[show abstract][hide abstract] ABSTRACT: The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(μ-acac-O,O,C^3)-(acac-O,O)_2]_2 and [R-Ir(acac-O,O)_2(L)] (R = acetylacetonato, CH_3, CH_2CH_3, Ph, or CH_2CH_2Ph, and L = H_2O or pyridine) Ir(III) complexes was studied by experimental methods. The system is selective for generating the anti-Markovnikov product of linear alkylarenes (61 : 39 for benzene + propylene and 98 : 2 for benzene + styrene). The reaction mechanism was found to follow a rate law with first-order dependence on benzene and catalyst, but a non-linear dependence on olefin. ^(13)C-labelling studies with CH_3^(13)CH_2-Ir-Py showed that reversible β-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the ^(13)C-label from the α to β-positions was found to be slower than the C–H activation of benzene (and thus formation of ethane and Ph-d_5-Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and β-hydride elimination (k_(CH): k_β) of 0.5. The comparable magnitude of these rates suggests a common rate determining transition state/intermediate, which has been shown previously with B3LYP density functional theory (DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O)_2-Sol (where Sol refers to coordinated solvent). This species then undergoes trans to cis isomerization of the acetylacetonato ligand to yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the regioselective and rate determining step), and then activation of the C–H bond of an incoming benzene to generate the product and regenerate the catalyst.
[show abstract][hide abstract] ABSTRACT: We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube (CNT) composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the “phase change” observed in experiments, indicating that such phase change is chemical in nature. The elastic–plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk response of resin polymer.
Journal of Applied Physics 01/2011; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: We evaluated the rheological characteristics of ι- and κ-Carrageenan in aqueous solutions. Viscosities strongly increased with increasing polymer concentration or salinity. Monovalent Na^+ cations were more effective in increasing viscosity than divalent Ca^(2+) cations. A more complex brine containing Na^+, Mg^(2+) and Ca^(2+) cations also showed high viscosities at a high salinity. We observed shear thinning behavior for these polysaccharide solutions. We explain these rheological phenomena with molecular processes, specifically the conformation change of the ι-Carrageenan from random coil to double helix. In the context of enhanced oil recovery, ι-Carrageenan solutions can reach much higher viscosities at high salinities than standard polyacrylamide solutions. Moreover, Carrageenans are renewable, nontoxic, green substances.
[show abstract][hide abstract] ABSTRACT: We use grand canonical Monte Carlo simulations with first principles based force fields to show that alkali metal (Li^+, Na^+, and K^+)-doped zeolitic imidazolate frameworks (ZIFs) lead to significant improvement of H_2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H_2, Na-ZIF-70 to 2.19 wt % H_2, and K-ZIF-70 to 1.62 wt % H_2, much higher than 0.74 wt % H_2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H_2 binding energies to the dopants. Moreover, the total H_2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H_2 uptake, which is the difference between the total adsorption at the charging pressure (say 100 bar) and the discharging pressure (say 5 bar), is the important factor for practical on-board hydrogen storage in vehicles. We show that delivery H_2 uptake leads to Na-ZIF-70 (1.37 wt %) > K-ZIF-70 (1.25 wt %) > Li-ZIF-70 (1.07 wt %) > ZIF-70 (0.68 wt %), which is different from the trend from the total and excess uptake. Moreover, the delivery uptake increases with increasing temperatures (i.e., 298 K > 273 K > 243 K)! To achieve high delivery H_2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H_2 binding energy needs not always lead to higher delivery H_2 uptake.
[show abstract][hide abstract] ABSTRACT: Selective catalysts that activate small molecules such as hydrocarbons, dioxygen, water, carbon dioxide and dihydrogen are central to new technologies for the use of alternative energy sources. For example, controlled hydrocarbon functionalization can lead to high impact technologies, but such catalysts require a level of molecular control beyond current means. The Center for Catalytic Hydrocarbon Functionalization facilitates collaborations among research groups in catalysis, materials, electrochemistry, bioinorganic chemistry and quantum mechanics to develop, validate and optimize new methods to rearrange the bonds of hydrocarbons, activate and transform water and carbon dioxide, implement enzymatic strategies into synthetic systems and design optimal environments for catalysis.
