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Caltech Authors (124,343 recursos)
Repository of works by Caltech published authors.

Type = Conference or Workshop Item

Mostrando recursos 1 - 20 de 636

1. Chemically specific dynamic bond percolation model for computational screening of polymer electrolytes - Webb, Michael; Savoie, Brett; Wang, Zhen-Gang; Miller, Thomas
Polymer electrolytes have significant promise for many lithium-ion battery applications because they are non-flammable, electrochem. stable, and easily manufd. However, significant improvements in the ionic cond. of even state- of-the-art polymer electrolytes is needed for technol. viability. The computationally- guided design of more conductive polymer electrolytes would save significant time and monetary resources, but these efforts require both a better understanding of ion- transport mechanisms in polymers and the development of tools for screening candidate polymers prior to synthesis and characterization. Using atomistic mol. dynamics (MD) simulations, we provide new insights into the mechanisms of lithium-ion diffusion in polymer electrolyte materials, including that the spatial distribution...

2. Reduction of CO2 on Cu and Au/W electrode surfaces: A study by differential electrochemical mass spectrometry - Javier, Alnald; Baricuatro, Jack; Kim, Youn-Geun; Soriaga, Manuel
This work describes results from an attempt to employ differential electrochem. mass spectrometry (DEMS) of selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary exptl. approach in the study of the reaction mechanism of the Cu-catalyzed electrochem. redn. of CO2. The results prompt the following empirical inferences: (i) CO is the first product of CO2 redn., as well as the first intermediate in more advanced reactions that include formation of pure and oxygenated hydrocarbons; this is in conformity with the (almost) unanimously held view. (ii) HCHO is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the prodn. of methane...

3. Observation and reactivity studies of an unusual RhI intermediate in H2 evolution catalysis - Aguirre Quintana, Luis; Gray, Harry; Winkler, Jay; Blakemore, James
Understanding mechanisms of hydrogen evolution is important for improving catalysts that generate clean fuels. Catalysts often operate via unknown mechanism(s) due to the difficulty of observing or isolating key intermediates in the 2e-/2H+ redn. process. For our study, we prepd. a family of RhI complexes bearing the pentamethylcyclopentadienyl (η5-Cp*) ligand in addn. to derivs. of 4,4'- bipyridyl or 1,10- phenanthroline (bound in the κ2 mode). Under carefully chosen conditions, these compds. formed intermediate species that suggest a previously unknown mechanism of H2 evolution involving protonhydride tautomerism driven by the Cp* ligand. This intermediate complex could be isolated in a MeCN soln., and even as a solid,...

4. Effects of the μ4-bridging atom in a tetranuclear iron cluster on the activation of nitric oxide - Reed, Christopher; Agapie, Theodor
A series of tetrairon μ_4-fluoride clusters, [LFe_3F(PhPz)_3Fe][OTf]_n (n=1-3), was synthesized using the previously reported 1,3,5- triarylbenzene framework, a multinucleating ligand contg. pyridine and alkoxide donors, which bind three of the four metal centers. These three metal sites contain six-coordinate iron atoms. 3-phenyl-pyrazolate ligands (PhPz) bridge the triiron core to a four-coordinate, site-differentiated iron center. These clusters have notably different redox properties than the closely related μ_4-oxo analogs, [LFe_3O(PhPz)_3Fe][OTf]_n (n=0-3). While for [LFe_3O(PhPz)_3Fe][OTf]_2 three reversible redox events are obsd. at -1.73 V (vs. Fc /Fc+), -0.72 V, and +0.02 V (Fe^(II)_4 Fe^(II)Fe^(III)_3), only two redox events are obsd. for the fluoride-bridged cluster, [LFe_3F(PhPz)_3Fe][OTf], at -0.52 V (Fe^(II)_4...

5. Scalable models of ion transport for electrolyte materials discovery - Savoie, Brett M.; Miller, Thomas F.
Solid polymeric electrolytes have the potential to greatly improve battery efficiency and stability, but ions like Li simply diffuse too slowly in current materials for them to be viable. The challenge for computationally designing new polymers lies in accurately modeling both the long- timescale of ion transport and the chem. specific ion- polymer couplings that govern ion motion. In this talk, I will show how short atomistic simulations can be combined with a generalizable mesoscopic ion transport model to reliably predict ion diffusivity across many chem. classes of polymers. This model has now been used to characterize several hundred new materials and the talk will...

