Investigations of the origin of stereocontrol in syndiospecific Ziegler-Natta polymerizations

In order to expand our understanding of the mechanism of stereocontrol in syndiospecific α-olefin polymerization, a family of Cs-symmetric, ansa-group 3 metallocenes was targeted as polymerization catalysts. The syntheses of new ansa-yttrocene and scandocene derivatives that employ the doubly [SiMe₂]- bridged ligand array (1,2-SiMe₂)₂{C₅H-3,5-(CHMe₂)₂} (where R = t- butyl, tBuThp; where R = i-propyl, iPrThp) are described. The structures of tBuThpY(µ-Cl)₂K(THF)₂, tBuThpSc(µ-Cl)₂K(Et₂O)₂, tBuThpYCH(SiMe₃)₂, Y₂{µ₂-(tBuThp)₂}(µ₂-H)₂, and tBuThpSc(µ-CH₃)₂ have been examined by single crystal X-ray diffraction methods. Ansa-yttrocenes and scandocenes that incorporate the singly [CPh₂]-bridged ligand array (CPh₂)(C₅H₄)(C₁₃H₈)(where C₅H₄ = Cp, cyclopentadienyl; where C₁₃H₈ = Flu, fluourenyl) have also been prepared. Select meallocene alkyl complexes are active single component catalysts for homopolymerization of propylene and 1-pentene. The scandocene tetramethylaluminate complexes generate polymers with the highes molecular weights of the series. Under all conditions examined atactic polymer microstructures are observed, suggesting a chain-end mechanism for stereocontrol.

A series of ansa-tantalocenes have been prepared as models for Ziegler-Natta polymerization catalysts. A singly bridged ansa-tantalocene trimethyl complex, Me₂Si(η⁵-C₅H₄)₂TaMe₃, has been prepared and used for the synthesis of a tantalocene ethylene-methyl complex. Addition of propylene to this ethylene-methyl adduct results in olefin exchange to give a mixture of endo and exo propylene isomers. Doubly-silylene bridged ansa-tantalocene complexes have been prepared with the tBuThp ligand; a tantalocene trimethyl complex and a tantalocene methylidene-methyl complex have been synthesized and characterized by X-ray diffraction. Thermolysis of the methylidene-methyl complex affords the corresponding ethylene-hydride complex. Addition of either propylene or styrene to this ethylene-hydride compound results in olefin exchange. In both cases, only one product isomer is observed. Studies of olefin exchange with ansa-tantalocene olefin-hydride and olefin-methyl complexes have provided information about the important steric influences for olefin coordination in Ziegler-Natta polymerization.

A tale of two cobalts

There are important problems to overcome if solar energy or other renewable energy sources are to be used effectively on a global scale. Solar photons must not only be harvested and converted into a usable form, but they must also be efficiently stored so that energy is available for use on cloudy days and at night. In this work, both the energy conversion and energy storage problems are addressed. Specifically, two cobalt complexes were designed and their reactivity probed for applications in energy conversion and storage. The first chapter describes a cobalt complex that is the first example of a dimeric cobalt compound with two singly proton-bridged cobaloxime units linked by a central BO4--bridge. Using electrochemical methods, the redox properties of the dimer were evaluated and it was found to be an electrocatalyst for proton reduction in acetonitrile.

Because hydrogen gas is difficult to handle and store, the hydrogenation of CO2 and later dehydrogenation of the liquid product, formic acid, has been proposed as a hydrogen storage system. Thus, a second complex, described in chapter two, supported by a triphosphine ligand framework was used as a catalyst precursor for this key dehydrogenation step. The studies here demonstrate the efficacy of the complex as a precatalyst for the desired reaction, with good conversion of starting formic acid to CO2 and H2. In order to better understand the properties of the triphosphine cobalt complex, a synthetic procedure for substituting electron donating groups (e.g., methoxy groups) onto the ligand was investigated, yielding a novel diphosphine cobalt(II) complex.

