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Caltech Authors (133.905 recursos)

Repository of works by Caltech published authors.

Type = Conference or Workshop Item

Mostrando recursos 1 - 20 de 673

  1. Lederman's Shoulder, Weinberg's Nose, and Other Lessons from the Past

    Politzer, H. David
    [No abstract]

  2. Crystallization-driven ordering and self-assembly in bottlebrush polymers

    Kim, Joey; Hadjichristidis, Nikos; Kornfield, Julia
    The natural building blocks of linear polymer are monomers. Similarly, the natural building blocks of bottblebrush polymers are linear polymers (commonly referred to as polymacromonomers). It is well established that many properties of linear polymers can be tuned by using different copolymer systems. Similarly, we expect tunability of bottlebrush polymers by using chem. distinct polymer chains. A collaborative effort with Hadjichristidis from KAUST led us to a deeper study on the arrangement of chem. distinct polymeric units within the bottlebrush architecture. We have organized these samples into four categories based on the geometric connectivity: homo-bottlebrush polymer (H) in which all side chains have identical chem. functionality,...

  3. Why isn't noble gas chemistry 30 years older? The failed (?) 1933 experiment of Yost and Kaye

    Labinger, Jay
    The first report of a noble gas compd., by Neil Bartlett in 1962, was quickly followed by a no. of others. Of course there ha been previous attempts, of which the best documented is probably that of Caltech inorg. chem. professor Donald Yost and student Albert Kaye; details of their unsuccessful expts. aimed at getting xenon to react with fluorine or chlorine were publ in JACS in 1933. After Bartlett"s success, a no. of commentators looked back to Yost"s paper and offered a variety of poss reasons for Yost"s failure. Examn. of the actual details of Yost's paper, in comparison with those of the...

  4. Tuning reactivity of biologically inspired clusters via metal and bridging anion composition

    Agapie, Theodor
    Complex inorg. cofactors perform challenging catalytic transformations in biol. systems, such as photosystem II and nitrogenase. The effect of cluster structure on the phys. and chem. properties of these active sites is not well understood. We have developed methodologies for the rational synthesis of tetranuclear homo- and hetero-metallic clusters. The effect of metal compn. on redn. potentials has been correlated to Lewis acidity for redox inactive metals. Clusters with open coordination site have been prepd. for small mol. activation. CH and CF oxygenation has been obsd. for putative terminal Fe-oxo species supported by tetranuclear clusters. Tetranuclear clusters with bridging fluoride and oxide interstitial ligands have been...

  5. Z-selective cross metathesis with 3(E)-1,3-dienes

    Cannon, Jeff; Luo, Shaoxiong; Engle, Keary; Taylor, Buck; Houk, Kendall; Grubbs, Robert
    A method for olefin metathesis between 3(E)-1,3-dienes and terminal olefins is described. The method employs a recently reported ruthenium-based catalyst contg. a cyclometalated NHC ligand and takes advantage of its pronounced selectivity for terminal olefins in the presence of internal E-olefins. A wide variety of functional groups can be tolerated. Applications of the reaction include direct synthesis of insect pheromones from commerical precursor.

  6. Necessity is the mother of invention: Natural products and the chemistry they inspire

    Reisman, Sarah
    The chem. synthesis of natural products provides an exciting platform from which to conduct fundamental research in chem. and biol. We are currently focused on devising concise approaches to complex polycyclic natural products such as ryanodol, acutumine, and members of the diterpenoid alkaloids. The densely packed arrays of heteroatoms and stereogenic centers that constitute these polycyclic targets challenge the limits of current technol. and inspire the development of new synthetic strategies and tactics. This seminar will describe our latest progress in both our methodol. and target-directed synthesis endeavors.

  7. Chemical origin of the colors of objects in the outer solar system and the Kuiper belt connection

    Brown, Michael
    The common small objects in the Kuiper belt come in two colors: red and very red. Such a strong bifurcation in what must be chemical properties is extremely hard to explain through standard ideas of outer solar system mixing, accretion and formation. We have proposed that, instead, all objects start with essentially identical chemical composition, but, after formation has ended and the nebula has dispersed, icy outer solar system objects all go through a cometary phase where surface volatiles are removed. Volatile removal is an exponential function of surface temperature, thus volatile evaporation lines will be imposed across the outer...

