Feb 16, 2015

Fast reaction with water dimers may limit the impact of the simplest Criegee intermediate on atmospheric chemistry [Also see Report by Chao et al.]
An intermediate in atmospheric ozone chemistry reacts more quickly with water dimers than with individual water molecules. [Also see Perspective by Okumura]
 Carbonyl oxides, or Criegee intermediates, are important transient species formed in the reactions of unsaturated hydrocarbons with ozone. Although direct detection of Criegee intermediates has recently been realized, the main atmospheric sink of Criegee intermediates remains unclear. We report ultraviolet absorption spectroscopic measurements of the lifetime of the simplest Criegee intermediate, CH2OO, at various relative humidity levels up to 85% at 298 kelvin. An extremely fast decay rate of CH2OO was observed at high humidity. The observed quadratic dependence of the decay rate on water concentration implied a predominant reaction with water dimer. On the basis of the water dimer equilibrium constant, the effective rate coefficient of the CH2OO + (H2O)2 reaction was determined to be 6.5 (±0.8) × 10−12 cubic centimeters per second. This work would help modelers to better constrain the atmospheric concentrations of CH2OO.
The traditional tools of physical organic chemistry benefit from modern data analysis techniques [Also see Research Article by Milo et al.]
Multidimensional analysis of a complex reaction offers predictive insight for rational catalyst optimization. [Also see Perspective by Lu]
 Knowledge of chemical reaction mechanisms can facilitate catalyst optimization, but extracting that knowledge from a complex system is often challenging. Here, we present a data-intensive method for deriving and then predictively applying a mechanistic model of an enantioselective organic reaction. As a validating case study, we selected an intramolecular dehydrogenative C-N coupling reaction, catalyzed by chiral phosphoric acid derivatives, in which catalyst-substrate association involves weak, noncovalent interactions. Little was previously understood regarding the structural origin of enantioselectivity in this system. Catalyst and substrate substituent effects were probed by means of systematic physical organic trend analysis. Plausible interactions between the substrate and catalyst that govern enantioselectivity were identified and supported experimentally, indicating that such an approach can afford an efficient means of leveraging mechanistic insight so as to optimize catalyst design.

Controlling the Size of Hot Injection Made Nanocrystals by Manipulating the Diffusion Coefficient of the Solute

Publication Date (Web): January 28, 2015 (Article)
DOI: 10.1021/ja509941g
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We investigate the relation between the chain length of ligands used and the size of the nanocrystals formed in the hot injection synthesis. With two different CdSe nanocrystal syntheses, we consistently find that longer chain carboxylic acids result in smaller nanocrystals with improved size dispersions. By combining a more in-depth experimental investigation with kinetic reaction simulations, we come to the conclusion that this size tuning is due to a change in the diffusion coefficient and the solubility of the solute. The relation between size tuning by the ligand chain length and the coordination of the solute by the ligands is further explored by expanding the study to amines and phosphine oxides. In line with the weak coordination of CdSe nanocrystals by amines, no influence of the chain length on the nanocrystals is found, whereas the size tuning brought about by phosphine oxides can be attributed to a solubility change. We conclude that the ligand chain length provides a practical handle to optimize the outcome of a hot injection synthesis in terms of size and size dispersion and can be used to probe the interaction between ligands and the actual solute.
 

Feb 11, 2015

The luminescence and electronic properties of inorganic nanocrystals depends on surface-layer structure
An initiator directs assembly of monomers by pulling them open to engage in external rather than internal hydrogen bonding.
Over the past decade, major progress in supramolecular polymerization has had a substantial effect on the design of functional soft materials. However, despite recent advances, most studies are still based on a preconceived notion that supramolecular polymerization follows a step-growth mechanism, which precludes control over chain length, sequence, and stereochemical structure. Here we report the realization of chain-growth polymerization by designing metastable monomers with a shape-promoted intramolecular hydrogen-bonding network. The monomers are conformationally restricted from spontaneous polymerization at ambient temperatures but begin to polymerize with characteristics typical of a living mechanism upon mixing with tailored initiators. The chain growth occurs stereoselectively and therefore enables optical resolution of a racemic monomer.
A long-sought reactive intermediate in hydrocarbon oxidation is observed via mass spectrometry.
Oxidation of organic compounds in combustion and in Earth’s troposphere is mediated by reactive species formed by the addition of molecular oxygen (O2) to organic radicals. Among the most crucial and elusive of these intermediates are hydroperoxyalkyl radicals, often denoted “QOOH.” These species and their reactions with O2 are responsible for the radical chain branching that sustains autoignition and are implicated in tropospheric autoxidation that can form low-volatility, highly oxygenated organic aerosol precursors. We report direct observation and kinetics measurements of a QOOH intermediate in the oxidation of 1,3-cycloheptadiene, a molecule that offers insight into both resonance-stabilized and nonstabilized radical intermediates. The results establish that resonance stabilization dramatically changes QOOH reactivity and, hence, that oxidation of unsaturated organics can produce exceptionally long-lived QOOH intermediates.
The strengthening of titanium is due to the interaction of oxygen solutes with the core of mobile screw dislocations.
Structural alloys are often strengthened through the addition of solute atoms. However, given that solute atoms interact weakly with the elastic fields of screw dislocations, it has long been accepted that solution hardening is only marginally effective in materials with mobile screw dislocations. By using transmission electron microscopy and nanomechanical characterization, we report that the intense hardening effect of dilute oxygen solutes in pure α-Ti is due to the interaction between oxygen and the core of screw dislocations that mainly glide on prismatic planes. First-principles calculations reveal that distortion of the interstitial sites at the screw dislocation core creates a very strong but short-range repulsion for oxygen that is consistent with experimental observations. These results establish a highly effective mechanism for strengthening by interstitial solutes.

