May 4, 2015

Oxygenation of Methylarenes to Benzaldehyde Derivatives by a Polyoxometalate Mediated Electron Transfer–Oxygen Transfer Reaction in Aqueous Sulfuric Acid
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.5b01745
Publication Date (Web): April 22, 2015
Copyright © 2015 American Chemical Society

The synthesis of benzaldehyde derivatives by oxygenation of methylarenes is of significant conceptual and practical interest because these compounds are important chemical intermediates whose synthesis is still carried out by nonsustainable methods with very low atom economy and formation of copious amounts of waste. Now an oxygenation reaction with a 100% theoretical atom economy using a polyoxometalate oxygen donor has been found. The product yield is typically above 95% with no “overoxidation” to benzoic acids; H2 is released by electrolysis, enabling additional reaction cycles. An electrocatalytic cycle is also feasible. This reaction is possible through the use of an aqueous sulfuric acid solvent, in an aqueous biphasic reaction mode that also allows simple catalyst recycling and recovery. The solvent plays a key role in the reaction mechanism by protonating the polyoxometalate thereby enabling the activation of the methylarenes by an electron transfer process. After additional proton transfer and oxygen transfer steps, benzylic alcohols are formed that further react by an electron transfer–proton transfer sequence forming benzaldehyde derivatives.

May 1, 2015

Photocatalytic Activity of Au/TiO2 Photocatalysts for H2 Evolution: Role of the Au Nanoparticles as a Function of the Irradiation Wavelength

  1. Marco Serra, 
  2. Dr. Josep Albero and
  3. Prof. Hermenegildo García*

DOI: 10.1002/cphc.201500141

Abstract

An investigation of hydrogen production with a series of Au/TiO2 photocatalysts reveals that the Au nanoparticles play different roles depending on the wavelength of the light irradiation. Under visible-light irradiation, the photoactivity is primarily controlled by the intensity of the Au surface plasmon band, whereas under UV irradiation the Au nanoparticles act as co-catalysts with TiO2.
Thumbnail image of graphical abstract

Serra, M., Albero, J. and García, H. (2015), Photocatalytic Activity of Au/TiO2 Photocatalysts for H2Evolution: Role of the Au Nanoparticles as a Function of the Irradiation Wavelength. ChemPhysChem. doi: 10.1002/cphc.201500141

Apr 30, 2015

Room-temperature sol–gel synthesis of organic ligand-capped ZnO nanoparticles

Probing the energetics of organic–nanoparticle interactions of ethanol on calcite

  1. Alexandra Navrotsky1
  1. Contributed by Alexandra Navrotsky, March 25, 2015 (sent for review November 3, 2014)

Significance

Organic ligand–inorganic nanoparticle (NP) interactions are crucial in both natural and engineering conditions. This paper reports a rich energetics of organic–NP binding as a function of molecular coverage for ethanol–nanocalcite system. A stepwise, yet gradually and continuously evolved energetics from weak associating to strong bonding to classical capping is revealed. Such information may reinforce our understanding of complex phenomena at organic–NP interfaces, and may also aid exploratory material scientists by providing solid, fundamental thermodynamic insights.

Apr 28, 2015

Energy Level Alignment at Titanium Oxide–Dye Interfaces: Implications for Electron Injection and Light Harvesting


