Recursos de colección

Caltech Authors (171.365 recursos)

Repository of works by Caltech published authors.

Type = Article

Mostrando recursos 1 - 20 de 22.524

  1. Orbital phase continuity principle and selection rules for concerted reactions

    Goddard, William A., III
    In recent years the GI method(1) for electronic wavefunctions has been used to improve upon Hartree- Fock and yet retain the orbital interpretation. In the GI method we allow every orbital to be different (no double occupation restriction) and yet retain the correct spin symmetry.(2) The result is one electron per orbital and two orbitals per bond, with the orbitals generally localized near one or two nuclei (for nonaromatic systems). These orbitals are solved for self-consistentcy; however, we find that for many molecules the orbitals tend to be qualitatively similar to the familiar valence-bond functions. For example, the π orbitals...

  2. The incorporation of quadratic convergence into open-shell self-consistent field equations

    Hunt, William J.; Goddard, William A., III; Dunning, Thom. H., Jr.
    The quadratically convergent approach to solving the correct SCF equations for general open-shell systems (with orthogonal orbitals, i.e., multiconfiguration-SCF or Hartree-Fock) is derived and used to discuss other less complicated approaches. Representative calculations on He ^21[(1s) (2s)] and H_2O 2^1A_1 [(1a_1)^2 (2a_1)^2 (1b_1)^2 (1b^2)_2 (3a_1) (4a_1)] are reported.

  3. A new type of wavefunction for BH

    Blint, R. J.; Goddard, W. A., III; Ladner, R. C.; Palke, W. E.
    The wavefunctions of BH have been obtained using a generalization of the valence bond and Hartree-Fock methods known as the Spin-Coupling Optimized GI (SOGI) method. The shapes of the bonding and especially the nonbonding orbitals are discussed as are several properties of the molecules.

  4. Exchange kinetic energy, contragradience, and chemical binding

    Wilson, C. Woodrow, Jr.; Goddard, William A., III
    The energy components of the G1 wavefunction for H_2 are carefully analyzed. It is found that the binding is dominated by the exchange part of the kinetic energy and that this term is dominant because of the contragradience of the orbitals in the bond region.

  5. Self-Assembled Monolayer Mechanism for Corrosion Inhibition of Iron by Imidazolines

    Ramachandran, Sunder; Tsai, Bao-Liang; Blanco, Mario; Chen, Huey; Tang, Youngchun; Goddard, William A., III
    Some of the most effective corrosion inhibitors for oil field pipeline applications are the oleic imidazoline (OI) class of molecules. However, the mechanism by which OIs inhibit corrosion is not known. We report atomistic simulations (quantum mechanics and molecular dynamics) designed to elucidate this mechanism. These studies lead to the self-assembled monolayer (SAM) model for corrosion inhibition, which explains the differences in corrosion inhibition efficiency for various OI molecules. The SAM model of OI inhibitors involves the following critical elements:  (i) The function of the OI is to form a self-assembled monolayer on the native oxide surface of iron; this...

  6. Comparison of the Behavior of Several Cobalt Porphyrins as Electrocatalysts for the Reduction of O_2 at Graphite Electrodes

    Song, Euihwan; Shi, Chunnian; Anson, Fred C.
    Four cobalt porphyrins were adsorbed on graphite electrodes and used to catalyze the electroreduction of O_2. The two porphyrins without substituent groups in the meso positions of the porphyrin ring operated at the most positive potentials and catalyzed the reduction of O_2 to both H_2O_2 and H_2O, but the H_2O did not result from significant reduction of H_2O_2. The porphyrins containing meso substituents catalyzed only the reduction of O_2 to H_2O_2. The catalysts that accomplish the four-electron reduction of O_2 are argued to consist of dimeric (or higher oligomeric) forms of the adsorbed porphyrins. The present results and those of...