The Art of Madness

The nineteenth century was not an entirely kind time for the female artist. Coming of age as the 1800’s bridged into its latter half, literary artists Elizabeth Stuart Phelps Ward, Charlotte Perkins Gilman, and Kate Chopin were all well aware of their uncharitable culture. Equipped with firm feminist bents and creative visions, each of three women produced a seminal work – The Story of Avis, “The Yellow Wallpaper,” and The Awakening, respectively – taking that atmosphere to task. In these stories, each of the three women produces a female protagonist who struggles for having been born simultaneously an artist and a woman. The writers pit their women’s desires against the restrictive latitude of their time and show how such conditions drive women to madness, as a result of which they are forced to either escape into the blind mind of insanity or deal daily with their pain and inescapable societal condemnation. In an age where “hysteria” was a frequent hit in the vernacular, Phelps, Gilman and Chopin use art and literature as mediums to show that, indeed, there is a method behind the madness.

Hydrogen-Atom Transfer Photochemistry of Tetrakis(µ-pyrophosphito)diplatinate(II)

In many senses, the hydrogen-atom transfer reactions observed with the triplet excited state of pyrophosphito-bridged platinum(II) dimers resemble the reactions of organic ketone nπ* states. The first two chapters describe our attempts to understand the reactivity differences between these two chromophores. Reactivity of the metal dimers is strongly regulated by the detailed nature of the ligands that ring the axial site, the hydrogen-abstraction center. A hydrogen-bonded network linking the ligands facilitates H-atom transfer quenching with alcohols through the formation of a hydrogen-bonded complex between the alcohol and a dimer. For substrates of equal C-H bond strength that lack a hydroxyl group (e.g., benzyl hydrocarbons), the quenching rate is several orders of magnitude slower.

The shape and size of the axial site, as determined by the ligands, also discriminate among quenchers by their steric characteristics. Very small quenchers quench slowly because of high entropies of activation, while very large ones have large enthalpic barriers. The two effects find a balance with quenchers of "just the right size."

The third chapter discusses the design of a mass spectrometer that uses positron annihilation to ionize neutral molecules. The mass spectrometer creates positron-molecule adducts whose annihilation produces fragmentation products that may yield information on the bonding of positrons in such complexes.

Electron tunneling in proteins and water

The subject of this thesis is electronic coupling in donor-bridge-acceptor systems. In Chapter 2, ET properties of cyanide-bridged dinuclear ruthenium complexes were investigated. The strong interaction between the mixed-valent ruthenium centers leads to intense metal-to-metal charge transfer bands (MMCT). Hush analysis of the MMCT absorption bands yields the electronic-coupling strength between the metal centers (H_(AB)) and the total reorganization energy (λ). Comparison of ET kinetics to calculated rates shows that classical ET models fail to account for the observed kinetics and nuclear tunneling must be considered.

In Chapter 3, ET rates were measured in four ruthenium-modified highpotential iron-sulfur proteins (HiPIP), which were modified at position His50, His81, His42 and His18, respectively. ET kinetics for the His50 and His81 mutants are a factor of 300 different, while the donor-acceptor separation is nearly identical. PATHWAY calculations corroborate these measurements and highlight the importance of structural detail of the intervening protein matrix.

In Chapter 4, the distance dependence of ET through water bridges was measured. Photoinduced ET measurements in aqueous glasses at 77 K show that water is a poor medium for ET. Luminescence decay and quantum yield data were analyzed in the context of a quenching model that accounts for the exponential distance dependence of ET, the distance distribution of donors and acceptors embedded in the glass and the excluded volumes generated by the finite sizes of the donors and acceptors.

In Chapter 5, the pH-dependent excited state dynamics of ruthenium-modified amino acids were measured. The [Ru(bpy)_(3)] ^(2+) chromophore was linked to amino acids via an amide linkage. Protonation of the amide oxygen effectively quenches the excited state. In addition. time-resolved and steady-state luminescence data reveal that nonradiative rates are very sensitive to the protonation state and the structure of the amino acid moiety.

Models of the Oxygen-Evolving Complex of Photosystem II

In the five chapters that follow, I delineate my efforts over the last five years to synthesize structurally and chemically relevant models of the Oxygen Evolving Complex (OEC) of Photosystem II. The OEC is nature’s only water oxidation catalyst, in that it forms the dioxygen in our atmosphere necessary for oxygenic life. Therefore understanding its structure and function is of deep fundamental interest and could provide design elements for artificial photosynthesis and manmade water oxidation catalysts. Synthetic endeavors towards OEC mimics have been an active area of research since the mid 1970s and have mutually evolved alongside biochemical and spectroscopic studies, affording ever-refined proposals for the structure of the OEC and the mechanism of water oxidation. This research has culminated in the most recent proposal: a low symmetry Mn₄CaO₅ cluster with a distorted Mn₃CaO₄ cubane bridged to a fourth, dangling Mn. To give context for how my graduate work fits into this rich history of OEC research, Chapter 1 provides a historical timeline of proposals for OEC structure, emphasizing the role that synthetic Mn and MnCa clusters have played, and ending with our Mn₃CaO₄ heterometallic cubane complexes.