Energy Level Alignment at Titanium Oxide–Dye Interfaces: Implications for Electron Injection and Light Harvesting by Laurent Lasser, Enrico Ronca, Mariachiara Pastore, Filippo De Angelis, Jérôme Cornil, Roberto Lazzaroni and David Beljonne via The Journal of Physical Chemistry C: Latest Articles (ACS Publications) http://ift.tt/1JON9FP http://pubs.acs.org
The location of hydrophilic and hydrophobic regions on tetrahedral nanoparticles determines how they pack together into a variety of structures [Also see Report by Huang et al.]
Tetrahedrally connected nanoparticles self-assemble into complex ordered phases. [Also see Perspective by Yang]
 Self-assembly of rigid building blocks with explicit shape and symmetry is substantially influenced by the geometric factors and remains largely unexplored. We report the selective assembly behaviors of a class of precisely defined, nanosized giant tetrahedra constructed by placing different polyhedral oligomeric silsesquioxane (POSS) molecular nanoparticles at the vertices of a rigid tetrahedral framework. Designed symmetry breaking of these giant tetrahedra introduces precise positional interactions and results in diverse selectively assembled, highly ordered supramolecular lattices including a Frank-Kasper A15 phase, which resembles the essential structural features of certain metal alloys but at a larger length scale. These results demonstrate the power of persistent molecular geometry with balanced enthalpy and entropy in creating thermodynamically stable supramolecular lattices with properties distinct from those of other self-assembling soft materials.
A catalytic oxidation points the way toward a more efficient method for making the key ingredient in Styrofoam.
Rising global demand for fossil resources has prompted a renewed interest in catalyst technologies that increase the efficiency of conversion of hydrocarbons from petroleum and natural gas to higher-value materials. Styrene is currently produced from benzene and ethylene through the intermediacy of ethylbenzene, which must be dehydrogenated in a separate step. The direct oxidative conversion of benzene and ethylene to styrene could provide a more efficient route, but achieving high selectivity and yield for this reaction has been challenging. Here, we report that the Rh catalyst (FlDAB)Rh(TFA)(η2–C2H4) [FlDAB is N,N′-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; TFA is trifluoroacetate] converts benzene, ethylene, and Cu(II) acetate to styrene, Cu(I) acetate, and acetic acid with 100% selectivity and yields ≥95%. Turnover numbers >800 have been demonstrated, with catalyst stability up to 96 hours.
Publication Date (Web): April 13, 2015 (Article)
DOI: 10.1021/jacs.5b02128
JACS ASAP
Abstract Image
We present a first-principle computational modeling investigation, based on density functional theory (DFT) and time-dependent DFT, on the structural, electronic, optical, and charge generation properties of the semiconductor/dye/catalyst heterointerfaces in a prototypical dye-sensitized photoanode for water oxidation. The investigated architecture comprises a Ru(II) dye-sensitized TiO2 substrate tethered to an IrO2 nanoparticle catalyst. Our realistic modeling strategy and quantitative analysis of the relevant interfacial hole/electron transfer reactions indicates the slow hole injection into IrO2 and the fast dye excited-state quenching to IrO2 as the primary sources of the relatively poor cell efficiency experimentally observed. On the basis of this atomistic and electronic structure information, we propose and computationally test, against a prototype dye, a new class of Ru(II) sensitizers, which show potentially improved photoelectrochemical performances. This study constitutes a first step toward the computer-assisted design of new and more efficient materials for solar fuels production through dye-sensitized photoelectrochemical cells.
 

Determination of Hydrogen Bond Structure in Water versus Aprotic Environments To Test the Relationship Between Length and Stability

Publication Date (Web): April 14, 2015 (Article)
DOI: 10.1021/ja512980h
JACS ASAP
Abstract Image
Hydrogen bonds profoundly influence the architecture and activity of biological macromolecules. Deep appreciation of hydrogen bond contributions to biomolecular function thus requires a detailed understanding of hydrogen bond structure and energetics and the relationship between these properties. Hydrogen bond formation energies (ΔGf) are enormously more favorable in aprotic solvents than in water, and two classes of contributing factors have been proposed to explain this energetic difference, focusing respectively on the isolated and hydrogen-bonded species: (I) water stabilizes the dissociated donor and acceptor groups much better than aprotic solvents, thereby reducing the driving force for hydrogen bond formation; and (II) water lengthens hydrogen bonds compared to aprotic environments, thereby decreasing the potential energy within the hydrogen bond. Each model has been proposed to provide a dominant contribution to ΔGf, but incisive tests that distinguish the importance of these contributions are lacking. Here we directly test the structural basis of model II. Neutron crystallography, NMR spectroscopy, and quantum mechanical calculations demonstrate that O–H···O hydrogen bonds in crystals, chloroform, acetone, and water have nearly identical lengths and very similar potential energy surfaces despite ΔGf differences >8 kcal/mol across these solvents. These results rule out a substantial contribution from solvent-dependent differences in hydrogen bond structure and potential energy after association (model II) and thus support the conclusion that differences in hydrogen bond ΔGf are predominantly determined by solvent interactions with the dissociated groups (model I). These findings advance our understanding of universal hydrogen-bonding interactions and have important implications for biology and engineering.
 