[show abstract][hide abstract] ABSTRACT: A novel memory device based on an electrically driven molecular rotor was fabricated and demonstrated to have bistable switching effects. The device showed an on/off ratio of approximately 10^4, a read window of about 2.5 V, and retention performance of greater than 10^4 s. The analysis of the device I–V characteristics suggests the source of the observed switching effects to be the redox-induced ligand rotation around the copper metal center, which is consistent with the observed temperature dependence of the switching behavior. This organic monolayer device holds a potential for nonvolatile high-density memory applications due to its scalability and reduced cost.
IEEE Electron Device Letters 10/2010; · 2.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: Tetrahedrally coordinated hydrogen-free amorphous diamond-like carbon coating (denoted as ta-C) presents ultralow friction under boundary lubrication conditions at 80 °C in presence of OH-containing molecules. To understand the mechanism of ultralow friction, we performed gas-phase lubrication experiments followed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses and this using two simple molecules: deuterated glycerol and hydrogen peroxide. The experiments were complemented by computer simulations using the ReaxFF reactive force field. These simulations suggest a ta-C surface rich in sp^2 carbon with some reactive sp^1 carbon atoms, in agreement with previous energy filtered transmission electron microscopy (EFTEM) results. Sliding simulations show that the carbon surface atoms react with glycerol and hydrogen peroxide to form OH-termination. Moreover, the hydroxylation is then followed by the chemical dissociation of some of the glycerol molecules leading to the formation of water. This is in agreement with the secondary ion mass spectrometry (SIMS) analyses and mass spectrometer results obtained with gas-phase lubrication experiments with the same molecules. Both experimental and computer simulations strongly suggest that the hydroxylation of the carbon surface is at the origin of ultralow friction together with the formation of water-rich film in the sliding interface.
[show abstract][hide abstract] ABSTRACT: The mechanism of benzene C−H bond activation by [Ir(μ-acac-O,O,C^3)(acac-O,O)(OAc)]_2 (4) and [Ir(μ-acac-O,O,C^3)(acac-O,O)(TFA)]_2 (5) complexes (acac = acetylacetonato, OAc = acetate, and TFA = trifluoroacetate) was studied experimentally and theoretically. Hydrogen−deuterium (H/D) exchange between benzene and CD_(3)COOD solvent catalyzed by 4 (ΔH^‡ = 28.3 ± 1.1 kcal/mol, ΔS^‡ = 3.9 ± 3.0 cal K^(−1) mol^(−1)) results in a monotonic increase of all benzene isotopologues, suggesting that once benzene coordinates to the iridium center, there are multiple H/D exchange events prior to benzene dissociation. B3LYP density functional theory (DFT) calculations reveal that this benzene isotopologue pattern is due to a rate-determining step that involves acetate ligand dissociation and benzene coordination, which is then followed by heterolytic C−H bond cleavage to generate an iridium-phenyl intermediate. A synthesized iridium-phenyl intermediate was also shown to be competent for H/D exchange, giving similar rates to the proposed catalytic systems. This mechanism nicely explains why hydroarylation between benzene and alkenes is suppressed in the presence of acetic acid when catalyzed by [Ir(μ-acac-O,O,C^3)(acac-O,O)(acac-C^3)]_2 (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF_(3)COOD solvent catalyzed by 5 (ΔH^‡ = 15.3 ± 3.5 kcal/mol, ΔS^‡ = −30.0 ± 5.1 cal K^(−1) mol^(−1)) results in significantly elevated H/D exchange rates and the formation of only a single benzene isotopologue, (C_(6)H_(5)D). DFT calculations show that this is due to a change in the rate-determining step. Now equilibrium between coordinated and uncoordinated benzene precedes a single rate-determining heterolytic C−H bond cleavage step.
