Physico-chemical aspects of polyethylene processing in an open mixer. Part 19: Mechanisms and kinetics of vinyl and vinylidene group formation

Vinyl and vinylidene group formation is detected in the initial stages of polyethylene processing. In the high temperature range (170-200 °C) the amount formed is small but significant. Formation of these double bonds is usually obscured by their rapid consumption. Bimolecular hydroperoxide decomposition does not seem to be an important source for these products in the early stages of processing. Vinyl and vinylidene group formation can be attributed mainly to intramolecular decomposition of special hydroperoxide groups. The data suggest vinyl groups to arise from secondary hydroperoxide groups formed in α-position to methyl branching. Intramolecular hydroperoxide decomposition involving a primary hydrogen atom from the methyl group yields a vinyl group and an aldehyde. Vinylidene groups seem to arise from secondary hydroperoxide groups formed in α-position to quaternary structures that necessarily include one methyl group. Intramolecular hydrogen abstraction of a primary hydrogen atom from the methyl group yields a vinylidene group and an aldehyde. The calculated rate parameters are in agreement with the thermochemical estimations relative to intramolecular abstraction of primary hydrogen atoms for both reactions. Vinyl groups are also formed on bimolecular hydroperoxide decomposition. The yield of vinylidene groups from the last reaction is negligible.     

A molecular orbital study of trans-polyenes to 18 double bonds

Calculations involving molecular orbitals have appeared in the chemical literature over the past few years, but all have used smalltrans-polyenes. We now report extended Huckel molecular orbital calculations ontrans-polyenes of up to 18 double bonds (36 carbons and 38 hydrogen atoms). Information obtained from these calculations include total energies, bond gaps, and net charges on each atom. Also found is that the band gap approaches 1.41 eV as the number of double bonds approaches infinity. This value is quite close to reported experimental values. Data are also presented for polyenes assuming equal C-C bond lengths.     

Photoinduced microstructural changes in 1,4-polyisoprene

An infrared and NMR study was made of the microstructural changes produced in thin films of purified cis- and trans-1,4-polyisoprene when irradiated with ultraviolet light in vacuo at room temperature. The major photochemical processes observed were cis–trans isomerization and loss of 1,4 double bonds, the latter process being accompanied by the formation of vinylidene and vinyl double bonds as well as some endlinking. Very surprisingly, the loss of original double bonds contributed also to a novel photocyclization which gave rise to cyclopropyl groups in the polyisoprene chain. The isomerization and the formation of cycloprophyl groups are presumed to proceed through triplet and biradical states of the 1,4 double bonds, while the vinylidene and vinyl double bonds must result from chain repture at the carbon–carbon bond joining successive isoprene units. Hydrogen abstraction and double-bond migration are of neglible importance in the overall photochemistry of polyisoprene.     

Poly[(μ3‐benzene‐1,4‐diacetato)[μ2‐1,4‐bis(1,2,4‐triazol‐1‐yl)butane]cadmium(II)]: self assembly into a three‐dimensional supramolecular framework based on [Cd(μ3‐benzene‐1,4‐diacetate)] double chains

The title compound, [Cd(C₁₀H₈O₄)(C₈H₁₂N₆)]_n, crystallizes with an asymmetric unit comprising a divalent Cd^II atom, a benzene‐1,4‐diacetate (PBEA²⁻) ligand and a complete 1,4‐bis(1,2,4‐triazol‐1‐yl)butane (BTB) ligand. [Cd(PBEA)]_n double chains, arranged parallel to the c axis, are formed through an exo‐tridentate binding mode of the PBEA²⁻ ligands. These [Cd(PBEA)]_n double chains are pillared by tethering BTB ligands, in which the BTB shows a trans–trans–trans conformation, to establish [Cd(PBEA)(BTB)]_n two‐dimensional coordination polymer (4,4)‐layer slab patterns. The three‐dimensional supramolecular architecture is formed by C—H...O hydrogen bonds and C—H...π interactions.     