  7. Molecular Recognition via Hydrogen Bonding at the Air−Water Interface:  An Isotherm and Fourier Transform Infrared Reflection Spectroscopy Study

    Weck, Marcus; Rink, Ralf; Ringsdorf, Helmut
    Molecular recognition in Langmuir monolayers at the air−water interface as a function of headgroup orientation and substrate using isotherms and in-situ Fourier transform infrared (FT-IR) reflection spectroscopy has been investigated. Isotherm measurements show that urea and 2,4,6-triaminopyrimidine (TAP) are specifically bound to barbituric acid lipid monolayers. As expected, TAP causes a larger shift in the limiting area of the isotherms than urea due to steric requirements. The peak positions of the CH stretching vibrations of the barbituric acid lipids indicate that the alkyl chains of barbituric acid lipids 1−3 are in a close-packed all-trans conformation both before and after the...

  8. Diacetylene Chelator Lipids as Support for Immobilization and Imaging of Proteins by Atomic Force Microscopy

    Dorn, Ingmar T.; Hofmann, Ulrich G.; Peltonen, Jouko; Tampé, Robert
    Chelator lipids represent a powerful and flexible tool to immobilize, orient, and crystallize histidine-tagged proteins at interfaces. To produce stable two-dimensional polymers that are biofunctional, we synthesized diacetylene lipids carrying a metal-chelating headgroup. These lipids were characterized at the air−water interface with respect to their thermodynamic properties, complex formation, and photopolymerization using film balance techniques combined with epi-fluorescence microscopy. Polymerized monolayers were transferred onto solid supports and reversible binding of histidine-tagged protein/DNA complexes was followed by atomic force microscopy. The versatility of the chelator lipid concept may open the possibility to examine structure and function of proteins or multiprotein assemblies...

  9. Orienting DNA Helices on Gold Using Applied Electric Fields

    Kelley, Shana O.; Barton, Jacqueline K.; Jackson, Nicole M.; McPherson, Lee D.; Potter, Aaron B.; Spain, Eileen M.; Allen, Michael J.; Hill, Michael G.
    Gold surfaces modified with thiol-derivatized DNA duplexes have been investigated as a function of applied electrochemical potential via atomic force microscopy (EC-AFM). At open circuit, monolayers of well-packed DNA helices form with a film depth of 45(3) Å. On the basis of the anisotropic dimensions of these 15 base pair duplexes (20 Å in diameter versus 50 Å in length), this corresponds to an average ∼45° orientation of the helical axis with respect to the gold surface. Under potential control, the monolayer thickness (and therefore the orientation of the helices) changes dramatically with applied potential. At potentials negative of ∼0.45...

  10. Surface-Enhanced Raman Scattering (SERS) from Azobenzene Self-Assembled “Sandwiches”

    Yu, Hua-Zhong; Zhang, Jin; Zhang, Hao-Li; Liu, Zhong-Fan
    A silver island film was thermally deposited on the top of an azobenzene self-assembled monolayer on gold, yielding a SERS-active system possessing a “sandwiched” structure of Ag|R_1−Azo−R_2S−|Au. For the first time, the positions of the Azo groups were controlled by using azobenzenealkanethiols with different terminal groups (R_1) or interchain spacers (R_2), to clarify the relationship between the SERS effect and the structural nature of the system. As will be shown, increases in distances of Azo groups from the gold substrate and from the silver film both cause the enhancement factor to decay exponentially, indicating that the enhancement correlates to both...

  11. Two‐dimensional electrophoresis of proteins as a tool in the metabolic engineering of cell cycle regulation

    Lee, Kelvin H.; Harrington, Michael G.; Bailey, James E.
    Metabolic engineering of cell cycle regulation addresses important biotechnological problems about serum removal from animal cell culture systems. Chinese hamster ovary cells stimulated to grow by fetal calf serum, insulin, or basic fibroblast growth factor were studied by two‐dimensional electrophoresis (2DE) and the resulting protein expression patterns were analyzed. Detailed 2DE protocols are provided and at least 24 gene products are identified which may play an important role in growth factor signaling. Moreover, a correlation between the expression of three proteins (cyclin D1, cyclin E, and E2F‐1) and mitogenic strength was found.

