Polymerization of fluorinated diacetylenes

Perfluoroalkylene diacetylenes, HC?C? (CF₂)_n? C?CH, underwent thermal polymerization at 250–350°C to give glassy polymers stable to 450°C. Partial polymerization of the volatile monomers gave oligomers that are processable at atmospheric pressure. Polymers with similar thermal stability were obtained by transition-metal-catalyzed polymerization of the monomers at moderate temperatures.     

Solid phase peptide templated glycosidic bond formation

Glycosylation reactions performed between a glycosyl donor and acceptor covalently linked to a peptide template both in the solution and solid phase give similar yields and product distributions. The adoption of a solid phase approach opens the way for the synthesis of libraries of peptide templates in an attempt to screen for particular peptide sequences that effect complete regio-and stereochemical control during glycosidic bond formation, whilst the use of second generation donors allows the possibility of an iterative approach.     

Polymerization of diacetylenes in multilayers

A series of amphiphilic diacetylene monocarbonic acids was synthesized, and their ability to form monolayers at the air–water interphase was investigated. Acids with total number of C atoms ≥20 and mp >45°C form surface states suitable to be used for buildup of multilayers by the Langmuir–Blodgett technique. Using the LB technique, multilayers of defined thickness were built up on quartz substrates. The multilayers were polymerized by exposure to a UV light source according to the mechanism of solid-state polymerization of diacetylenes without destruction of the layer structure and with retention of the packing in the individual layers. Thus well-defined polymer multilayers were obtained. The monomer and polymer multilayers were characterized by UV spectra, x-ray small-angle diffraction, and interference microscopic and electron microscopic techniques. The polyconjugated backbones of the polymer chains are all stretched out in the plane of the substrate but not over macroscopic dimensions because the multilayers have a crystalline domain structure. The polymer multilayer films are quite stable under ambient conditions.     

Polymerization of mono- and multilayer forming diacetylenes

Summary Heptadeca-2,4-diine-1-ol and Heptadeca-2,4-diine-1-acid were synthesized and their polymerization behaviour in the solid state was investigated. Both compounds polymerize thermally below the melting point and photochemically under the action of uv-light to form polymers via 1,4-addition to the conjugated triple bonds with a backbone of conjugated double-and triple-bonds. The photoresponse spectra resemble the absorption spectra exhibiting maxima of photoreactivity in the absorption band of the conjugated triple bonds of the monomers.Both compounds satisfy the general criteria of formation of monomeric monolayer films at the air-water-interface of a Langmuir-trough. The monolayers could be transfered onto quartz or metal substrates by the Langmuir-Blodgett-technique and multilayers of the monomers could be built up. These multilayers were polymerized by exposure to high energy-radiation and thus, multilayers of the crystalline, well ordered polymer were obtained. The polymer multilayers were studied by electron diffraction and ir-spectroscopy. The polymer has the same structure as the one obtained by solid-state polymerization of the crystals. A model of the structure of the monomeric and polymeric multilayers is proposed, which is based on the combined evidences from the known chemical structure, the diffraction and spectroscopic data and the formation of the layered structure.

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Zusammenfassung Heptadeca-2,4-diin-1-ol und Heptadeca-2,4-diin-1-säure wurden dargestellt und das Polymerisationsverhalten im festen Zustand untersucht. Beide Verbindungen polymerisieren thermisch und photochemisch bei Bestrahlung mit UV-Licht und bilden dabei über eine 1,4-Addition an die konjugierten Dreifachbindungen Polymere mit einer Hauptkette aus konjugierten Doppel- und Dreifachbindungen. Die Photoantwortspektren ähneln den Absorptionsspektren und Maxima der Photoreaktivität liegen in dem Bereich der Absorption der konjugierten Dreifachbindungen der Monomeren. Beide Verbindungen lassen sich an der Grenzfläche Gas-Wasser eines Langmuir-Troges spreiten. Die Monoschichten konnten nach der Langmuir-Blodgett-Technik auf Quarz- oder Metallunterlagen überführt und auf these Weise Multischichten der Monomeren erzeugt wurden. Diese Multischichten wurden durch Bestrahlung mit energiereicher Strahlung polymerisiert und dadurch Multischichten des kristallinen, geordneten Polymeren erhalten. Die Multischichten des Polymeren wurden mit Elektronenbeugung und IR-Spektroskopie untersucht. Die Polymeren hatten die gleiche chemische Struktur wie die aus den Kristallen der Monomeren erhaltenen. Auf der Grundlage der bekannten chemischen Struktur der Polymeren, der Beugungs- und spektroskopischen Daten und aus der Tatsache, daß sich Monoschichten erhalten lassen, wurde ein Modell für die Struktur der Multischichten der Monomeren und der Polymeren entwickelt.