dl‐Cysteinium semioxalate

Two chiral counterparts (l ‐ and d ‐cysteinium cations related by an inversion centre) are present in the structure of the title compound, C₃H₈NO₂S⁺·C₂HO₄⁻, with a 1:1 cation–anion ratio. The carboxy group of the cysteinium cation is protonated in the trans position relative to the amino group. The crystal structure is built up of double layers, in which dimers of cysteinium cations are connected to each other not directly, but via bridges of twisted semioxalate anions linked to each other via O—H...O hydrogen bonds forming infinite chains. An interesting feature of the crystal structure is the absence of either S—H...S or S—H...O hydrogen bonds.     

A non-charged,easily polarizable,intramolecular, hydrogen bond in 2-(α-pyridyl N-oxide)ethane sulphonic acid

Non-charged intramolecular hydrogen bonds formed in 2-(α-pyridyl N-oxide)ethane sulphonic acid cause strong continuous absorption; thus, these hydrogen bonds are easily polarizable. A double minimum proton potential with a deeper well at the oxygen atom of the NO group is present in these hydrogen bonds.     

Introduction of unsaturated pendant groups to polyethylene by γ-ray irradiation under a 1,3-butadiene atmosphere

Ultra-high-molecular-weight polyethylene (Mˉ _v: 5 × 10⁶, 100-times elongated film) was irradiated with γ-rays under a 1,3-butadiene atmosphere at room temperature. Electron paramagnetic resonance (EPR) measurements indicated that the radicals formed on the polyethylene substrate during the irradiation were short-lived. EPR, Fourier transform IR spectroscopy, solid-state NMR, and differential scanning calorimetry of the as-irradiated materials indicated that butadiene molecules were covalently bound to the polyethylene chains as pendant groups bearing trans-vinylene and vinyl functions in a ratio of 3:1. Some crosslinks among the pendants, or between pendants and the main chains were produced. The number of unsaturated pendants introduced (including bridges) per carbon atom of the polyethylene main chain was dependent on the irradiation dose and the butadiene pressure, and was 0.096 butadiene units for 10 kGy irradiation under a 304 kPa butadiene atmosphere. The unsaturated pendants or bridges on the polyethylene chain thus introduced may be good targets to functionalize polyethylene by covalent modification. Received: 22 February 1999 Accepted in revised form: 30 June 1999     

(Z)‐6‐{[1,3‐Dihydroxy‐2‐(hydroxy­methyl)propan‐2‐ylamino]methyl­ene}‐2‐methoxy‐4‐[(E)‐o‐tolyl­diazenyl]­cyclo­hexa‐2,4‐dienone

The title compound, C₁₉H₂₃N₃O₅, adopts the keto–amine tautomeric form with the hydr­oxy H atom located on the N atom, where it is involved in a strong intra­molecular N—H⋯O hydrogen bond. The compound exhibits trans geometry with respect to the azo N=N double bond, with a dihedral angle between the two benzene rings of 38.03 (6)°. The packing of the mol­ecules in the crystal structure is determined by O—H⋯O and C—H⋯O hydrogen bonds. A comparison with closely related compounds is given.     

The photochemistry of α,β-acetylenic ketones. II. Formation of furan derivatives

The photochemical reactions of α,β-acetylenic ketones have been examined. Irradiation of 1-p-substituted phenyl-2-propyn-1-ones 2–4 in primary alcohols gave 2,5-disubstituted furans 2a–4c. The formation of furans can be explained in terms of cyclization, followed by dehydration of the 1:1-adduct of acetylenic ketone and alcohol, which was formed initially by hydrogen atom abstraction from alcohol by the excited acetylenic ketone. Irradiation of 1-p-tolyl-2-propyn-1-one ( 2 ) in ethanol-d₁ yielded 2-methyl-5-p-tolylfuran ( 2b ) containing no deuterium. This result was consistent with a mechanism that involves hydrogen atom abstraction from alcohol by the carbon of triple bond rather than abstraction by carbonyl oxygen.     

