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Caltech Authors (168.318 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 21.511

  1. Spectroscopy and Electrochemistry of mer-[RuCl_3(dmso)(tmen)]. Dimethylsulfoxide Is Sulfur-Bonded to Ru(II), Ru(III), and Ru(IV)

    Rack, Jeffrey J.; Gray, Harry B.
    The discovery that halo−ruthenium(sulfoxide) complexes exhibit anticancer activity has stimulated interest in the nature of bonding of metal ions to dimethylsulfoxide (dmso). Both M−S and M−O bonds are observed in metal complexes containing dmso, with the former mode prevalent with “soft” metal centers. The importance of dπ-S back-bonding in S-bonded complexes has been addressed by several investigators. Especially revealing was the finding by Taube and co-workers that S to O linkage isomerism can be induced by oxidation of pentaammineruthenium(II) to ruthenium(III), thereby suggesting that dπ-S bonding is a stabilizing factor only in the lower oxidation state. However, the observation that...

  2. Visible Region Photooxidation on TiO_2 with a Chromophore−Catalyst Molecular Assembly

    Treadway, Jospeh A.; Moss, John A.; Meyer, Thomas J.
    Nanocrystalline films of TiO_2 derivatized by adsorption of polypyridyl complexes of RuII have provided a basis for a family of solution photoelectrochemical devices. At the molecular level, a key feature in these devices is irreversible photoinjection into the TiO_2 conduction band following metal-to-ligand charge-transfer (MLCT) excitation giving Ru^(III). Reduction of Ru^(III) by I^- followed by I_3^- reduction at the anode completes the cell. It should be possible to exploit this basic scheme for the photoproduction of high-energy chemicals, and we report preliminary results on such an example based on an adsorbed chromophore−oxidant molecular assembly.

  3. Structures of [M_2(dimen)_4](Y)_2 (M = Rh, Ir; dimen = 1,8-Diisocyanomenthane; Y = PF_6, Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, B(C_6H_5)_4) Crystals Featuring an Exceptionally Wide Range of Metal−Metal Distances and Dihedral Twist Angles

    Exstrom, Christopher L.; Britton, Doyle; Mann, Kent R.; Hill, Michael G.; Miskowski, Vincent M.; Schaefer, William P.; Gray, Harry B.; Lamanna, William M.
    The binuclear complexes [M_2(dimen)_4](Y)_2 (M = Rh, Ir; dimen = 1,8-diisocyanomenthane; Y = PF_6, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and B(C_6H_5)_4) have face-to-face structures with M−M distances between 3.60 and 4.48 Å, and square-planar unit twist angles between 0 and 17.2°. Ligand flexing and out-of-plane bending of the metal centers accommodate M−M distances longer than 3.9 Å; addition of a torsional deformation produces a twisted conformation for shorter M−M distances (<3.9 Å). Spectroscopic data indicate that there are two or more deformational isomers of Ir_2(dimen)_4^(2+) in solution.

  4. Cobalt meso-Tetrakis(N-methyl-4-pyridiniumyl)porphyrin Becomes a Catalyst for the Electroreduction of O_2 by Four Electrons When [(NH_3)_5Os]^(n+) (n = 2, 3) Groups Are Coordinated to the Porphyrin Ring

    Shi, Chunnian; Anson, Fred C.
    In a series of recent reports evidence has been offered to support the proposal that coordination of (NH_3)_5Ru^(2+) groups to ligand sites introduced on the meso positions of certain cobalt porphyrins enhances their abilities to act as electrocatalysts for the reduction of O_2 to H_2O. The enhancement is believed to be the result of back-bonding by the Ru(II) centers into the pendant ligands and the cobalt porphyrin to which they are linked. The catalytic mechanism that has been proposed leaves the coordinated Ru centers in their +2 oxidation state as the O_2 molecule coordinated to the Co(II) center of the...

  5. Polymer Brushes: Scaling, Compression Forces, Interbrush Penetration, and Solvent Size Effects

    Martin, J. I.; Wang, Z.-G.
    This work discusses the results of a self-consistent-field calculation of conformational and thermodynamic properties of polymers end-grafted to a surface in athermal solvents. Three primary issues are addressed. First, we address the question raised recently by Carignano and Szleifer as to whether the second virial treatment of previous numerical and analytical self-consistent-field theories provides an adequate description of polymer brushes. We show that, for grafted chains that are sufficiently long, there exists a broad range of grafting densities where the lateral pressure and the brush thickness both scale as predicted by the second virial treatment. For shorter chains (of 100...