[show abstract][hide abstract] ABSTRACT: We investigate four different types of surfactants for effectiveness in tertiary oil recovery (TOR). The selected surfactant formulations were tested for enhanced oil recovery using coreflood tests on Berea sandstones. In addition to the corefloods, one sandpack surfactant flood was performed. The porous media were conditioned to residual waterflood oil saturation prior to surfactant slug injection. This was followed by polymer drive slug injection, and incremental oil recovery was measured against time.
The tested formulations were selected after an extensive research effort including measuring interfacial tensions (IFT) and adsorption behavior on kaolinite clay. Effective were low 1-naphthol concentrations dissolved in 1-butanol in alkyl polyglycoside surfactant formulations which led to significant additional incremental oil recovery (40% TOR) due to dramatic reductions in IFT. Three other types of surfactants in this study include:
• a di-tridecyl sulfosuccinic acid ester,
• coconut diethanolamide, and
• alkylpropoxy sulfate sodium salts
which led to TOR of 15%, 75% and 35–50%, respectively.
These results indicate that a wide variety of surfactants can meet the technical requirements as enhanced oil recovery (EOR) agents.
[show abstract][hide abstract] ABSTRACT: The degree of elasticity for the impact of a particle with a rigid wall is normally characterized with the restitution parameter, R. We examine such impact behavior of Cu nanoparticles with molecular dynamics simulations, for different particle sizes (1–15 nm in radius) and impact velocities (25–200 m s^(−1)). The impact can be ultra-elastic (R > 1) or inelastic (R < 1). Ultra-elastic or inelastic impact may occur for the smallest nanoparticles soly due to fluctuations, and the impact is inelastic but can be highly elastic (R ~ 0.9–1) for larger sizes. R decreases with increasing size and impact velocity in general. Impact-induced structure transitions (e.g., dislocations) can be reversible and induce irreversible heating regardless of their reversibility. Such heating along with remnant plasticity is the key mechanism for impact inelasticity. Inelastic impact may occur with little remnant plasticity.
[show abstract][hide abstract] ABSTRACT: We measured interfacial tensions (IFT) of aqueous alkyl polyglucoside (APG) systems formulated with sorbitan ester-type cosurfactants against n-octane. The study focused on low to ultra-low IFT systems which are relevant for enhanced oil recovery (EOR). In addition, we measured equilibrium adsorption concentrations of these surfactants and cosurfactants onto kaolinite clay, commonly found in oil reservoirs. We present one surfactant EOR laboratory flood experiment with one selected APG-sorbitan ester formulation with which we recovered 94% of initial oil in place (IOIP).
[show abstract][hide abstract] ABSTRACT: The essential components of the Stoddart−Heath-type rotaxane molecular switch tunnel junction devices are the aromatic shuttle and stations, which are attached to other components, such as linkers, stoppers, and anchors. In this study, we explored a possibility of whether the molecular switch can be made simple by leaving only the π-stacked aromatic key components between two metal electrodes. The current−voltage (I−V) characteristics of these simplest model devices were calculated from density functional theory using the nonequilibrium matrix Green’s function formalism. When the aromatic components are in direct contact with the surfaces, the I−V characteristics depend dramatically on the orientation of the π stacks, and spatial or temporal changes in the orientation can decrease the device robustness. The robustness can be restored by introducing a buffer layer or a covalent bond (say bulky stoppers and anchors as well as titanium adhesion layers) between the π stack and the electrodes, and this is, indeed, what has been done in the actual fabrication of the working devices. We also propose an alternative strategy to build a simple switch that uses controlled orientation change as the basis for switching.