The thermal reaction of 2-pentene around 500°C at low extent of reaction

The thermal reaction of 2-pentene (cis or trans) has been performed in a static system over the temperature range of 470°–535°C at low extent of reaction and for initial pressures of 20–100 torr. The main products of decomposition are methane and 1,3-butadiene. Other minor primary products have been monitored: trans-2-pentene, trans- and cis-2-butenes, ethane, 1,3-pentadienes, 3-methyl-1-butene, propylene, 1-butene, hydrogen, ethylene, and 1-pentene. The initial orders of formation, 0.8–1.1 for most of the products and 1.5–1.8 for 1-pentene, increase with temperature. The formation of the products and the influence of temperature on their orders can be essentially explained by a free radical chain mechanism. But cis–trans or trans–cis isomerization and hydrogen elimination from cis-2-pentene certainly involve both molecular and free radical processes. The formation of 1-pentene mainly occurs from the abstraction of the hydrogen atom of 2-pentene by resonance stabilized free radicals (C₅H₉.).     

Peculiarities of electroreduction of trans-2-allyl-6-methyl(allyl,phenyl)-1,2,3,6-tetrahydropyridines

Electrochemical reduction of trans-2-allyl-6-R-1,2,3,6-tetrahydropyridines (R = Me, All, and Ph) on the mercury cathode in anhydrous DMF (with 0.1 M Bu₄NClO₄ as the supporting electrolyte) resulted in catalytic hydrogen evolution, while in the case of anhydrous DMF the electrochemical activity of the endocyclic double bond was dictated by the nature of the R substituent at the carbon atom neighboring the double bond. The electrocatalytic hydrogenation of the piperideines under study on the Ni (Ni_disp/Ni) cathode in 40% aqueous DMF in the presence of a tenfold excess of AcOH yielded the corresponding trans-2-propyl-6-R¹-piperidines (R¹ = Me, Pr, and Ph). Using trans-2,6-diallyl-1,2,3,6-tetrahydropyridine as an example, the conditions (with annealed copper as the cathode) for selective hydrogenation of the double bonds in allyl substituents with preservation of the endocyclic double bond were found.     

Cocrystallization of melaminium levulinate monohydrate

Crystals of 2,4,6‐tri­amino‐1,3,5‐triazin‐1‐ium levulinate (4‐oxo­pentanoate) monohydrate, C₃H₇N₆⁺·C₅H₇O₃⁻·H₂O, are formed via self‐assembled hydrogen bonding by cocrystallization of mel­amine and levulinic acid. Two N—H⋯N hydrogen bonds and four N—H⋯O hydrogen bonds connect two melaminium entities such that each of two pairs of N—H⋯O bonds bridges two H atoms belonging to the amine groups of two different melaminium cations via the carbonyl O atom of one levulinate mol­ecule.     

Covalently Linked Bis(Amido-Corroles): Inter- and Intramolecular Hydrogen-Bond-Driven Supramolecular Assembly

Four bis-corroles linked by diamide bridges were synthesized through peptide-type coupling of a trans-A₂B-corrole acid with aliphatic and aromatic diamines. In the solid state, the hydrogen-bond pattern in these bis-corroles is strongly affected by the type of solvent used in the crystallization process. Although intramolecular hydrogen bonds play a decisive role, they are supported by intermolecular hydrogen bonds and weak N−H⋅⋅⋅π interactions between molecules of toluene and the corrole cores. In an analogy to mono(amido-corroles), both in crystalline state and in solutions, the aliphatic or aromatic bridge is located directly above the corrole ring. When either ethylenediamine or 2,3-diaminonaphthalene are used as linkers, incorporation of polar solvents into the crystalline lattice causes a roughly parallel orientation of the corrole rings. At the same time, both NHCO⋅⋅⋅NH corrole hydrogen bonds are intramolecular. In contrast, solvation in toluene causes a distortion with one of the hydrogen bonds being intermolecular. Interestingly, intramolecular hydrogen bonds are always formed between the –NHCO– functionality located further from the benzene ring present at the position 10-meso. In solution, the hydrogen-bonds pattern of the bis(amido-corroles) is strongly affected by the type of the solvent. Compared with toluene (strongly high-field shifted signals), DMSO and pyridine disrupt self-assembly, whereas hexafluoroisopropanol strengthens intramolecular hydrogen bonds.     