  12. Component Dynamics in Miscible Blends:  Equally and Unequally Entangled Polyisoprene/Polyvinylethylene

    Arendt, B. H.; Krishnamoorti, R.; Kornfield, J. A.; Smith, S. D.
    Miscible blends of 1,4-polyisoprene (PIP) and polyvinylethylene (PVE) are studied using rheo-optical methods to extract the dynamics of each component in the blend. Since orientational coupling contributes to the birefringence, but not the stress, simultaneous analysis of these observables for bidisperse blends is used to determine the coupling coefficient, ε. No systematic dependence of ε on composition or temperature was detected. The dynamics of the blend components extracted using the mean value of ε are in good agreement with those observed previously for equally entangled blends:  blending causes only small changes in the component entanglement molecular weights but dramatically alters...

  13. Dynamics of Disordered Diblocks of Polyisoprene and Polyvinylethylene

    Arendt, B. H.; Krishnamoorti, R.; Kannan, R. M.; Seitz, K.; Kornfield, J. A.; Roovers, J.
    The apparent thermorheological simplicity of disordered diblocks is reconciled with the failure of time−temperature superposition of corresponding miscible blends by examining the relaxation of the constituent blocks using rheo-optical techniques. Diblocks of 1,4-polyisoprene (PIP) and polyvinylethylene (PVE) are examined over a range of temperatures for two compositions (φ_(PIP) = 0.25 and 0.75). Unlike blends of PIP and PVE, the block copolymers appear to obey time−temperature superposition on the basis of their viscoelastic properties. However, departure from thermorheological simplicity is exposed in their stress−optical behavior. In particular, the copolymer rich in the high T_g component (φ_(PIP) = 0.25) shows distinct temperature...

  14. Utility of a Ruthenium Metathesis Catalyst for the Preparation of End-Functionalized Polybutadiene

    Hillmyer, Marc A.; Nguyen, SonBinh T.; Grubbs, Robert H.
    The polymerization of cyclooctadiene (COD) in the presence of an allylic difunctionalized chain transfer agent (1) by a ruthenium metathesis catalyst (Ru) was accomplished. Deprotection of the resulting acetate end-functionalized polybutadiene was performed, leading to commercially important 1,4-hydroxytelechelic polybutadiene (HTPBD) with number average functionalities close to 2.0. The polymerizations were performed at high monomer concentrations or in the absence of solvent, and the robust nature of Ru allowed the use of high monomer to catalyst ratios (∼0.01 mol % Ru relative to total olefin). Investigation of the metathesis of olefin alcohols with Ru showed that side reactions may complicate the...

  15. Ruthenium Alkylidene Initiated Living Ring-Opening Metathesis Polymerization (ROMP) of 3-Substituted Cyclobutenes

    Maughon, Bob R.; Grubbs, Robert H.
    The synthesis and living ring-opening metathesis polymerization (ROMP) of substituted cyclobutenes were investigated with the functional group tolerant initiators (PCy_3)_2Cl_2Ru=CHCHCPh_2 (1) and (PCy_3)_2Cl_2Ru=CHPh (2). Synthetic methodology was developed for the synthesis of a wide variety of 3-functionalized cyclobutenes containing ether, ester, alcohol, amine, amide, and carboxylic acid substituents. Coordination of these functional groups to the propagating carbene was observed, resulting in the formation of a chelated propagating species with concomitant loss of one phosphine ligand from the metal center. Studies aimed at understanding this chelation and its effect on the polymerization were undertaken. On the basis of these results, the...

  16. On the Nature of the Perforated Layer Phase in Undiluted Diblock Copolymers

    Qi, Shuyan; Wang, Zhen-Gang
    The nature, stability, and mechanism of formation of the block copolymer perforated lamellar structure are elucidated. This structure is shown to develop from the anisotropic fluctuations of the lamellar phase when it reaches its spinodal. It is proposed that there can be two different perforated lamellar structures, one based on a hexagonal close packed (hcp) lattice and one based on a body-centered cubic (bcc) lattice, with nearly degenerate free energy. In the framework of a Leibler-type free energy functional, it is shown that the perforated lamellar structure is only pseudostable (corresponding to a saddle point in the free energy surface)...