6. Probing binding interactions of agonists at a nicotinic acetylcholine receptor subtype important to addiction and Parkinson's disease - Post, Michael R.; Lester, Henry; Dougherty, Dennis A.
Nicotine addiction is still a major problem in American society. Sixteen percent of adults report smoking regularly, resulting in 96 billion dollars in healthcare costs per yr. Parkinson's disease is incurable, with 6,000 new diagnoses each year - it is the 14th leading cause of death in the US. Parkinson's disease and nicotine addiction are disorders of the dopamine pathway, making α6β2- contg. subtypes of the nicotinic acetylcholine receptor (nAChR) , which are ligand gated ion channels localized to dopaminergic neurons, an attractive target. Advanced knowledge of the binding site, which sits at the interface of α6 and β2, could lead to design of agonists...

7. Electrochemical CO2 reduction catalyzed by Mn catalysts: DFT investigations point to strategies for overpotential reduction and activity improvement - Lam, Yan Choi; Nielsen, Robert J.; Goddard, William A.; Gray, Harry B.
Exptl., [(L) Mn(CO)_3]- (where L = bis alkyl- substituted bipyridine) has been obsd. to catalyze the electrochem. redn. of CO_2 to CO in the presence of trifluoroethanol (TFEH). Our DFT calcns. (B3LYP- d3 with continuum solvation) of the free energies of reaction and activation show that the highly exergonic hydrogen bonding between TFEH and TFE (homoconjugation) plays a crit. role in reaction thermodn. and kinetics. The analgous 2,2'- bipyrimidine complex is predicted to have a lower (by ca. 0.5 V) onset overpotential, but at the expense of max. catalytic activity. As a strategy to improve catalytic activity and onset overpotential, a series of 2,2'- and...

8. Variable binding modes of pyridine in molybdenum complexes supported by novel P-pyridine-P ligands - Wan, Ruomeng; Horak, Kyle; Buss, Joshua A.; Agapie, Theodor
Low oxidn. state molybdenum complexes bind N_2 and in some cases reduce it to NH_3. We have previously demonstrated that molybdenum complexes supported by terphenyl diphosphine ligand platforms bind and activate small mols. We report herein the synthesis and characterization of mononuclear Mo complexes supported by two isomers of a novel ligand framework: 3,5-bis(2-(diisopropylphosphino)phenyl)pyridine (PNP^(3,5)) and 2, 6-bis(2-(diisopropylphosphino) phenyl) pyridine (PNP^(2,6)). Metalation of the phosphines with Mo(CO)_3 (MeCN)_3 results in the formation of MoPNP(CO)_3 complexes. The PNP^(3,5) isomer shows binding via the pyridine π system, while the PNP^(2,6) via the nitrogen lone pair. Oxidn. results in Mo species. Upon photolysis, CO ligand substitution occurs to give...

9. Multi-layer anode for saline wastewater treatment: Manipulating oxidant generation pathway by TiO_2 thin film deposition - Yang, Yang; Hoffmann, Michael R.
Nowadays, both developed and developing countries are suffering from water scarcity due to the rapid urbanization and global climate change. The on-site reuse of water is a possible soln., which calls for the development of various decentralized wastewater treatment techniques. Electrochem. oxidn. is a promising candidate due to its modular design, convenient operation, and small carbon footprint. In the past decades, the IrO_x based DSA electrode showed excellent performance on saline wastewater treatment. However, the high cost and low energy efficiency impede the widespread application. In this study, uniform TiO_2 thin film was deposited onto the IrO_x electrode by novel spray- pyrolysis process. Instead of...

10. Selective two-electron reduction of carbon monoxide mediated by molybdenum complexes supported on an asymmetric phenol phosphine ligand - Riduan, Siti Nurhanna; Buss, Joshua A.; Agapie, Theodor
Our group has recently reported a monometallic Molybdenum complex supported by a terphenyl diphosphine ligand capable of undergoing multielectron reductive processes. Various oxidn. states of the metal center are stabilized by the ability of the ligand to accommodate different binding modes. For example, the arene π-system displays η^2, η^4, η^6 binding while one of the phosphine arms reversibly dissocs. To further study the significance of the hemilability of the ligand donor arms, a monometallic Mo dicarbonyl complex supported by an asym. terphenyl phosphine- phenol ligand (POH) framework has been isolated. Deprotonation and treatment of the (PMo(CO)_2OH) complex with KC_8 or K(C_(10)H_8) yields a reduced Mo...