In Chapter 2, the triarylbenzene ligand framework used throughout my work is introduced, and trinuclear clusters of Mn, Co, and Ni are discussed. The ligand scaffold consistently coordinates three metals in close proximity while leaving coordination sites open for further modification through ancillary ligand binding. The ligands coordinated could be varied, with a range of carboxylates and some less coordinating anions studied. These complexes’ structures, magnetic behavior, and redox properties are discussed.

Chapter 3 explores the redox chemistry of the trimanganese system more thoroughly in the presence of a fourth Mn equivalent, finding a range of oxidation states and oxide incorporation dependent on oxidant, solvent, and Mn salt. Oxidation states from Mn^II₄ to Mn^IIIMn^IV₃ were observed, with 1-4 O^2– ligands incorporated, modeling the photoactivation of the OEC. These complexes were studied by X-ray diffraction, EPR, XAS, magnetometry, and CV.

As Ca²⁺ is a necessary component of the OEC, Chapter 4 discusses synthetic strategies for making highly structurally accurate models of the OEC containing both Mn and Ca in the Mn₃CaO₄ cubane + dangling Mn geometry. Structural and electrochemical characterization of the first Mn₃CaO₄ heterometallic cubane complex— and comparison to an all-Mn Mn₄O₄ analog—suggests a role for Ca²⁺ in the OEC. Modification of the Mn₃CaO₄ system by ligand substitution affords low symmetry Mn₃CaO₄ complexes that are the most accurate models of the OEC to date.

Finally, in Chapter 5 the reactivity of the Mn₃CaO₄ cubane complexes toward O- atom transfer is discussed. The metal M strongly affects the reactivity. The mechanisms of O-atom transfer and water incorporation from and into Mn₄O₄ and Mn₄O₃ clusters, respectively, are studied through computation and ¹⁸O-labeling studies. The μ3-oxos of the Mn₄O₄ system prove fluxional, lending support for proposals of O^2– fluxionality within the OEC.

Nonclassical excitation and quantum interference in a three level atom

Non-classical properties and quantum interference (QI) in two-photon excitation of a three level atom (|1〉), |2〉, |3〉) in a ladder configuration, illuminated by multiple fields in non-classical (squeezed) and/or classical (coherent) states, is studied. Fundamentally new effects associated with quantum correlations in the squeezed fields and QI due to multiple excitation pathways have been observed. Theoretical studies and extrapolations of these findings have revealed possible applications which are far beyond any current capabilities, including ultrafast nonlinear mixing, ultrafast homodyne detection and frequency metrology. The atom used throughout the experiments was Cesium, which was magneto-optically trapped in a vapor cell to produce a Doppler-free sample. For the first part of the work the |1〉 → |2〉 → |3〉 transition (corresponding to the 6S_1/2F = 4 → 6P_3/2F' = 5 → 6D_5/2F" = 6 transition) was excited by using the quantum-correlated signal (Ɛ_s) and idler (Ɛ_i) output fields of a subthreshold non-degenerate optical parametric oscillator, which was tuned so that the signal and idler fields were resonant with the |1〉 → |2〉 and |2〉 → |3〉 transitions, respectively. In contrast to excitation with classical fields for which the excitation rate as a function of intensity has always an exponent greater than or equal to two, excitation with squeezed-fields has been theoretically predicted to have an exponent that approaches unity for small enough intensities. This was verified experimentally by probing the exponent down to a slope of 1.3, demonstrating for the first time a purely non-classical effect associated with the interaction of squeezed fields and atoms. In the second part excitation of the two-photon transition by three phase coherent fields Ɛ₁ , Ɛ₂ and Ɛ₀, resonant with the dipole |1〉 → |2〉 and |2〉 → |3〉 and quadrupole |1〉 → |3〉 transitions, respectively, is studied. QI in the excited state population is observed due to two alternative excitation pathways. This is equivalent to nonlinear mixing of the three excitation fields by the atom. Realizing that in the experiment the three fields are spaced in frequency over a range of 25 THz, and extending this scheme to other energy triplets and atoms, leads to the discovery that ranges up to 100's of THz can be bridged in a single mixing step. Motivated by these results, a master equation model has been developed for the system and its properties have been extensively studied.