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With 11 figures and 1 table     

Polymerization and characterization of polyfunctional amphiphilic diacetylenes

Amphiphilic diacetylenes of the type HC?C? (CH₂)_n? C?C? C?C? (CH₂)₈? COOH (n=6 and 12) were synthesized. The solid-state polymerization of the bulk crystals, mono-layers on the gas-water interface, and multilayers were investigated. The diacetylene moieties underwent polymerization upon irradiation by UV or γ-rays, showing different polymerizability depending on the material form. The polymerization of the terminal acetylene groups was not observed. Enhanced polymerizability and stability of the monolayers on subphases containing LiOH are explained by a proposed model of the monolayers. © 1995 John Wiley & Sons, Inc.     

A molecular orbital explanation of bond distance variation caused by hydrogen bond formation

For hydrogen bond systems X–D–HA–Y, a simple molecular orbital model is proposed to understand the mechanism of the bond distance variations caused by the hydrogen bond formation. This model explains the bond distance variations for X–D and A–Y as follows. Electrostatic potential that the electrons in a molecule receive from other molecules causes the changes in atomic orbital energy differences between the bonded atoms. Then, the changes in the orbital energy differences make the bond orders larger or smaller and consequently the bond distances vary. The validity of this model has been confirmed by the effective fragment potential method, using the test systems of (HCOOH)₂, HCONH₂ (formamide) crystal and BF₃·2H₂O crystal.     

Solvent effects on hydrogen bond formation

Enthalpies of solution have been used to calculate transfer enthalpies for phenol, pyridine, and DMSO between the solvent cyclohexane and the solvents CCl₄, benzene, and CHCl₃. By use of model compounds, enthalpies due to interactions with phenol, pyridine, and DMSO have been determined. These enthalpies are used to calculate the effect of solvation relative to cyclohexane on hydrogen bonded complexes in CCl₄ and benzene solvents. Correlations with enthalpies due to interactions and frequency shifts for the hydroxyl stretch in these solvents have also been made.     

C-H Group as proton donor by formation of a weak hydrogen bond

A systematic CNDO/2 study has been carried out on the weak C-H...F, C-H...O, and C-H...N hydrogen bonds. An H-bond formation has been revealed for proton donors with negative excess charge on the hydrogen. The concept of the positive character of the hydrogen as well as some essential features of the hydrogen bond have been discussed. Some useful correlations have been found and the importance of the charge shifts has been emphasized.     

A spectroscopic study of hydrogen bond formation by silatranes and their analogues

The IR absorption shifts of OH and OD stretching vibrations upon interaction of silatranes XSi(OCH(CH₃)CH₂)₃N and their monocyclic analogues of the type R₂(OCHR′CH₂)₂NR$?with phenol and deuteromethanol, respectively, were measured. In the systems involving silatranes these values are higher than in systems with the corresponding ethoxysilanes. The equilibrium constants and thermodynamic parameters of the interaction of the compounds studied with phenol in n-heptane were measured by electronic spectroscopy. The interaction of phenol with the compounds studied shows two lines correlating with the thermodynamic parameters ΔH - f(ΔS). One of the lines is plotted by alkoxysilanes, cyclic and acyclic ethers. The second line corresponds to the compounds having a O-C-C-N group. This enables a conclusion to be drawn that in a non-polar medium the basic centre of silatranes and their monocyclic analogues is different from the oxygen in ethers and alkoxysilanes.     

Activation of electron transfer reduction of p-benzoquinone derivatives by intermolecular regioselective hydrogen bond formation

Electron transfer reduction of p-benzoquinones by cobalt tetraphenylporphyrin is enhanced significantly by the presence of o-bis(phenylcarbamoylmethyl)benzene (o-L) due to the regioselective hydrogen bond formation between the corresponding semiquinone radical anions and o-L, whereas m- and p-isomers (m-L and p-L) have no effect on the electron transfer equilibrium or the rate.     

Immobilization of polymer-protected metal colloid catalysts by the formation of polymer hydrogen bond complexes

poly(N-vinyl-2-pyrrolidone)-(PVP) and polyvinyl alcohol-(PVA) protected nanoscopic noble metal colloidal catalysts were immobilized to prepare the corresponding heterogeneous catalysts by forming polymer hydrogen bond complexes of polyacrylic acid (PAA) with PVP or PVA. A PVP–PAA–Pd catalyst was found to be very active and selective for the partial hydrogenation of cyclopentadiene to cyclopentene.     