11. Rolling revolution in analytical isotope geochemistry - Eiler, John
The many subjects of isotope geochem. have two things in common: they generally require anal. of isotope ratios at low natural abundances (∼10^(-2)-10^(-6)) and high precision (∼10^(-3)-10(-6)). For the first 50 years of this field (ca. 1950-2000), these requirements were met by a narrow range of technologies - mostly low mass resoln. (M/ΔM of a few hundred) TIMS and gas source IRMS - that limited the field to a few simple mols. and elements. Over the last 15 years this field has been transformed by the emergence of a wide range of new instruments and methods: plasma-source, high-resoln. MC-ICPMS has dramatically expanded the list of...

12. Towards an energy landscape of G protein-coupled receptor (GPCR) activation using hybrid methods - Dong, Sijia S.; Abrol, Ravinder; Goddard, William A.
The prediction of various GPCR conformations along their activation pathways has been a challenge for computational biophysicists. The shortage of exptl. active structures has impaired the study of GPCR activation mechanisms. We have developed a hybrid computational method that predicts multiple GPCR conformations systematically, including the active ones. This is one of the very few methods that can predict the high-energy active conformations, capable of coupling to the G protein, starting from an inactive conformation. We are also able to generate, to the best of our knowledge, the first quant. energy profile of GPCR activation consistent with the qual. energy landscape from expts. Our hybrid...

13. Homogeneous Ta/Ir tandem catalytic alkane/alkene coupling - Bercaw, John E.; Labinger, Jay A.; Leitch, David; Steelman, Keith
Light alkanes and alkenes are abundant but under-utilized energy carriers due to their high volatility and low energy d. A tandem catalytic approach for the coupling of alkanes and alkenes has been developed in order to upgrade these light hydrocarbons into heavier fuel mols. This process involves alkane dehydrogenation by pincer- ligated iridium complexes, and alkene dimerization by a Cp*TaCl_2(alkene) catalyst. These two homogeneous catalysts operate with up to 60 /30 cooperative turnovers (Ir/Ta) in the dimerization of 1- hexene /n- heptane, giving C_(13)/C_(14) products in 40 % yield. Because there is no kinetically relevant interaction between the two catalysts, the tandem mechanism can be...

14. Small molecule activation with metal complexes supported by ligands with pendant arene moieties - Agapie, Theodor
Multidentate ligands connected by terphenyl linkers have been employed to support mono and multimetallic complexes. The central arene of the terphenyl unit acts as a labile donor and coordinates in a variety of modes as a function of metal and oxidn. state. This arene can be non- innocent with respect to redox and H-transfer chem. Variants with catechol and hydroquinone moieties show multi- proton, multi- electron transfer chem. Addnl., pendant phosphine arms can reversibly dissoc., facilitating reactivity. The reactivity of these complexes for the activation of small mols. such as H_2, N_2, CO_2, O_2, and CO will be discussed.

15. Extending the π-system: Modulation of arene and phosphine donor lability in polyarene diphosphine-supported molybdenum complexes and its application to small molecule reactivity - Low, Choon Heng; Buss, Joshua A.; Agapie, Theodor
Previous work in our group has shown that low-valent molybdenum centers supported by a p-terphenyl diphosphine ligand are capable of activating and transforming a variety of small mols., including mediating the two- and four- electron reductive coupling of carbon monoxide. The hemilabile nature of the central arene and the phosphine arms has been postulated be important to their reactivities. To further probe the role of hemilabile arene binding, the use of polyarene-linked diphosphine ligands (1,4-naphthalenediyl, L^(naph), and 1,4-anthracenediyl, L^(anth)) is investigated. Previously, such polyarenes have shown greater propensity for η^4-arene binding to the non-fused carbons. Metalation with Mo(CO)_3(MeCN)_3 followed by oxidn. gives the complex [L^(naph)Mo(CO)_2][OTf]_2 with...