  17. Synthesis of Discotic Columnar Side-Chain Liquid Crystalline Polymers by Ring-Opening Metathesis Polymerization (ROMP)

    Weck, Marcus; Mohr, Bernhard; Maughon, Bob R.; Grubbs, Robert H.
    Discotic liquid crystalline polymers bearing alkoxy-substituted triphenylene moieties in the side-chain were synthesized by ring-opening metathesis polymerization using a well-defined ruthenium initiator. To elucidate the effect of backbone flexibility on the mesomorphism, norbornene and cyclobutene monomers containing triphenylene moieties were synthesized, which yielded the relatively rigid poly(norbornene)s and the more flexible poly(butadiene)s, respectively, after polymerization. To further increase the backbone flexibility, the poly(butadiene)s were hydrogenated using Crabtree's catalyst to yield triphenylene-substituted poly(butane)s. The mesomorphic behavior of the polymers has been investigated by differential scanning calorimetry (DSC) and powder diffraction X-ray scattering (wide angle X-ray scattering, WAXS). All polymers bearing a...

  18. Synthesis of Polymer Network Scaffolds from l-Lactide and Poly(ethylene glycol) and Their Interaction with Cells

    Han, Dong Keun; Hubbell, Jeffrey A.
    New lactide-based poly(ethylene glycol) (PEG) polymer networks (GL−PEGs) have been prepared by UV photopolymerization using two nontoxic macromers, triacrylated lactic acid oligomer emanating from a glycerol center (GL) and monoacrylated PEG. These materials have been developed for use as polymer scaffolds in tissue engineering, which have cell-adhesion resistant, ligand-immobilizable, and biodegradable characteristics. The GL−PEG cross-linked polymer networks obtained were glassy and transparent, and the gel content was approximately 90%, irrespective of the degree of polymerization of lactide on the glycerol center and the amount and molecular weight of the PEG acrylate incorporated. All networks showed relatively low swelling in water,...

  19. Rouse and Reptation Dynamics of Linear Polybutadiene Chains Studied by ^2H NMR Transverse Relaxation

    Klein, Philip G.; Adams, Christine H.; Brereton, Michael G.; Ries, Michael E.; Nicholson, Timothy M.; Hutchings, Lian R.; Richards, Randal W.
    Deuterium NMR has been used to investigate two different types of dynamics of linear polybutadiene chains in the melt. The transverse relaxations of short Rouse chains of molecular weight 640−3000 were biexponential, which has been attributed to separate decays of the methylene and methine deuterons. Interpretation of the transverse relaxation rates using a model for Rouse dynamics, combined with molecular simulations, gave the shortest Rouse unit as approximately 4.4 monomers and the shortest Rouse time as 8.3 × 10^(-7) s. The reptation dynamics of higher molecular weight entangled chains were investigated using ABA isotopic triblock copolymers, of total molecular weight...

  20. Polymeric Biomaterials with Degradation Sites for Proteases Involved in Cell Migration

    West, Jennifer L.; Hubbell, Jeffrey A.
    A new class of biodegradable polymeric biomaterials is described which exhibits degradation by specific enzymes that are localized near cell surfaces during cell migration. These materials are telechelic BAB block copolymers of the water-soluble biocompatible polymer poly(ethylene glycol) (the A block, abbreviated PEG hereafter) and short oligopeptides that are cleavage sequences for targeted enzymes (the B block). These copolymers are further capped at each end with reactive acrylate groups to allow polymerization to form cross-linked hydrogel networks. Two materials were synthesized, one specifically degraded by collagenase and the other by plasmin. The incorporation of peptides that induce cell adhesion to...

Aviso de cookies: Usamos cookies propias y de terceros para mejorar nuestros servicios, para análisis estadístico y para mostrarle publicidad. Si continua navegando consideramos que acepta su uso en los términos establecidos en la Política de cookies.