  8. Enantioselective Ni-catalyzed reductive cross-coupling reactions

    Reisman, Sarah
    Nickel-catalyzed reductive cross-coupling reactions have emerged as direct bond formation. These reactions typically tolerate an array of functional groups, and employ inexpensive, earth abundant metals as stoichiometric reductants. May require the use of pre-generated organometallic reagents. In order to realize the catalyzed reductive cross-coupling reactions, it is crit. to develop enantioselective transformations. This seminar will describe our recent successes in the develop enantioselective cross-coupling reactions.

  9. Genetically encoded nanostructures for non-invasive imaging of biological systems

    Shapiro, Mikhail
    Many important biological processes – ranging from simple metabolism to complex cognition – take place deep inside living organisms, yet our ability to study them in this context is limited. Technologies such as fluorescent and luminescent proteins enable exquisitely precise imaging of genetically specific cellular function in small and translucent specimens, but are limited by the poor penetration of light into larger tissues. In contrast, most non-invasive technologies such as magnetic resonance imaging (MRI) and ultrasound – while based on energy forms that penetrate tissue effectively – lack the needed molecular precision. Our work attempts to bridge this gap by...

  10. Ring-expansion metathesis polymerization of cycloolefins to highly pure cyclic polymers

    Edwards, Julian; Zhang, Hefeng; Hadjichristidis, Nikos; Pasch, Harald; Vlassopoulos, Dimitris; Grubbs, Robert
    The means to produce cyclic polymers in high purity and at large scale remains an elusive goal. Although ring-closure methods have been successful in producing rings free of linear chains, scalability is an unavoidable obstacle. We report a ring-expansion method for synthesizing cyclic polyalkenamers in high purity and at scale ( > 25 g / reaction) using a recyclable heterogeneous ruthenium-based olefin metathesis catalyst. This ring-expansion metathesis polymn. (REMP) catalyst is an analog of the 2nd Generation Hoveyda-Grubbs catalyst and is covalently bound to a silica surface via dual tethers from the N-heterocyclic carbene (NHC) and benzylidene. Cyclic polyalkenamers were synthesized from a variety of cycloolefins,...

  11. Aerosol-cloud-climate interactions from a modeling perspective

    Wang, Yuan
    Atm. aerosols interact directly and indirectly with the Earth"s radiation budget a effect, aerosols scatter and absorb solar radiation. Also, aerosols indirectly impac development, lifetime, pptn., and albedo. Current understanding of the aerosol uncertain, constituting the greatest uncertainty in climate predictions. In this pr to quantify the climate impacts of aerosol-cloud-radiation interactions using will be reviewed.

  12. Proximal effects in bimetallic catalysts for olefin polymerization, in cross metathes and in multiblock polymers

    Radlauer, Madalyn; Agapie, Theodor; Hillmyer, Marc
    Proximity and sterics play a large role in polymn. and polymer chem. In the bimetallic polymn. catalysts where the two metal centers are held on the sam ligand framework, the proximity of the second metal changes reactivity a enhancements of polar group tolerance or isoselecitivity in olefin polymn. are no systems with distal metal centers or in monometallic analogs. In cross metathes chains, high E-stereoregularity and head-to-tail regioregularity are achieved w generation catalysts when there is allylic substitution directing reactivity through polymers, the proximity of incompatible blocks due to a covalent connection system. This frustration can produce interesting and complicated morphologies where thermodn. penalties are expected to minimize the...

  13. Self-assembly of brush block copolymers to nanostructured materials

    Chang, Alice; Bates, Christopher; Matsen, Mark; Grubbs, Robert
    Brush block copolymers possess unique structural features and display extensiv work, we harness these properties of the brush polymer architecture nanostructured materials. We have explored how the properties of brush self-assembly, and in turn, how self-assembly enables mesoscale structural c properties, including reflectivity, cond., and mech. toughness. Herein we electrolytes based on ABA brush triblock copolymers and (2) a new equil. mo triblock terpolymers. We have demonstrated routes to control the structu materials, as well as guide future materials design.

  14. Genetic programming of molecular, cellular and materials assembly

    Tirrell, David
    The expression of artificial genes allows programming of mol. structure and intermol. interactions. This lecture will explore the use of programmed intermol. interactions to control the assembly of macromol. networks and microbial consortia.