Norbornenyl- and nortricyclyl free radicals

The number, symmetry, and product-forming capabilities of the intermediates in the photoinitiated reductions of endo- and exo-5- bromonorbornene and 2-bromonortricyclene with tri-n-butyltin hydride at temperatures between -10° and 22° were investigated.

Three mechanisms were evaluated:

1. The 5-norbornenyl- and 2-nortricyclyl radicals isomerize reversibly with the former producing nortricyclene by abstraction of hydrogen from tri-n-butyltin hydride.

2. The 5-norbornenyl- and 2-nortricyclyl radicals isomerize reversibly, but some norbornene can be formed from the 2-nortricyclyl radical or some nortricyclene can be formed from the 5-norbornenyl radical by abstraction of hydrogen.

3. There is intervention of a "bridged" radical which may be for med reversibly or irreversibly from the 5-norbornenyl- and 2-nortricyclyl radicals.

Within small error limits, the ratios of norbornene to nortricyclene as a function of the concentration of tri-n-butyltin hydride are consistent with the first mechanism.

In the reductions with tri-n-butyltin deuteride, primary deuterium isotope effects of 2. 3 and 2. 1 for the abstraction of deuterium by the 2-nortricyclyl- and 5-norbornenyl radicals, respectively, were found. The primary deuterium isotope effects were invariant with the concentration of tri-n-butyltin deuteride, although the ratios of norbornene to nortricyclene changed appreciably over this range. This is consistent with the first mechanism, and can accommodate the formation of either product from more than one intermediate only if the primary kinetic deuterium isotope effects are nearly equal for all reactions leading to the single product.

The reduction of endo-5-bromonorbornene-5, 6, 6-d₃ with tri-n-butyltin hydride or tri-n-butyltin deuteride leads to both unrearranged and rearranged norbornenes. The ratios of unrearranged to rearranged norbornene require that the 5-norbornenyl-5, 6, 6-d₃ radical isomerize to an intermediate with the symmetry expected of a nortricyclyl free radical. The results are consistent with mechanism 1, but imply a surprising normal secondary kinetic deuterium isotope effect of about 1.25 for the abstraction of hydrogen by the 5-norbornenyl- 5, 6, 6-d₃ radical.

Approximate calculations show that there does not appear to be any substantial difference in the stabilities of the 5-norbornenyl and 2-nortricyclyl radicals.

Although the results can not exclude a small contribution by a mechanism other than mechanism 1, no such contribution is required to adequately explain the results.

TeO2对氟铝酸盐玻璃性质和结构的影响

以10MgF2—20CaF2—10SrF2—10BaF2—15YF3—35AlF3氟铝酸盐玻璃为基玻璃引入不同含量的TeO2得到了新的氟碲铝酸盐玻璃．用差热分析方法研究了TeO2对氟铝酸盐玻璃性能的影响，通过拉曼光谱和红外吸收谱来研究玻璃的结构变化．差热分析表明TeO2的增加使玻璃开始析晶温度瓦升高，融化温度％降低，成玻璃能力增加．玻璃结构分析表明氟碲铝酸盐玻璃的结构中存在[FnAl-O—AlFn]、[TeO3]、[TeO2F]和[TeOF2]等多面体，这些多面体由F^-和O^2-离子连接．这种新的氟碲铝

Trypsin inhibitory loop is an excellent lead structure to design serine protease inhibitors and antimicrobial peptides

The disulfide-bridged hendecapeptide ( CWTKSIPPKPC) loop, derived from an amphibian skin peptide, is found to have strong trypsin inhibitory capability. This loop, called the trypsin inhibitory loop ( TIL), appears to be the smallest serine protease inhib

Magnetization process of a single chain of nanomagnets

We have investigated the magnetization reversal process of a single chain of identical nanomagnetic dots fabricated from 30 nm thick Ni 80Fe20. The structures consist of two 5 μm wide support wires bridged with a single chain of identical dots of diameter δ in the range 100-250 nm. For fields applied perpendicular to the single chain, we observed an unusual size dependent hysteretic behavior in the magnetoresistance curve at high field. This is due to the magnetostatic interaction arising from the proximity of the magnetic charges. We are able to deduce from a simple micromagnetic simulation that the reversal process in the chain of dots with δ=100nm is mediated by a collective process of nearly coherent spin rotation. The magnetotransport measurements along the chain reveal a complex magnetization reversal process in the chain of nanomagnets. © 2002 American Institute of Physics.