Taking temperature at the nanoscale

  • Christian Colliex
Science 6 February 2015: 611-612.
A local probe technique can determine temperature with nanometer-scale resolution [Also see Report by Mecklenburg et al.]
Electron microscopy measurement of the bulk plasmon of aluminum provides an accurate temperature probe. [Also see Perspective by Colliex]
Modern microelectronic devices have nanoscale features that dissipate power nonuniformly, but fundamental physical limits frustrate efforts to detect the resulting temperature gradients. Contact thermometers disturb the temperature of a small system, while radiation thermometers struggle to beat the diffraction limit. Exploiting the same physics as Fahrenheit’s glass-bulb thermometer, we mapped the thermal expansion of Joule-heated, 80-nanometer-thick aluminum wires by precisely measuring changes in density. With a scanning transmission electron microscope and electron energy loss spectroscopy, we quantified the local density via the energy of aluminum’s bulk plasmon. Rescaling density to temperature yields maps with a statistical precision of 3 kelvin/hertz1/2, an accuracy of 10%, and nanometer-scale resolution. Many common metals and semiconductors have sufficiently sharp plasmon resonances to serve as their own thermometers.

Solvent-Induced Oriented Attachment Growth of Air-Stable Phase-Pure Pyrite FeS2 Nanocrystals

Publication Date (Web): February 3, 2015 (Communication)
DOI: 10.1021/ja512979y
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We report here the selective synthesis of air-stable phase-pure pyrite FeS2 nanocubes, spheroidal nanocrystals, and microspheres by solvent-induced oriented attachment (OA). It was found that the solvents could control the OA process and thus the morphologies of the products. Solvent exchange experiments and detailed Raman analysis revealed that 1-octanol contributed to the long-term stability of these pyrite nanomaterials.
 

Water-Soluble Fe(II)–H2O Complex with a Weak O–H Bond Transfers a Hydrogen Atom via an Observable Monomeric Fe(III)–OH

Publication Date (Web): January 22, 2015 (Article)
DOI: 10.1021/ja5068405
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Water-Soluble Fe(II)–H2O Complex with a Weak O–H Bond Transfers a Hydrogen Atom via an Observable Monomeric Fe(III)–OH

Department of Chemistry, University of Washington, Campus Box 351700, Seattle, Washington 98195-1700, United States
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja5068405
Publication Date (Web): January 22, 2015
Copyright © 2015 American Chemical Society

Abstract

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Understanding the metal ion properties that favor O–H bond formation versus cleavage should facilitate the development of catalysts tailored to promote a specific reaction, e.g., C–H activation or H2O oxidation. The first step in H2O oxidation involves the endothermic cleavage of a strong O–H bond (BDFE = 122.7 kcal/mol), promoted by binding the H2O to a metal ion, and by coupling electron transfer to proton transfer (PCET). This study focuses on details regarding how a metal ion’s electronic structure and ligand environment can tune the energetics of M(HO–H) bond cleavage. The synthesis and characterization of an Fe(II)–H2O complex, 1, that undergoes PCET in H2O to afford a rare example of a monomeric Fe(III)–OH, 7, is described. High-spin 7 is also reproducibly generated via the addition of H2O to {[FeIII(OMe2N4(tren))]2-(μ-O)}2+ (8). The O–H bond BDFE of Fe(II)–H2O (1) (68.6 kcal/mol) is calculated using linear fits to its Pourbaix diagram and shown to be 54.1 kcal/mol less than that of H2O and 10.9 kcal/mol less than that of [Fe(II)(H2O)6]2+. The O–H bond of 1 is noticeably weaker than the majority of reported Mn+(HxO–H) (M = Mn, Fe; n+ = 2+, 3+; x = 0, 1) complexes. Consistent with their relative BDFEs, Fe(II)–H2O (1) is found to donate a H atom to TEMPO, whereas the majority of previously reported Mn+–O(H) complexes, including [MnIII(SMe2N4(tren))(OH)]+ (2), have been shown to abstract H atoms from TEMPOH. Factors responsible for the weaker O–H bond of 1, such as differences in the electron-donating properties of the ligand, metal ion Lewis acidity, and electronic structure, are discussed.
 