  6. SiO^-•••HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites

    Koller, Hubert; Lobo, Raul F.; Burkett, Sandra L.; Davis, Mark E.
    The high-silica zeolites NON, DDR, MTW, AFI, and MFI synthesized with organic quaternary ammonium cations show in the as-synthesized form a line at 10.2 ± 0.2 ppm in the ^1H MAS NMR spectra that does not originate from the organic structure directing agents (SDA's). This signal is assigned to SiO^-•••HOSi hydrogen bonds between defect sites with an O•••O distance of 2.7 Å. The presence of Q^3 sites is confirmed by ^(29)Si MAS NMR. The intensity of the line at 10.2 ppm in the ^1H MAS NMR spectra decreases when the positive charge of the quaternary ammonium cations is balanced by...

  7. Morphology Control in Electrochemically Grown Conducting Polymer Films. 3. A Comparative Study of Polyaniline Films on Bare Gold and on Gold Pretreated with p-Aminothiophenol

    Sabatani, Eyal; Gafni, Yael; Rubinstein, Israel
    Polyaniline (PANI) films deposited galvanostatically on gold electrodes precoated with self-assembled monolayers of p-aminothiophenol (PATP) exhibit significantly higher optical densities than similar polymer films deposited on bare gold, as measured by in situ ellipsometry. At the same time there is no change in the total mass of the deposit or in the amount of polymer, as indicated by in situ quartz crystal microbalance measurements and by ex situ Rutherford backscattering results, respectively. It is therefore concluded that PANI grown on Au/PATP is considerably denser than PANI grown on bare Au. Failure of other self-assembled monolayers to produce a similar effect...

  8. Quenching of Tris(2,2'-bipyridine)ruthenium(II) Luminescence by Cobalt(III) Polypyridyl Complexes in Different Sites in and on Clays

    Awaluddin, Amir; DeGuzman, Roberto N.; Kumar, Challa V.; Suib, Steven L.; Burkett, Sandra L.; Davis, Mark E.
    The effect of quenching of tris(2,2'-bipyridine)ruthenium(II) [Ru(bpy)_3^(2+)] luminescence by Co(phen)_3^(3+) in various locations in and on bentolite-H clay has been investigated. Ru(bpy)_3^(2+) incorporated into the interlayer region of bentolite-H by ion exchange exhibited spectral properties that were distinct from the Ru(bpy)_3^(2+) incorporated by incipient wetness methods. For example, the samples showed different diffraction, luminescence emission, and lifetime decays. The method of preparation, the nature of counterion (chloride or cyanide), and the presence of a quencher [Co(phen)_3^(3+) influence the emission properties. For example, incipient wetness methods allow deposition on outer surfaces (OUT) of the clay whereas ion-exchange leads to deposition primarily...

  9. Fourier Transform Ion Cyclotron Resonance Spectrometric Studies of Lewis Acid-Base Chemistry in Monochlorosilane

    Murthy, Srihari; Beauchamp, J. L.
    Fourier transform ion cyclotron resonance spectrometry is used to determine reaction pathways and rate constants for the reactions of the Lewis acids, SiH_nCl_(3-n)^+ (n = 1-3), with SiH_3Cl. SiH_2Cl^+ is observed to participate in both an H^- transfer reaction and a disproportionation reaction with SiH_3Cl. The ions SiH_3^+ and SiHCl_2^+ are observed to participate in only H^- transfer reactions with SiH_3Cl. The H^- affinity of SiH_2-Cl^+ is determined to be 1.4 ± 0.3 kcal mol^(-1) greater than that of SiHCl_2^+. Within experimental uncertainty, the values of the H^- affinities of the chlorosilyl ions lie within 1.7 kcal mol^(-1) of each...

  10. Future systems and control research in synthetic biology

    Del Vecchio, Domitilla; Qian, Yili; Murray, Richard M.; Sontag, Eduardo D.
    Synthetic biology is the application of engineering principles to the fundamental components of biology, with the aim of creating systems with novel functionalities that can be used for energy, environment, and medical applications. While the potential impact of this new technology is enormous, there are challenges that we need to overcome before the impact of synthetic biology can be fully realized. Many of these challenges fall beyond the scope of molecular biology and are indeed “system-level” problems, where very little research is being performed. This paper identifies pressing challenges in synthetic biology that can be formulated as systems and control...