A novel antimicrobial peptide from amphibian skin secretions of Odorrana grahami

A novel antimicrobial peptide named odorranain-NR was identified from skin secretions of the diskless odorous frog, Odorrana grahami. It is composed of 23 amino acids with an amino acid sequence of GLLSGILGAGKHIVCGLTGCAKA. Odorranain-NR was classified into a novel family of antimicrobial peptide although it shared similarity with amphibian antimicrobial peptide family of nigrocin. Odorranain-NR has an unusual intramolecular disulfide-bridged hexapeptide segment that is different from the intramolecular disulfide-bridged heptapeptide segment at the C-terminal end of nigrocins. Furthermore, the -AKA fragment at the C-terminal of odorranain-NR is also different from nigrocins. Three different cDNAs encoding two odorranain-NR precursors and only one mature odorranain-NR was cloned from the cDNA library of the skin of O. grahami. This peptide showed antimicrobial activities against tested microorganisms except Escherichia coli (ATCC25922). Its antimicrobial mechanisms were investigated by transmission electron microcopy. odorranain-NR exerted its antimicrobial functions by various means depending on different microorganisms. (C) 2008 Elsevier Inc. All rights reserved.

Highly Functionalized Daphnane Diterpenoids from Trigonostemon thyrsoideum

Trigonothyrins A-C (1-3), which are highly functionalized daphnane diterpenoids, were isolated from the stems of Trigonostemon thyrsoideum. Compounds 1-3 represent the first examples of daphnanes with an oxygen-bridged four-membered-ring system, and a linkage mode of 12,13,14-orthoester. Compound 3 was observed to inhibit HIV-1 induced cytopathic effects. The EC50 value was 2.19 mu g/mL, and the therapeutic index (TI) was more than 90.

Control of material transport through pulse shape manipulation-a development toward designer pulses

The variety of laser systems available to industrial laser users is growing and the choice of the correct laser for a material target application is often based on an empirical assessment. Industrial master oscillator power amplifier systems with tuneable temporal pulse shapes have now entered the market, providing enormous pulse parameter flexibility in an already crowded parameter space. In this paper, an approach is developed to design interaction parameters based on observations of material responses. Energy and material transport mechanisms are studied using pulsed digital holography, post process analysis techniques and finite-difference modelling to understand the key response mechanisms for a variety of temporal pulse envelopes incident on a silicon (1/1/1) substrate. The temporal envelope is shown to be the primary control parameter of the source term that determines the subsequent material response and the resulting surface morphology. A double peak energy-bridged temporal pulse shape designed through direct application of holographic imaging data is shown to substantially improve surface quality. © 2014 IEEE.

Non-destructive optical characterization of phase separation in bulk heterojunction organic photovoltaic cells

The dithiophene donor-acceptor copolymers that are bridged either with carbon (C-PCPDTBT) or silicon atoms (Si-PCPDTBT) belong to a promising family of materials for use in photoactive layers for organic photovoltaic cells (OPVs). In this work, we implement the non-destructive Spectroscopic Ellipsometry technique in the near infrared to the far ultraviolet spectral region in combination with advanced theoretical modeling to investigate the vertical distribution of the C-PCPDTBT and Si-PCPDTBT polymer and fullerene ([6,6]-phenyl C71-butyric acid methyl ester - PC70BM) phases in the blend, as well as the effect of the polymer-to-fullerene ratio on the distribution mechanism. It was found that the C-PCPDTBT:PC70BM blends have donor-enriched top regions and acceptor-enriched bottom regions, whereas the donor and acceptor phases are more homogeneously intermixed in the Si-PCPDTBT:PC70BM blends. We suggest that the chemical incompatibility of the two phases as expressed by the difference in their surface energy, may be a key element in promoting the segregation of the lower surface phase to the top region of the photoactive layer. We found that the increase of the photoactive layer thickness reduces the polymer enrichment at the cathode, producing a more homogeneous phase distribution of donor and acceptor in the bulk that leads to the increase of the OPV efficiency. © 2014 Elsevier B.V.