Jan 30, 2015

Real-Time Measurement of the Vertical Binding Energy during the Birth of a Solvated Electron

Publication Date (Web): January 22, 2015 (Article)
DOI: 10.1021/ja511571y
 
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Using femtosecond time-resolved two-photon photoelectron spectroscopy, we determine (i) the vertical binding energy (VBE = 0.8 eV) of electrons in the conduction band in supported amorphous solid water (ASW) layers, (ii) the time scale of ultrafast trapping at pre-existing sites (22 fs), and (iii) the initial VBE (1.4 eV) of solvated electrons before significant molecular reorganization sets in. Our results suggest that the excess electron dynamics prior to solvation are representative for bulk ASW.
 

Jan 29, 2015

Crash program aims to teach computers to read journals and hatch new ideas.
The physics Nobel laureate Frank Wilczek has famously predicted that in 100 years, the best physicist will be a machine. Now the U.S. Defense Advanced Research Projects Agency (DARPA) is working toward that vision in a different arena: cancer research. Last summer, the agency launched a $45 million program called Big Mechanism, aimed at developing computer systems that will read research papers, integrate the information into a computer model of cancer mechanisms, and frame new hypotheses for flesh-and-blood scientists (or even other robots) to test—all by the end of 2017. 

Photochemical Nitrogen Conversion to Ammonia in Ambient Conditions with FeMoS-Chalcogels

Publication Date (Web): January 15, 2015 (Article)
DOI: 10.1021/ja512491v


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In nature, nitrogen fixation is one of the most important life processes and occurs primarily in microbial organisms containing enzymes called nitrogenases. These complex proteins contain two distinct subunits with different active sites, with the primary N2 binding site being a FeMoS core cluster that can be reduced by other nearby iron–sulfur clusters. Although nitrogen reduction to ammonia in biology does not require the absorption of light, there is considerable interest in developing catalyst materials that could drive the formation of ammonia from nitrogen photochemically. Here, we report that chalcogels containing FeMoS inorganic clusters are capable of photochemically reducing N2 to NH3 under white light irradiation, in aqueous media, under ambient pressure and room temperature. The chalcogels are composed of [Mo2Fe6S8(SPh)3]3+ and [Sn2S6]4– clusters in solution and have strong optical absorption, high surface area, and good aqueous stability. Our results demonstrate that light-driven nitrogen conversion to ammonia by MoFe sulfides is a viable process with implications in solar energy utilization and our understanding of primordial processes on earth.

Jan 28, 2015

Parahydrogen-Induced Polarization by Pairwise Replacement Catalysis on Pt and Ir Nanoparticles

Publication Date (Web): January 15, 2015 (Article)
DOI: 10.1021/ja511476n


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Pairwise and random addition processes are ordinarily indistinguishable in hydrogenation reactions. The distinction becomes important only when the fate of spin correlation matters, such as in parahydrogen-induced polarization (PHIP). Supported metal catalysts were not expected to yield PHIP signals given the rapid diffusion of H atoms on the catalyst surface and in view of the sequential stepwise nature of the H atom addition in the Horiuti–Polanyi mechanism. Thus, it seems surprising that supported metal hydrogenation catalysts can yield detectable PHIP NMR signals. Even more remarkably, supported Pt and Ir nanoparticles are shown herein to catalyze pairwise replacement on propene and 3,3,3-trifluoropropene. By simply flowing a mixture of parahydrogen and alkene over the catalyst, the scalar symmetrization order of the former is incorporated into the latter without a change in molecular structure, producing intense PHIP NMR signals on the alkene. An important indicator of the mechanism of the pairwise replacement is its stereoselectivity, which is revealed with the aid of density matrix spectral simulations. PHIP by pairwise replacement has the potential to significantly diversify the substrates that can be hyperpolarized by PHIP for biomedical utilization.
Publication Date (Web): January 8, 2015 (Perspective)
DOI: 10.1021/ja5112749