Interplay between hydrogen bond formation and multicenter pi-electron delocalization: intermolecular hydrogen bonds

The interplay between aromatic electron delocalization and intermolecular hydrogen bonding is thoroughly investigated using multicenter delocalization analysis. The effect on the hydrogen bond strength of aromatic electron delocalization within the acceptor and donor molecules is determined by means of the interaction energies between monomers, calculated at the B3LYP/6-311++G(d,p) level of theory. This magnitude is compared to variations of multicenter electron delocalization indices and covalent hydrogen bond indices, which are shown to correlate perfectly with the relative values of the interaction energies for the different complexes studied. The multicenter electron delocalization indices and covalent bond indices have been computed using the quantum theory of atoms in molecules approach. All the hydrogen bonds are formed with oxygen as the acceptor atom; however, the atom bonded to the donor hydrogen has been either oxygen or nitrogen. The water-water complex is taken as reference, where the donor and acceptor molecular environments are modified by substituting the hydrogens and the hydroxyl group by phenol, furan, and pyrrole aromatic rings. The results here shown match perfectly with the qualitative expectations derived from the resonance model.     

Transfer of electron density as a result of hydrogen bond formation

It has been found that the amount of charge transfer between donor and acceptor molecules in four sets of hydrogen‐bonded complexes may be adequately described as an exponential function of the equilibrium distance between the hydrogen atom and the nearest atom of the acceptor molecule. The exponential factors of the transfer are of the same order but somewhat larger than the factors found otherwise in the investigations of dynamic electron transfer. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Sensitivity of monomer nuclear quadrupole coupling constants to hydrogen bond formation

Ab initio calculations on several representative complexes have been carried out to show the sensitivity of quadrupole coupling constants to hydrogen bond formation, and particularly the sensitivity to orientation in the complex. Spectroscopic structural determinations based on nuclear quadrupole coupling constants are examined for five representative systems. It appears that nuclear quadrupole coupling constants may give orientational angles accurately, but inaccuracy may amount to ≈ 15° in certain instances.     

Activation of electron-transfer reduction of oxygen by hydrogen bond formation of superoxide anion with ammonium ion

A hydrogen bond formed between the superoxide anion and the ammonium ion (NH4+) accelerates electron transfer from the C60 radical anion to oxygen significantly, whereas the tetra-n-butylammonium ion has no ability to form a hydrogen bond with the superoxidie anion, exhibiting no acceleration of the electron-transfer reduction of oxygen. The second-order rate constant of electron transfer from C60*- to O2 increases linearly with increasing concentration of NH4+. This indicates that O2*- produced in the electron transfer from C60 to O2 is stabilized by 1:1 complex formation between O2*- and NH4+. The 1:1 complex formed between O2*- and NH4+ was detected by ESR. The binding of O2*- with NH4+ results in a positive shift of the reduction potential of O2 with increasing concentration of NH4+, leading to the acceleration of electron transfer from C60*- to O2.     

The hydrogen bond

The concept of the hydrogen bond is a century old but remains youthful because of its vital role in so many branches of science and because of continued advances in experiment, theory and simulation. We discuss the structural and energetic characteristics of normal hydrogen bonds X–H···Y as well as some exceptions to the normal, including proton-shared and ion-pair bonds. We consider the harmonic and anharmonic vibration of X–H in a variety of complexes, and demonstrate that there is no fundamental difference between blue-shifting and red-shifting bonds. We discuss water clusters and liquid water and indicate possible directions of future progress.     

Protein nanopatterns by oxime bond formation

Patterning proteins on the nanoscale is important for applications in biology and medicine. As feature sizes are reduced, it is critical that immobilization strategies provide site-specific attachment of the biomolecules. In this study, oxime chemistry was exploited to conjugate proteins onto nanometer-sized features. Poly(Boc-aminooxy tetra(ethylene glycol) methacrylate) was synthesized by free radical polymerization. The polymer was patterned onto silicon wafers using an electron beam writer. Trifluoroacetic acid removal of the Boc groups provided the desired aminooxy functionality. In this manner, patterns of concentric squares and contiguous bowtie shapes were fabricated with 150-170-nm wide features. Ubiquitin modified at the N-terminus with an α-ketoamide group and N(ε)-levulinyl lysine-modified bovine serum albumin were subsequently conjugated to the polymer nanopatterns. Protein immobilization was confirmed by fluorescence microscopy. Control studies on protected surfaces and using proteins presaturated with O-methoxyamine indicated that attachment occurred via oxime bond formation.     

Borrowing hydrogen: a catalytic route to C-C bond formation from alcohols

Ruthenium complexes have been shown to perform efficient transfer hydrogenation reactions between alcohols and alkenes; in combination with an in situ Wittig reaction, indirect formation of C-C bonds has been achieved from alcohols.     

Practical,high-yield synthesis of thiol-terminated diacetylenes for formation of conductive monolayers

Self-assembled monolayers of thiol terminated conjugated diacetylenes can be cross-linked using ultraviolet light to form highly conjugated polydiacetylenic conductive monolayers [1]; however, the reported syntheses of the diacetylene monomers present numerous problems that prevent the wide spread application of these in functional materials. We report a redesigned four-step synthesis that proceeds in 75–80% overall yields and allows gram scale production of an array of thiol terminated conjugated diacetylenes, thereby allowing examination and application of these low-dimensional conductive materials.