Publication Date (Web): April 28, 2015 (Article)
DOI: 10.1021/ja512388n
JACS ASAP
 
Abstract Image
Tyramine β-monooxygenase (TβM) belongs to a family of physiologically important dinuclear copper monooxygenases that function with a solvent-exposed active site. To accomplish each enzymatic turnover, an electron transfer (ET) must occur between two solvent-separated copper centers. In wild-type TβM, this event is too fast to be rate limiting. However, we have recently shown [Osborne, R. L.; et al. Biochemistry 2013, 52, 1179] that the Tyr216Ala variant of TβM leads to rate-limiting ET. In this study, we present a pH–rate profile study of Tyr216Ala, together with deuterium oxide solvent kinetic isotope effects (KIEs). A solvent KIE of 2 on kcat is found in a region where kcat is pH/pD independent. As a control, the variant Tyr216Trp, for which ET is not rate determining, displays a solvent KIE of unity. We conclude, therefore, that the observed solvent KIE arises from the rate-limiting ET step in the Tyr216Ala variant, and show how small solvent KIEs (ca. 2) can be fully accommodated from equilibrium effects within the Marcus equation. To gain insight into the role of the enzyme in the long-range ET step, a temperature dependence study was also pursued. The small enthalpic barrier of ET (Ea = 3.6 kcal/mol) implicates a significant entropic barrier, which is attributed to the requirement for extensive rearrangement of the inter-copper environment during PCET catalyzed by the Tyr216Ala variant. The data lead to the proposal of a distinct inter-domain pathway for PCET in the dinuclear copper monooxygenases.
 

Apr 27, 2015

Structural Characteristics and Eutaxy in the Photo-Deposited Amorphous Iron Oxide Oxygen Evolution Catalyst


Structural Characteristics and Eutaxy in the Photo-Deposited Amorphous Iron Oxide Oxygen Evolution Catalyst by Joshua A. Kurzman, Kevan E. Dettelbach, Andrew J. Martinolich, Curtis P. Berlinguette and James R. Neilson via Chemistry of Materials: Latest Articles (ACS Publications) http://ift.tt/1GnzrMH http://pubs.acs.org

Reversible Chemochromic MoO3 Nanoribbons through Zerovalent Metal Intercalation

Abstract Image

Mengjing Wang and Kristie J. Koski *
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
ACS Nano, 2015, 9 (3), pp 3226–3233
DOI: 10.1021/acsnano.5b00336

Molybdenum trioxide (α-MoO3) is a 2D layered oxide with use in electrochromic and photochromic devices owing to its ability to reversibly change color between transparent and light blue with electrochemical or hydrogen intercalation. Despite its significant application potential, MoO3 performance is largely limited by the destructiveness of these intercalation techniques, insignificant coloration, and slow color response. We demonstrate a reversible chemochromic method, using intercalation of zerovalent metals into α-MoO3 nanoribbons (Sn, ∼2 at. %; Co, ∼4 at. %), to chemically alter MoO3 from transparent white to a deep blue indigo, resulting in enhanced coloration and chemically tunable optical properties. We present two strategies to reversibly tune the color response of MoO3 nanoribbons. Chromism can be reversed (i) by complete oxidative deintercalation with hydrogen peroxide or iodine or (ii) through a temperature-driven disorder–order phase transition of the intercalated zerovalent metal.

Revealing Energy Level Structure of Individual Quantum Dots by Tunneling Rate Measured by Single-Electron Sensitive Electrostatic Force Spectroscopy

Abstract Image
Antoine Roy-Gobeil , Yoichi Miyahara *, and Peter Grutter
Department of Physics, McGill University, 3600 rue University, Montreal, Quebec H3A2T8, Canada
Nano Lett., 2015, 15 (4), pp 2324–2328
DOI: 10.1021/nl504468a

We present theoretical and experimental studies of the effect of the density of states of a quantum dot (QD) on the rate of single-electron tunneling that can be directly measured by electrostatic force microscopy (e-EFM) experiments. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the dot, which is detected through its back-action on the capacitavely coupled cantilever, depends on the tunneling rate of the QD to a back-electrode. The density of states of the QD can therefore be measured through its effect on the energy dependence of tunneling rate. We present experimental data on individual 5 nm colloidal gold nanoparticles that exhibit a near continuous density of state at 77 K. In contrast, our analysis of already published data on self-assembled InAs QDs at 4 K clearly reveals discrete degenerate energy levels.

Polyimide Dendrimers Containing Multiple Electron Donor–Acceptor Units and Their Unique Photophysical Properties

Polyimide Dendrimers Containing Multiple Electron Donor–Acceptor Units and Their Unique Photophysical Properties


Angew. Chem. Int. Ed. 2015, 54, 1-6.

Dr. Francesca M. Toma, Dr. Fausto Puntoriero, Dr. Toan V. Pho, Marcello La Rosa, Dr. Young-Si Jun, Dr. Bertrand J. Tremolet de Villers, James Pavlovich, Prof. Galen D. Stucky, Prof. Sebastiano Campagna and Prof. Fred Wudl.