  15. Rhodium-conjugate fluorescent probes for diagnostic of mismatched DNA

    Nano, Adela; Barton, Jacqueline
    In healthy cells, DNA base pair mismatches, as well as some other base pair anomalies derived from DNA base damage, are cor. through the mismatch repair (MMR) machinery. If left unrepaired, mismatches cause mutations, some of which promote cancerous transformations. Currently, there is a lack of effective mol. sensors for early cancer diagnosis. Therefore, developing mol. probes capable of detecting and signaling DNA base mismatches directly would be a powerful tool to this end. Previous studies have shown that rhodium metalloinsertors bind mismatches in double-strained DNA with high specificity. Nevertheless, these rhodium complexes show poor luminescence properties. Here we describe the design and study of rhodium...

  16. Overcoming challenges in selective Wacker-type oxidation reactions

    Chu, Crystal; Ziegler, Daniel; Carr, Brian; Wickens, Zachary; Grubbs, Robert
    The Wacker oxidn. has proven to be a powerful reaction for the oxidn. of simple olefins to Me ketones. However, important limitations restrict its application in the lab. setting. Typically, oxidns. of simple terminal olefins occur with Markovnikov selectivity, while achieving high aldehyde selectivity has historically been more challenging. Addnl., internal olefins are generally unreactive under Wacker oxidn. conditions. Our group has recently devoted research efforts toward addressing these limitations. Specifically, during studies of a dicationic palladium catalyst capable of oxidizing internal olefins, it was obsd. that electron-withdrawing trifluoromethyl groups are capable of significantly enhancing selectivity for distal, rather than proximal, oxidn.; even modest aldehyde selectivity...

  17. Effects of alkyl group and NHC ligand variation with ruthenium-based olefin metathesis catalysts bearing chelating ortho-alkoxy benzylidenes

    Luo, Shaoxiong; Engle, Keary; Liu, Peng; Dong, Xiaofei; Taylor, Buck; Takase, Michael; Houk, Kendall; Grubbs, Robert
    A series of second-generation ruthenium olefin metathesis catalysts was investigated using a combination of reaction kinetics, X-ray crystallog., NMR spectroscopy, and DFT calcns. in order to det. the relationship between the structure of the chelating ortho-alkoxybenzylidene and the obsd. initiation rate. Included in this series were twenty new catalysts contg. an array of ortho-alkoxy benzylidenes and NHC ligands. The initiation rates of this series of catalysts were detd. using a UV/Vis assay. All new catalysts were obsd. to be faster-initiating than the corresponding isopropoxy control, and the dicyclohexylmethyl catalyst was found to be among the fastest-initiating Hoveyda-type catalysts reported to date. Anal. of the X-ray structures...

  18. Reactivity of metal complexes supported by ligands with functionalized pendant ar

    Agapie, Theodor
    Multidentate phosphines connected by terphenyl linkers have been employ multimetallic complexes. The central arene of the terphenyl unit acts as a labile variety of modes as a function of metal and oxidn. state. Addnl., this arene can to redox and H-transfer chem. Mo complexes with P-arene-P pincers show reductive coupling of two CO mols. demonstrating the ability of such ligands to s activation chem. Variants with catechol and hydroquinone moieties perform transfer chem. Mechanistic studies of reactions with O_2 show that second-sphere O_2 activation steps. The coordination chem. of these systems and reactivit discussed.

  19. Consequences of low-χ block copolymer design

    Chang, Alice; Bates, Christopher; Matsen, Mark; Grubbs, Robert
    We report the discovery of a new equil. triblock terpolymer morphol. Enabled design, we developed a strategy to decouple the domain sequence from the synth all previously reported periodic materials accessed by block copolymer self blocks dets. the phase and domain connectivity of the material. For an ABC model demands periodic domains with ABCB repeat units. We explored low-χ bl series of ABC triblock terpolymers having grafted polylactide (PLA), polystyrene (P (PEO) side chains. Partial mixing of the PLA and PEO end blocks results in morphol. (LAMP) with ACBC connectivity. Consequences of partial mixing decrease in domain spacing with increasing mol. wt. (and increasing PEO characterized by small-angle X-ray...

  20. High Order Super Nested Arrays

    Liu, Chun-Lin; Vaidyanathan, P. P.
    Mutual coupling between sensors has a negative impact on the estimation of directions of arrival (DOAs). Sparse arrays such as nested arrays, coprime arrays, and minimum redundancy arrays (MRAs) have less mutual coupling than uniform linear arrays (ULAs). These sparse arrays also have a difference coarray of size O(N^2), where N is the number of sensors, and can therefore resolve O(N^2) uncorrelated source directions. The various sparse arrays proposed in the literature have their pros and cons. The nested array is practical and easy to use but has a dense ULA part which suffers from mutual coupling effects like the...

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