystals

Three series of calamitic liquid crystals have been prepared, consisting of a mesogenic core attached to which is a perfluorinated chain via a thioester linkage, and a hydrocarbon chain containing a terminal double bond. The rigid core is either a monophenyl, biphenyl or phenyl benzoate group. The mesomorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of the mesogenic core and of the hydrocarbon chain length on mesomorphic behaviour was studied. Increasing the length of the alkyl chain strongly reduces the mesomorphic behaviour while increasing the number of aromatic rings in the core increases the transition temperatures, with the widest LC range observed for derivatives with the phenyl benzoate core. The introduction of a single ring as the mesogenic core is considered of great interest in the development of low cost liquid crystal materials.     

Evaluation of structural change during surface pressure relaxation in Langmuir monolayer of zinc stearate by infrared external reflection spectroscopy

In situ polarized infrared external reflection spectra of the Langmuir monolayer of zinc stearate on a water surface were recorded during surface pressure relaxation, and the molecular structure of the monolayer was evaluated. The wavenumber of the antisymmetric methylene stretching band decreased during surface pressure relaxation. This result indicated that the shift to a highly compact packing of the hydrocarbon chain in the Langmuir monolayer occurred on a time scale of several minutes. Moreover, a wavenumber change in the antisymmetric carboxylate stretching band was observed. The O–C–O angle of the carboxylate group and the binding nature of the zinc cation to the carboxylate group appeared to have changed during the relaxation. The orientation angle of the molecule and the time scale of relaxation mechanisms were also discussed.     

Polyethylene chain growth on zinc catalyzed by olefin polymerization catalysts: a comparative investigation of highly active catalyst systems across the transition series

Highly active transition metal ethylene polymerization catalysts across the transition series have been investigated for their ability to catalyze chain growth on zinc. In reactions of various catalysts with ZnEt(2), product distributions range from Schulz-Flory to Poisson, with several catalysts showing intermediate behavior. A statistical modeling program is introduced to correlate product distributions with the relative rates of propagation, chain transfer to zinc, and beta-H transfer. Six regimes have been identified, ranging from Schulz-Flory to pure Poisson where chain transfer to metal is the only termination process, through to combined alkane/alkene distributions where beta-H transfer is competitive with chain transfer to metal. It is concluded that, while catalyzed chain growth (CCG) is favored by a reasonable match between the bond dissociation energies of both the main group and transition metal alkyl species, the M-C bond energies of the bridging alkyl species, and hence the stabilities of any hetero-bimetallic intermediates or transition states, are key. The latter are strongly influenced by the steric environment around the participating metal centers, more bulky ligands leading to a weakening of the bonds to the bridging alkyl groups; CCG is thus usually more favored for sterically hindered catalysts.     

Polyunsaturated docosahexaenoic vs docosapentaenoic acid-differences in lipid matrix properties from the loss of one double bond

Insufficient supply to the developing brain of docosahexaenoic acid (22:6n3, DHA), or its omega-3 fatty acid precursors, results in replacement of DHA with docosapentaenoic acid (22:5n6, DPA), an omega-6 fatty acid that is lacking a double bond near the chain's methyl end. We investigated membranes of 1-stearoyl(d(35))-2-docosahexaenoyl-sn-glycero-3-phosphocholine and 1-stearoyl(d(35))-2-docosapentaenoyl-sn-glycero-3-phosphocholine by solid-state NMR, X-ray diffraction, and molecular dynamics simulations to determine if the loss of this double bond alters membrane physical properties. The low order parameters of polyunsaturated chains and the NMR relaxation data indicate that both DHA and DPA undergo rapid conformational transitions with correlation times of the order of nanoseconds at carbon atom C(2) and of picoseconds near the terminal methyl group. However, there are important differences between DHA- and DPA-containing lipids: the DHA chain with one additional double bond is more flexible at the methyl end and isomerizes with shorter correlation times. Furthermore, the stearic acid paired with the DHA in mixed-chain lipids has lower order, in particular in the middle of the chain near carbons C(10)(-)(12), indicating differences in the packing of hydrocarbon chains. Such differences are also reflected in the electron density profiles of the bilayers and in the simulation results. The DHA chain has a higher density near the lipid-water interface, whereas the density of the stearic acid chain is higher in the bilayer center. The loss of a single double bond from DHA to DPA results in a more even distribution of chain densities along the bilayer normal. We propose that the function of integral membrane proteins such as rhodopsin is sensitive to such a redistribution.