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Understanding the mechanisms of chemical reactions, especially catalysis, has been an important and active area of computational organic chemistry, and close collaborations between experimentalists and theorists represent a growing trend. This Perspective provides examples of such productive collaborations. The understanding of various reaction mechanisms and the insight gained from these studies are emphasized. The applications of various experimental techniques in elucidation of reaction details as well as the development of various computational techniques to meet the demand of emerging synthetic methods, e.g., C–H activation, organocatalysis, and single electron transfer, are presented along with some conventional developments of mechanistic aspects. Examples of applications are selected to demonstrate the advantages and limitations of these techniques. Some challenges in the mechanistic studies and predictions of reactions are also analyzed.

Introducing Amphiphilicity to Noble Metal Nanoclusters via Phase-Transfer Driven Ion-Pairing Reaction

Publication Date (Web): January 13, 2015 (Article)
DOI: 10.1021/jacs.5b00090

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Amphiphilicity is a surface property that has yet to be explored for the noble metal nanoclusters (NCs). This article shows how amphiphilicity may be added to sub-2-nm metal NCs by patching hydrophilic NCs (e.g., Au25(MHA)18 NCs where MHA is 6-mercaptohexanoic acid) with hydrophobic cations (e.g., cetyltrimethylammonium ion, CTA+) to about half of a monolayer coverage. Specifically we demonstrate the preparation of amphiphilic Au25(MHA)18@xCTA NCs (x = 6–9 where x is the number of CTA+ per NC) by the phase-transfer (PT) driven ion-paring reaction between CTA+ and −COO (derived from the deprotonation of the terminal carboxyl group of MHA). Due to the coexistence of flexible hydrophilic MHA and hydrophobic MHA···CTA ligands in comparable amounts on the NC surface, the Au25(MHA)18@xCTA NCs (x = 6–9) exhibit good amphiphilicity, which enabled them to dissolve in solvents with distinctly different polarities and to self-assemble like a molecular amphiphile. Consequently, the amphiphilic Au25(MHA)18@xCTA NCs (x = 6–9) could self-organize into stacked bilayers at the air–liquid interface, similar to the formation of lyotropic liquid crystalline phases by common surfactants. The good solubility and molecular-amphiphile-like self-assembly properties can significantly increase the utility of noble metal NCs in basic and applied research.
Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS2 Nanosheets–Carbon Nanotubes for Hydrogen Evolution Reaction
Publication Date (Web): January 14, 2015 (Article)
DOI: 10.1021/ja511572q
 
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Hydrogen evolution reaction (HER) from water through electrocatalysis using cost-effective materials to replace precious Pt catalysts holds great promise for clean energy technologies. In this work we developed a highly active and stable catalyst containing Co doped earth abundant iron pyrite FeS2 nanosheets hybridized with carbon nanotubes (Fe1–xCoxS2/CNT hybrid catalysts) for HER in acidic solutions. The pyrite phase of Fe1–xCoxS2/CNT was characterized by powder X-ray diffraction and absorption spectroscopy. Electrochemical measurements showed a low overpotential of ∼0.12 V at 20 mA/cm2, small Tafel slope of ∼46 mV/decade, and long-term durability over 40 h of HER operation using bulk quantities of Fe0.9Co0.1S2/CNT hybrid catalysts at high loadings (∼7 mg/cm2). Density functional theory calculation revealed that the origin of high catalytic activity stemmed from a large reduction of the kinetic energy barrier of H atom adsorption on FeS2 surface upon Co doping in the iron pyrite structure. It is also found that the high HER catalytic activity of Fe0.9Co0.1S2 hinges on the hybridization with CNTs to impart strong heteroatomic interactions between CNT and Fe0.9Co0.1S2. This work produces the most active HER catalyst based on iron pyrite, suggesting a scalable, low cost, and highly efficient catalyst for hydrogen generation.
 

“Darker-than-Black” PbS Quantum Dots: Enhancing Optical Absorption of Colloidal Semiconductor Nanocrystals via Short Conjugated Ligands

Publication Date (Web): January 9, 2015 (Article)
DOI: 10.1021/ja510739q


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Colloidal quantum dots (QDs) stand among the most attractive light-harvesting materials to be exploited for solution-processed optoelectronic applications. To this aim, quantitative replacement of the bulky electrically insulating ligands at the QD surface coming from the synthetic procedure is mandatory. Here we present a conceptually novel approach to design light-harvesting nanomaterials demonstrating that QD surface modification with suitable short conjugated organic molecules permits us to drastically enhance light absorption of QDs, while preserving good long-term colloidal stability. Indeed, rational design of the pendant and anchoring moieties, which constitute the replacing ligand framework leads to a broadband increase of the optical absorbance larger than 300% for colloidal PbS QDs also at high energies (>3.1 eV), which could not be predicted by using formalisms derived from effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/QD orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical band gap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms, which mix with the highest occupied states of the QDs. More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components.