[show abstract][hide abstract] ABSTRACT: We report the H_2 uptake behavior of 10 zeolitic−imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H_2 and ZIF in the GCMC were based on ab initio quantum mechanical (QM) calculations (MP2) aimed at correctly describing London dispersion (van der Waals attraction). Thus these predictions of H_2 uptake are based on first principles (non empirical) and hence applicable to new framework materials for which there is no empirical data. For each of these 10 ZIFs we report the total and excess H_2 adsorption isotherms up to 100 bar at both 77 and 300 K. We report the hydrogen adsorption sites in the ZIFs and the relationships between H_2 uptake amount, isosteric heat of adsorption (Q_(st)), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H_2 adsorption behaviors in contrast to the metal−organic frameworks (MOFs). This is because ZIFs leads to greater diversity in the adsorption sites (depending on both organic linkers and zeolite topologies) than in MOFs. In particular, the ZIFs uptake larger amounts of H_2 at low pressure because of the high H_2 adsorption energy, and ZIFs have a variety of H_2 adsorption sites. For example, ZIF-11 has an initial Q_(st) value of ~15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H_2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs.
[show abstract][hide abstract] ABSTRACT: We studied the influence of molecular structural elements of alkyl polyglycoside (APG) surfactants on the interfacial tension (IFT) in aqueous formulations against n-octane. This included the analysis of alkyl and aryl chain length, type and number of sugar-ring head, anomers, addition of cosolvents and effect of salt addition. We found that longer alkyl or aryl chains lead to lower IFT, consistent with data recorded for commercial (mixed) APGs. APGs with only one sugar-ring head had lower IFT than their analog maltose derivates (two-ring head). Intriguingly the stereochemistry of the sugar head (i.e. galactose versus glucose) and the type of anomer showed a significant influence on IFT. The n-octyl-alpha-D-glucopyranoside anomer had a lower IFT than the corresponding p-anomer. 1-octanol and 1-hexanol were efficient cosolvents consistent with the datasets observed for commercial APGs. Salt addition reduced IFT. Functional groups (aldehyde, amide-methoxy) integrated into the molecular architecture of the APG skeleton were efficient in terms of significantly reducing IFT, suggesting a strategy for the molecular design of advanced APG surfactants. We discuss the results in the context of the hydrophilic-lipophilic deviation (HLD) concept, which we modified so that IFT values are discussed instead of phase behavior.
[show abstract][hide abstract] ABSTRACT: Thiols (RSH) or thiolates (RS) are one of the most popular anchor groups used for attaching molecules to gold electrodes in molecular junction devices. Operation of these devices under oxidizing conditions may lead to oxidation of the thiolate anchor groups. Herein we investigate the plausibility of this process as a potential source of current fluctuation. Density functional theory calculations on various oxide derivatives of ethanethiolate on gold clusters (EtSO_n/Au_(13); n = 0−3) suggest that oxidation of thiolate anchor groups (Au−S) into sulfoxides (Au−SO) should be unlikely under ambient conditions. Under oxidizing conditions, sulfoxides can form and oxidize further into sulfinates (Au−SO_2) and sulfonates (Au−SO_3) favorably via oxygen transfer from surface oxides or other active oxygen species. Nonequilibrium Green’s function calculations on model devices with thiolate, sulfoxide, and sulfonate anchor groups suggest that thiolates show essentially the same insulating current−voltage characteristics before and after oxidation. However, oxidation of thiolates into sulfonates can increase the length of the electron transfer pathway (that is, the molecule), and this type of oxidation-induced change (molecular lengthening, SAM thickening, and possibly SAM disordering) can affect the robustness of the molecular junction devices.
[show abstract][hide abstract] ABSTRACT: We present a technique for using quantum Monte Carlo (QMC) to obtain high quality energy differences. We use generalized valence bond (GVB) wave functions, for an intuitive approach to capturing the important sources of static correlation, without needing to optimize the orbitals with QMC. Using our modifications to Walker branching and Jastrows, we can then reliably use diffusion quantum Monte Carlo to add in all the dynamic correlation. This simple approach is easily accurate to within a few tenths of a kcal/mol for a variety of problems, which we demonstrate for the adiabatic singlet-triplet splitting in methylene, the vertical and adiabatic singlet-triplet splitting in ethylene, 2+2 cycloaddition, and Be_2 bond breaking.