16. Tetranuclear complexes as precursors for the rational design of pentanuclear oxido clusters reminiscent of the oxygen evolving complex in photosystem II - Lee, Heui Beom; Tsui, Emily Y.; Agapie, Theodor
The oxygen evolving complex in Photosystem II contains a multimetallic [Mn_4CaO_5] active site. Previously, a pseudo-C_3 sym. tris(dipyridyl alkoxide) ligand based on a 1,3,5- triarylbenzene framework was used for the synthesis of trimetallic clusters. Here, substitution of a single dipyridyl alkoxide moiety for a carboxylate moiety leads to the formation of tetrametallic complexes [M_4(μ^3-OH)] (M = Mn, Fe, Co). Starting from [Mn_4(μ^3-OH)], pentametallic Mn oxido cluster complexes [M'Mn_4O_2] incorporating redox inactive metals (M' = Ca^+, Sr^(2+), Y^(3+), La^(3+)) have been prepd. Structure, redox properties, and spectroscopy of such complexes will be discussed.

17. Metal-carbon bond functionalization in the context of methane oxidation - Nielsen, Robert J.; Cheng, Mujeng; Goddard, William A.
Metal-mediated alkane functionalization requires catalyzing C-H cleavage and carbon- heteroatom bond formation, each of which pose high, potentially rate- limiting barriers. To complement the traditional nucleophilic attack mechanism for functionalizing M-CH_3 bonds, we have used computed free energy surfaces to develop the attack of electrophiles on metalcarbon bonds. Recently we described the conversion of a vanadium- Ph bond to phenoxide via migration of the Ph group to a sep. vanadate oxo group. This formally homolytic M-C bond cleavage results in the one- electron redn. of both vanadium centers. We predict that some one-electron oxidants will allow this mechanism to be transferred to late transition metals compatible...

18. Nitrogen fixation with iron complexes - Peters, Jonas C.; Del Castillo, Trevor J.; Rittle, Jonathan; Thompson, Niklas B.
Nitrogen redn. to NH_3 is a requisite transformation for life. While it is widely appreciated that the Fe- rich cofactors of nitrogenase enzymes facilitate this transformation, how they do so remains poorly understood. It is known that Fe is the only transition metal essential to all nitrogenases, and recent biochem. and spectroscopic data have implicated Fe as the likely site of N_2 binding in FeMo-co. These observations motivate studies of functional Fe catalysts as models. In this talk, Fe complexes that catalyze the redn. of N_2 to NH_3 will be discussed. Our lab's most recent data for various Fe complexes, aimed at improving catalysis, providing...

19. DFT study of an unusual proton-relay role for Cp* in hydrogen evolution catalysis - Johnson, Samantha I.; Corona, Sydney L.; Blakemore, James D.; Winkler, Jay R.; Gray, Harry B.; Goddard, William A.
Understanding mechanisms of the hydrogen evolution reaction (HER) is crucial to designing efficient catalysts for the prodn. of solar fuels. Cp*Rh(bpy) (Cp* = η^5- pentamethylcyclopentadienyl; bpy = κ^2-2,2'- bipyridyl) generates hydrogen in the presence of acid. However, the nature of the elementary steps leading to H-H formation has not been clear, as chem. characterization of intermediates in the catalytic reaction has been difficult to obtain. Here, we present a joint exptl.- computational study that addresses this challenge. D. functional theory (DFT) calcns. demonstrate that the catalyst first undergoes a 2e- redn. to form a Rh^I complex. Subsequently, in presence of acid, the Rh complex undergoes...

20. Effect of interlayer anions on [NiFe]-LDH nanosheet water oxidation activity - Hunter, Bryan M.; Winkler, Jay R.; Gray, Harry B.; Mueller, Astrid M.
Powering the planet with sustainable, carbon-neutral fuels affects every aspect of human life. Sunlight-driven water splitting is an attractive soln. to provide environmentally benign hydrogen fuel. Global scalability demands that all photoelectrode and catalyst materials consist of earth- abundant elements. The water oxidn. half reaction requires four coupled electron and proton transfer steps, for which robust and efficient electrocatalysts are needed. We have shown previously that [NiFe] - layered double hydroxide (LDH) nanosheets are highly active water oxidn. catalysts [Hunter, Blakemore, Deimund, Gray, Winkler, Mueller, J. Am. Chem. Soc. 2014, 136, 13118]. They were synthesized by pulsed laser ablation in liqs. (PLAL), a medium-throughput method...

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