Department of Chemistry and Biochemistry, Department of Materials, Center for Polymers and Organic Solids, University of California, Santa Barbara, CA.

Dipartimento di Scienze Chimiche, Università di Messina and Centro Interuniversitario per la Conversione Chimica dell'Energia Solare (SOLARCHEM, Sezione di Messina), Messina,Italy.

Abstract:

A high-yielding synthesis of a series of polyimide dendrimers, including decacyclene- and perylene-containing dendrimer D6, in which two types of polyimide dyes are present, is reported. In these constructs, the branching unit is represented by trisphenylamine, and the solubilizing chains by N-9-heptadecanyl-substituted perylene diimides. The photophysical properties of the dendrimers have been studied by absorption, steady-state, and time-resolved emission spectroscopy and pump–probe transient absorption spectroscopy. Photoinduced charge-separated (CS) states are formed on the femtosecond timescale upon visible excitation. In particular, in D6, two different CS states can be formed, involving different subunits that decays independently with different lifetimes (ca. 10–100 ps).





Apr 24, 2015

CO2-free electric power circulation via direct charge and discharge using the glycolic acid/oxalic acid redox couple

R. Watanabe,ab   M. Yamauchi,*ab   M. Sadakiyo,ab   R. Abebc and  T. Takeguchibd  
Energy Environ. Sci., 2015, Advance Article

DOI: 10.1039/C5EE00192G
Received 20 Jan 2015, Accepted 24 Feb 2015
First published online 24 Feb 2015



The establishment of an efficient electric power distribution method is the key to realising a sustainable society driven by renewable-energy-based electricity, such as solar photovoltaics, wind turbine, and wave electricity, in view of supply instability. Here, we demonstrate an electric power circulation method that does not emit CO2 and is based on the glycolic acid (GC)/oxalic acid (OX) redox couple. Direct electric power storage in GC ensures considerably high energy density storage and good transportability through OX electroreduction with significantly high selectivity (>98%) using pure anatase-type titania (TiO2) spheres under mild conditions in the potential region of −0.5 to −0.7 V vs. the RHE at 50 °C. The most desirable characteristic of this electroreduction is the suppression of hydrogen evolution even in acidic aqueous media (Faraday efficiency of 70–95%, pH 2.1). We also successfully generated power without CO2emissions via selective electrooxidation of GC with an alkaline fuel cell.

A quantitative analysis of the efficiency of solar-driven water-splitting device designs based on tandem photoabsorbers patterned with islands of metallic electrocatalysts

Hydrogens detected by subatomic resolution protein crystallography in a [NiFe] hydrogenase

Link 

Abstract: The enzyme hydrogenase reversibly converts dihydrogen to protons and electrons at a metal catalyst1. The location of the abundant hydrogens is of key importance for understanding structure and function of the protein2, 3, 4, 5, 6. However, in protein X-ray crystallography the detection of hydrogen atoms is one of the major problems, since they display only weak contributions to diffraction and the quality of the single crystals is often insufficient to obtain sub-ångström resolution7. Here we report the crystal structure of a standard [NiFe] hydrogenase (~91.3 kDa molecular mass) at 0.89 Å resolution. The strictly anoxically isolated hydrogenase has been obtained in a specific spectroscopic state, the active reduced Ni-R (subform Ni-R1) state. The high resolution, proper refinement strategy and careful modelling allow the positioning of a large part of the hydrogen atoms in the structure. This has led to the direct detection of the products of the heterolytic splitting of dihydrogen into a hydride (H) bridging the Ni and Fe and a proton (H+) attached to the sulphur of a cysteine ligand. The Ni–H and Fe–H bond lengths are 1.58 Å and 1.78Å, respectively. Furthermore, we can assign the Fe–CO and Fe–CN ligands at the active site, and can obtain the hydrogen-bond networks and the preferred proton transfer pathway in the hydrogenase. Our results demonstrate the precise comprehensive information available from ultra-high-resolution structures of proteins as an alternative to neutron diffraction and other methods such as NMR structural analysis.

Structure of [lsqb]NiFe[rsqb] hydrogenase from D. vulgaris Miyazaki F and proposed catalytic cycle. 
 

Apr 23, 2015

Determining the Oxidation State of Small, Hydroxylated Metal-Oxide Nanoparticles with Infrared Absorption Spectroscopy


Determining the Oxidation State of Small, Hydroxylated Metal-Oxide Nanoparticles with Infrared Absorption Spectroscopy by Xing Huang and Matthew J. Beck via Chemistry of Materials: Latest Articles (ACS Publications) http://ift.tt/1cE95cx http://pubs.acs.org