Jan 26, 2015

Publication Date (Web): January 10, 2015 (Article)
DOI: 10.1021/ja5100405

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A series of π-bound Mo−quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal–arene interactions between Mo and the π system of the quinonoid fragment. The Mo–catechol complex (2a) was found to react within minutes with 0.5 equiv of O2 to yield a Mo–quinone complex (3), H2O, and CO. Si- and B-protected Mo–catecholate complexes also react with O2 to yield 3 along with (R2SiO)n and (ArBO)3 byproducts, respectively. Formally, the Mo–catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O2 oxygens. Unreactive by itself, the Mo–dimethyl catecholate analogue reduces O2 in the presence of added Lewis acid, B(C6F5)3, to generate a MoI species and a bis(borane)-supported peroxide dianion, [[(F5C6)3B]2O22–], demonstrating single-electron-transfer chemistry from Mo to the O2 moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O2 with pendant acids weaker than B(C6F5)3. Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O2 is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O2 species.
Publication Date (Web): January 8, 2015 (Article)
DOI: 10.1021/ja5119545
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Structures and reactivities of gaseous Fe(CN)63–(H2O)n were investigated using infrared photodissociation (IRPD) kinetics, spectroscopy, and computational chemistry in order to gain insights into how water stabilizes highly charged anions. Fe(CN)63–(H2O)8 is the smallest hydrated cluster produced by electrospray ionization, and blackbody infrared dissociation of this ion results in loss of an electron and formation of smaller dianion clusters. Fe(CN)63–(H2O)7 is produced by the higher activation conditions of IRPD, and this ion dissociates both by loss of an electron and by loss of a water molecule. Comparisons of IRPD spectra to those of computed low-energy structures for Fe(CN)63–(H2O)8 indicate that water molecules either form two hydrogen bonds to the trianion or form one hydrogen bond to the ion and one to another water molecule. Magic numbers are observed for Fe(CN)63–(H2O)n for n between 58 and 60, and the IRPD spectrum of the n = 60 cluster shows stronger water molecule hydrogen-bonding than that of the n = 61 cluster, consistent with the significantly higher stability of the former. Remarkably, neither cluster has a band corresponding to a free O–H stretch, and this band is not observed for clusters until n ≥ 70, indicating that this trianion significantly affects the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells. These results provide new insights into the role of water in stabilizing high-valency anions and how these ions can pattern the structure of water even at long distances.
 

Jan 23, 2015

Facet-Dependent Photoelectrochemical Performance of TiO2 Nanostructures: An Experimental and Computational Study

Publication Date (Web): January 6, 2015 (Article)
DOI: 10.1021/ja5111078
 
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The behavior of crystalline nanoparticles depends strongly on which facets are exposed. Some facets are more active than others, but it is difficult to selectively isolate particular facets. This study provides fundamental insights into photocatalytic and photoelectrochemical performance of three types of TiO2 nanoparticles with predominantly exposed {101}, {010}, or {001} facets, where 86–99% of the surface area is the desired facet. Photodegradation of methyl orange reveals that {001}-TiO2 has 1.79 and 3.22 times higher photocatalytic activity than {010} and {101}-TiO2, respectively. This suggests that the photochemical performance is highly correlated with the surface energy and the number of under-coordinated surface atoms. In contrast, the photoelectrochemical performance of the faceted TiO2 nanoparticles sensitized with the commercially available MK-2 dye was highest with {010}-TiO2 which yielded an overall cell efficiency of 6.1%, compared to 3.2% for {101}-TiO2 and 2.6% for {001}-TiO2 prepared under analogous conditions. Measurement of desorption kinetics and accompanying computational modeling suggests a stronger covalent interaction of the dye with the {010} and {101} facets compared with the {001} facet. Time-resolved THz spectroscopy and transient absorption spectroscopy measure faster electron injection dynamics when MK-2 is bound to {010} compared to other facets, consistent with extensive computational simulations which indicate that the {010} facet provides the most efficient and direct pathway for interfacial electron transfer. Our experimental and computational results establish for the first time that photoelectrochemical performance is dependent upon the binding energy of the dye as well as the crystalline structure of the facet, as opposed to surface energy alone.