  11. Experimental and Theoretical Studies of Co(CH_4)_x^+ with x = 1-4

    Haynes, Chris L.; Armentrout, P. B.; Perry, Jason K.; Goddard, William A., III
    The sequential bond energies of Co(CH_4)_x^+, x = 1-4, are determined experimentally and theoretically. Collision-induced dissociation studies conducted using a guided ion beam tandem mass spectrometer yield 0 K bond energies of D_o[Co^+-Xe] = 0.85 ± 0.07 eV, D_o[Co^+-CH_4] = 0.93 ± 0.06 eV, D_o[Co(CH_4)^+-CH_4] = 1.00 ± 0.05 eV, D_0[Co(CH_4)_2^+-CH_4] = 0.41 ± 0.05 eV, and D_o[Co(CH-4)_3^+-CH_4] = 0.67 ± 0.06 eV. These numbers are in excellent agreement with ab initio calculations which lead to values of D_e[Co^+-(CH_4)] = 0.93 eV, D_e[Co(CH_4)^+-CH_4] = 1.00 eV, D_e[Co(CH_4)_2+-CH_4] ≈ 0.40 eV, and D_e[Co(CH_4)_3^+-CH_4] ≈ 0.70 eV. We find that the nonmonotonic...

  12. Limits on the Corrosion Rate of Si Surfaces in Contact with CH_3OH-Ferrocene^(+/0) and CH_3OH-1,1'-Dimethylferrocene^(+/0) Solutions

    Shreve, Gary A.; Karp, Chris D.; Pomykal, Katherine E.; Lewis, Nathan S.
    Although Si/CH^3OH contacts have been extensively investigated and reported to provide highly efficient photoelectrochemical energy conversion devices, a recent study using the scanning electrochemical microscope (SECM) has claimed that, in CH_3OH solutions, Si surfaces in contact with 4.57 mM ferrocenium (Fc^+) were etched in the dark at a mass-transport-limited rate. The reported etching rate constant of > 0.37 cm s^-1) at 4.57 mM ferrocenium corresponds to an equivalent corrosion current density of > 240 mA cm^(-2) and to a Si etch rate of > 75 nm s^(-1). The presence of such severe corrosion was inferred from an unexpectedly large feedback...

  13. Viscosity of Supercooled Sulfuric Acid Solutions

    Williams, Leah R.; Long, Forrest S.
    The physical properties of supercooled sulfuric acid solutions are of interest because sulfate aerosols play a role in heterogeneous stratospheric chemistry and possibly in the nucleation of polar stratospheric cloud particles. We report measurements of the viscosity of sulfuric acid solutions at temperatures between 200 and 300 K and for concentrations between 30 and 80 wt %. These results can be used to estimate diffusion constants and to calculate nucleation rates in sulfuric acid at low temperatures. The current measurements give higher viscosities at lower temperatures than previous approximations. This in turn decreases estimated diffusion constants and nucleation rates.

  14. Nitrobenzene "Caged" Compounds as Irreversible Photoreductants: A Rational Approach to Studying Photoinduced Intermolecular Electron-Transfer Reactions in Proteins

    DiMagno, Theodore J.; Stowell, Michael H. B.; Chan, Sunney I.
    Nitrobenzene “caged” compounds are well-known for their use in delivering biologically active substrates to a reaction mixture after photoexcitation. We have discovered that they also behave as photoreductants from the triplet state after photoexcitation. To explore the properties of these newly discovered photoreductants, a series of substituted nitrobenzene derivatives have been synthesized. These nitrobenzene derivatives exhibit many desirable characteristics suitable for the physiological photoreduction of different proteins, and examples are shown for the photoreduction of cytochrome c and other heme proteins. The observed rate constant for photoreduction of cytochrome c, k_(obs), ranges from 300 to 36 000 s^(-1) for the...

  15. The Generation of the Distant Kuiper Belt by Planet Nine from an Initially Broad Perihelion Distribution

    Khain, Tali; Batygin, Konstantin; Brown, Michael E.
    The observation that the orbits of long-period Kuiper Belt objects (KBOs) are anomalously clustered in physical space has recently prompted the Planet Nine hypothesis—the proposed existence of a distant and eccentric planetary member of our solar system. Within the framework of this model, a Neptune-like perturber sculpts the orbital distribution of distant KBOs through a complex interplay of resonant and secular effects, such that in addition to perihelion-circulating objects, the surviving orbits get organized into apsidally aligned and anti-aligned configurations with respect to Planet Nine's orbit. In this work, we investigate the role of Kuiper Belt initial conditions on the...