Restriction of Spin Probe Motions in Polymer Composites Due to Chemical Factors

Abstract

Hyperfine splitting constants of the nitroxyl radical, with and without hydrogen bonds to the surrounding molecules, have been calculated using the UHF method on a 6-31G* base. In polyethylene filled with silica, hydrogen bonds are formed between nitroxyl radicals and —OH groups of the filler. The formation of hydrogen bonds leads to a change in the A _zz value from 3.33 mT for an isolated nitroxyl radical to 3.83 mT for a radical with a hydrogen bond. The relevant values as measured experimentally are 3.4 and 4.0 mT, respectively. The same procedure was used to calculate the theoretical A _zz value for a nitroxyl radical interacting with polyamide via a hydrogen bond. The value was found to be 3.63 mT (experimental value = 3.6 mT). Hydrogen bond formation results in a restricted motion of the nitroxyl radical in a polymeric medium.     

Crystal and molecular structure of pyridazine-3-carboxylic acid hydrochloride and zinc(II) pyridazine-3-carboxylate tetrahydrate

Crystals of pyridazine-3-carboxylic acid hydrochloride contain almost planar molecular sheets in which the cations, composed of acid molecules each with a hydrogen atom attached to one of the ring-nitrogen atoms, interact with chloride anions via a network of weak hydrogen bonds. Van der Waals interactions between sheets are indicated by the intersheet spacing of 3.47?Å. The crystal structure of di(aqua-O)bis(trans-pyridazine-3-carboxylato-N,O)zinc(II) dihydrate is composed of monomeric molecules in which the zinc(II) ion at the center of symmetry is coordinated by two ligand molecules each via its N,O bonding moiety. The ligand molecules and the metal ion form a trans-planar configuration. Two water oxygen atoms, above and below the plane, complete a distorted octahedron. A network of weak hydrogen bonds holds the monomers together.     

Head‐to‐head dimers in the zwitterion of 1‐hydroxy‐1‐phosphono‐3‐(1‐piperidino)propylidene‐1‐phosphonate (PHPBP)

The title compound, C₈H₁₉NO₇P₂, is a member of the bis­phosphonate family of therapeutic compounds. PHPBP has inner‐salt character, consisting of a negatively charged PO₃ group and a positively charged N atom. The six‐membered piperidine ring adopts an almost‐perfect chair conformation. The hydroxyl group and the N atom have gauche and trans conformations in relation to the O—C—C—C—N backbone, respectively. Hydrogen bonding is the main contributor to the packing in the crystal, which consists of head‐to‐head dimers formed through phosphonyl–phosphonyl hydrogen bonds, while O—H⋯O and N—H⋯O interactions join the dimers into a plane parallel to crystallographic b and c axes.     

Supramolecular architectures in cytosinium 6‐chloronicotinate monohydrate and 5‐bromo‐6‐methylisocytosinium hydrogen sulfate