[show abstract][hide abstract] ABSTRACT: We develop a finite-difference time-domain (FDTD) method for simulating the dynamics of graphene electrons, denoted GraFDTD. We then use GraFDTD to study the temporal behavior of a single localized electron wave packet, showing that it exhibits optical-like dynamics including the Goos–Hänchen effect [ F. Goos and H. Hänchen, Ann. Phys. 436, 333 (1947)] at a heterojunction, but the behavior is quantitatively different than for electromagnetic waves. This suggests issues that must be addressed in designing graphene-based electronic devices analogous to optical devices. GraFDTD should be useful for studying such complex time-dependent behavior of a quasiparticle in graphene.
[show abstract][hide abstract] ABSTRACT: Standard implementations of density functional theory (DFT) describe well strongly bound molecules and solids but fail to describe long-range van der Waals attractions. We propose here first-principles-based augmentation to DFT that leads to the proper long-range 1/R^6 attraction of the London dispersion while leading to low gradients (small forces) at normal valence distances so that it preserves the accurate geometries and thermochemistry of standard DFT methods. The DFT-low gradient (DFT-lg) formula differs from previous DFT-D methods by using a purely attractive dispersion correction while not affecting valence bond distances. We demonstrate here that the DFT-lg model leads to good descriptions for graphite, benzene, naphthalene, and anthracene crystals, using just three parameters fitted to reproduce the full potential curves of high-level ab initio quantum mechanics [CCSD(T)] on gas-phase benzene dimers. The additional computational costs for this DFT-lg formalism are negligible.
[show abstract][hide abstract] ABSTRACT: This work presents a ReaxFF reactive force-field for use in molecular dynamics simulations of the ZnO–
water system. The force-field parameters were fitted to a data-set of energies, geometries and charges
derived from quantum-mechanical B3LYP calculations. The presented ReaxFF model provides a good
fit to the QM reference data for the ZnO–water system that was present in the data-set. The force-field
has been used to study how water is adsorbed, molecularly or dissociatively, at monolayer coverage
on flat and stepped ZnO surfaces, at three different temperatures (10 K, 300 K, and 600 K). The stepped
surfaces were created by introducing steps along the (0001)-direction on the (1010)-surface. Equilibrium
between molecular and dissociated water was observed on the (1010) terraces, resulting in a half
dissociated, half molecular water monolayer. The equilibrium between dissociated and molecular water
on the surface was found to be reached quickly (<10 ps). When water molecules desorb and the coverage
falls, the 1:1 water–hydroxyl ratio is maintained on (1010) terraces, while steps remain largely hydroxylated.
The results show that structures that promote hydrogen bonding are favored and that the presence
of steps promotes an increased level of hydroxylation in the water monolayers.
[show abstract][hide abstract] ABSTRACT: Mechanically bonded macromolecules constitute a class of challenging synthetic targets in polymer science. The controllable intramolecular motions of mechanical bonds, in combination with the processability and useful physical and mechanical properties of macromolecules, ultimately ensure their potential for applications in materials science, nanotechnology and medicine. This tutorial review describes the syntheses and properties of a library of diverse mechanically bonded macromolecules, which covers (i) main-chain, side-chain, bridged, and pendant oligo/polycatenanes, (ii) main-chain oligo/polyrotaxanes, (iii) poly[c2]daisy chains, and finally (iv) mechanically interlocked dendrimers. A variety of highly efficient synthetic protocols—including template-directed assembly, step-growth polymerisation, quantitative conjugation, etc.—were employed in the construction of these mechanically interlocked architectures. Some of these structures, i.e., side-chain polycatenanes and poly[c2]daisy chains, undergo controllable molecular switching in a manner similar to their small molecular counterparts. The challenges posed by the syntheses of polycatenanes and polyrotaxanes with high molecular weights are contemplated.
Chemical Society Reviews 12/2009; · 24.89 Impact Factor