  16. Mitigation of severe urban haze pollution by a precision air pollution control approach

    Yu, Shaocai; Liu, Pengfei; Wang, Liqiang; Wu, Yujie; Wang, Si; Liu, Kai; Zhu, Tong; Zhang, Yuanhang; Hu, Min; Zeng, Liming; Zhang, Xiaoye; Cao, Junji; Alapaty, Kiran; Wong, David C.; Pleim, Jon; Mathur, Rohit; Rosenfeld, Daniel; Seinfeld, John H.
    Severe and persistent haze pollution involving fine particulate matter (PM_(2.5)) concentrations reaching unprecedentedly high levels across many cities in China poses a serious threat to human health. Although mandatory temporary cessation of most urban and surrounding emission sources is an effective, but costly, short-term measure to abate air pollution, development of long-term crisis response measures remains a challenge, especially for curbing severe urban haze events on a regular basis. Here we introduce and evaluate a novel precision air pollution control approach (PAPCA) to mitigate severe urban haze events. The approach involves combining predictions of high PM_(2.5) concentrations, with a hybrid...

  17. SILVERRUSH. V. Census of Lyα, [O III] λ5007, Hα, and [C II] 158 μm Line Emission with ~1000 LAEs at z = 4.9–7.0 Revealed with Subaru/HSC

    Harikane, Yuichi; Capak, Peter L.; Faisst, Andreas L.; Davidzon, Iary
    We investigate Lyα, [O III] λ5007, Hα, and [C II] 158 μm emission from 1124 galaxies at z = 4.9–7.0. Our sample is composed of 1092 Lyα emitters (LAEs) at z = 4.9, 5.7, 6.6, and 7.0 identified by Subaru/Hyper-Suprime-Cam (HSC) narrowband surveys covered by Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) and 34 galaxies at z = 5.148–7.508 with deep ALMA [C II] 158 μm data in the literature. Fluxes of strong rest-frame optical lines of [O III] and Hα (Hβ) are constrained by significant excesses found in the SPLASH 3.6 and 4.5 μm photometry. At z = 4.9,...

  18. HAT-P-11: Discovery of a Second Planet and a Clue to Understanding Exoplanet Obliquities

    Yee, Samuel W.; Petigura, Erik A.; Fulton, Benjamin J.; Knutson, Heather A.; Batygin, Konstantin; Bakos, Gáspár Á.; Hartman, Joel D.; Hirsch, Lea A.; Howard, Andrew W.; Isaacson, Howard; Kosiarek, Molly R.; Sinukoff, Evan; Weiss, Lauren M.
    HAT-P-11 is a mid-K dwarf that hosts one of the first Neptune-sized planets found outside the solar system. The orbit of HAT-P-11b is misaligned with the star's spin—one of the few known cases of a misaligned planet orbiting a star less massive than the Sun. We find an additional planet in the system based on a decade of precision radial velocity (RV) measurements from Keck/High Resolution Echelle Spectrometer. HAT-P-11c is similar to Jupiter in its mass (M_p sin i = 1.6 ± 0.1 M J ) and orbital period (P = 9.3^(+1.0)_(-0.5) year), but has a much more eccentric orbit...

  19. OGLE-2017-BLG-0329L: A Microlensing Binary Characterized with Dramatically Enhanced Precision Using Data from Space-based Observations

    Han, C.; Calchi Novati, S.; Beichman, C.; Carey, S.
    Mass measurements of gravitational microlenses require one to determine the microlens parallax π_E, but precise π_E measurement, in many cases, is hampered due to the subtlety of the microlens-parallax signal combined with the difficulty of distinguishing the signal from those induced by other higher-order effects. In this work, we present the analysis of the binary-lens event OGLE-2017-BLG-0329, for which π_E is measured with a dramatically improved precision using additional data from space-based Spitzer observations. We find that while the parallax model based on the ground-based data cannot be distinguished from a zero-π E model at the 2σ level, the addition...

  20. Phonon Networks with Silicon-Vacancy Centers in Diamond Waveguides

    Lemonde, M.-A.; Meesala, S.; Sipahigil, A.; Schuetz, M. J. A.; Lukin, M. D.; Loncar, M.; Rabl, P.
    We propose and analyze a novel realization of a solid-state quantum network, where separated silicon-vacancy centers are coupled via the phonon modes of a quasi-one-dimensional diamond waveguide. In our approach, quantum states encoded in long-lived electronic spin states can be converted into propagating phonon wave packets and be reabsorbed efficiently by a distant defect center. Our analysis shows that under realistic conditions, this approach enables the implementation of high-fidelity, scalable quantum communication protocols within chip-scale spin-qubit networks. Apart from quantum information processing, this setup constitutes a novel waveguide QED platform, where strong-coupling effects between solid-state defects and individual propagating phonons...

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