Aminopyrimidine derivatives are biologically important as they are components of nucleic acids and drugs. The crystals of two new salts, namely cytosinium 6‐chloronicotinate monohydrate, C₄H₆N₃O⁺·C₆H₃ClNO₂⁻·H₂O, ( I ), and 5‐bromo‐6‐methylisocytosinium hydrogen sulfate (or 2‐amino‐5‐bromo‐4‐oxo‐6‐methylpyrimidinium hydrogen sulfate), C₅H₇BrN₃O⁺·HSO₄⁻, ( II ), have been prepared and characterized by single‐crystal X‐ray diffraction. The pyrimidine ring of both compounds is protonated at the imine N atom. In hydrated salt ( I ), the primary R₂²(8) ring motif (supramolecular heterosynthon) is formed via a pair of N—H…O(carboxylate) hydrogen bonds. The cations, anions and water molecule are hydrogen bonded through N—H…O, N—H…N, O—H…O and C—H…O hydrogen bonds, forming R₂²(8), R₃²(7) and R₅⁵(21) motifs, leading to a hydrogen‐bonded supramolecular sheet structure. The supramolecular double sheet structure is formed via water–carboxylate O—H…O hydrogen bonds and π–π interactions between the anions and the cations. In salt ( II ), the hydrogen sulfate ions are linked via O—H…O hydrogen bonds to generate zigzag chains. The aminopyrimidinium cations are embedded between these zigzag chains. Each hydrogen sulfate ion bridges two cations via pairs of N—H…O hydrogen bonds and vice versa, generating two R₂²(8) ring motifs (supramolecular heterosynthon). The cations also interact with one another via halogen–halogen (Br…Br) and halogen–oxygen (Br…O) interactions.     

Synthesis,characterization, crystal structure,and theoretical studies on Schiff-base compound 6-[(5-Bromopyridin-2-yl)iminomethyl]phenol

The title Schiff-base compound, 6-[(5-Bromopyridin-2-yl)iminomethyl]phenol (1), has been synthesized and characterized by elemental analyses, FT-IR, UV–Vis and ¹H-NMR spectroscopy, and X-ray single crystal diffraction. In the gas phase four isomers were found for title compound. Density functional (DFT) calculations have been carried out and it was found that the A isomer is the most stable one. The protonated imine N atom is involved in intra- and inter-molecular hydrogen bonds with the phenoxide group and H aromatic atoms, respectively. The title compound displays a trans configuration about the C=N double bond.     

Tetraaquabis[5‐(3‐pyridyl‐κN)pyrimidine]zinc(II) bis(trifluoromethanesulfonate): a novel cationic complex and three‐dimensional hydrogen‐bonded network

The title compound, [Zn(C₉H₇N₃)₂(H₂O)₄](CF₃O₃S)₂, contains an octahedral [ZnL₂(H₂O)₄]²⁺ cationic complex with trans geometry (Zn site symmetry ), and each 5‐(3‐pyridyl)pyrimidine (L) ligand is coordinated in a monodentate fashion through the pyridine N atom. In the extended structure, these complexes, with both hydrogen‐bond acceptor (pyrimidine) and donor (H₂O) functions, are linked to each other by intermolecular water–pyrimidine O—H...N hydrogen‐bonding interactions, resulting in a double chain along the crystallographic a axis. The trifluoromethanesulfonate anions are integrated into the chains via O—H...O hydrogen bonds between the coordinated water and sulfonate O atoms. These double chains are associated into a novel three‐dimensional network through interchain water–pyrimidine O—H...N hydrogen bonds. The asymmetric ligand plays an important role in constructing this unusual supramolecular structure.     

Density functional theory and topological analysis on the hydrogen bonding interactions in cysteine‐thymine complexes

Hydrogen bonding interactions between amino acids and nucleic acid bases constitute the most important interactions responsible for the specificity of protein binding. In this study, complexes formed by hydrogen bonding interactions between cysteine and thymine have been studied by density functional theory. The relevant geometries, energies, and IR characteristics of hydrogen bonds (H‐bonds) have been systematically investigated. The quantum theory of atoms in molecule and natural bond orbital analysis have also been applied to understand the nature of the hydrogen bonding interactions in complexes. More than 10 kinds of H‐bonds including intra‐ and intermolecular H‐bonds have been found in complexes. Most of intermolecular H‐bonds involve O (or N) atom as H‐acceptor, whereas the H‐bonds involving C or S atom usually are weaker than other ones. Both the strength of H‐bonds and the structural deformation are responsible for the stability of complexes. Because of the serious deformation, the complex involving the strongest H‐bond is not the most stable structures. Relationships between H‐bond length (ΔR_X‐H), frequency shifts (Δv), and the electron density (ρ_b) and its Laplace (?²ρ_b) at bond critical points have also been investigated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011