Dielectric metal/metal oxide nanocomposites: modeling response properties at multiple scales

Nanocomposites with metallic inclusions show great promise as tunable functional materials, particularly for applications where high permittivities are desirable, such as charge-storage. These applications strain quantum mechanical computational approaches, as any representative sample of the material includes hundreds if not thousands of atoms. Many continuum methods offer some predictive power for matrix-inclusion composites, but cannot be directly applied to composites with small inclusions, for which quantum and interfacial effects dominate. Here, we develop an adjustable finite element approach to calculate the permittivities of composites consisting of a metal-oxide matrix with nanometer-scale silver inclusions, by introducing an interfacial layer in the model. The approach involves solving the Laplace equation with Dirichlet and Neumann boundary conditions. We demonstrate that such a continuum model, when appropriately informed using quantum mechanical results, can capture many of the relevant polarization effects in a metal/metal oxide nanocomposite, including those that contain arbitrarily-small inclusions, at a fraction of the computational cost of performing the full quantum mechanics

Tuning the Dielectric Response in a Nanocomposite Material through Nanoparticle Morphology

The introduction of metal cluster dopants and molecular-scale inclusions in metal oxide matrices provides an opportunity for exploring new high-k solid-state dielectrics with tunable response. The quantum properties of molecular nanoparticles depend strongly on their size and shape, a characteristic that can be exploited in changing the response properties of a material, while the small nanoparticle size can help limit the usual issues of conduction and leakage. Here, we model the polarization of molecular-scale silver inclusions in magnesium oxide, using the Modern Theory of Polarization and Car-Parinello Molecular Dynamics (CPMD). Several trends are considered, including nanoparticle size, shape and orientation relative to the applied field. Dielectric permittivity enhancements of 30-100% were observed with inclusion sizes varying from 8 to 32 atoms, considering both rod-like and disk-like inclusions, with alignment either parallel or perpendicular to the external field. Currents calculated using the modern theory of polarization with periodic boundary conditions can experience box edge jumps due to the distortion of the matrix during the simulations - an approach for addressing these issues in CPMD calculations is outlined within

Accelerated Molecular Dynamics for Structural Prediction in Protein/Peptide Binding: The SLICE Method

The work in this submission presents a molecular simulation technique that is able to overcome induced-fit binding issues with a uniquely challenging binding site. This method differs from commercially available methods in that we use a combined software approach that allows users to patch together inexpensive and academically available programs. Our method also differs fundamentally in how the potential energy of the interaction is explored and lends itself to the success in modeling our test system. As an added benefit, our test system is CBX8, an epigenetic reader protein and speculative target for chemotherapy. The method was not only successful in matching crystal structure data, but uncovered a number of induced-fit structural features of the binding motif that are useful in the design of potential inhibitors for this protein.</div

A generally applicable quantitative reactivity model for nucleophilic aromatic substitution built from simple descriptors

We report a multivariate linear regression model able to make accurate predictions for the rate and regioselectivity of nucleophilic aromatic substitution (SNAr) reactions based on the electrophile structure. This model uses a diverse training/test set from experimentally-determined relative SNAr rates between benzyl alcohol and 74 unique electrophiles, including heterocycles with multiple substitution patterns. There is a robust linear relationship between the experimental SNAr free energies of activation and three molecular descriptors that can be obtained computationally: the LUMO energy of the electrophile; the average molecular electrostatic potential (ESP) at the carbon undergoing substitution; and the sum of average ESP values for the ortho and para atoms relative to the reactive center. Despite using only simple descriptors calculated from ground state wavefunctions, this model demonstrates excellent correlation with previously measured SNAr reaction rates, and is able to accurately predict site selectivity for multihalogenated substrates: 91% prediction accuracy across 82 individual examples. The excellent agreement between predicted and experimental outcomes makes this easy-to-implement reactivity model a potentially powerful tool for synthetic planning

Surface-site reactivity in small-molecule adsorption: A theoretical study of thiol binding on multi-coordinated gold clusters

Background: The adsorption of organic molecules on metal surfaces has a broad array of applications, from device engineering to medical diagnosis. The most extensively investigated class of metal–molecule complexes is the adsorption of thiols on gold.Results: In the present manuscript, we investigate the dependence of methylthiol adsorption structures and energies on the degree of unsaturation at the metal binding site. We designed an Au20 cluster with a broad range of metal site coordination numbers, from 3 to 9, and examined the binding conditions of methylthiol at the various sites.Conclusion: We found that despite the small molecular size, the dispersive interactions of the backbone are a determining factor in the molecular affinity for various sites. Kink sites were preferred binding locations due to the availability of multiple surface atoms for dispersive interactions with the methyl groups, whereas tip sites experienced low affinity, despite having low coordination numbers

A reactivity map for oxidative addition enables quantitative predictions for multiple catalytic reaction classes

Making accurate, quantitative predictions of chemical reactivity based on molecular structure is an unsolved problem in chemical synthesis, particularly for complex molecules. We report a generally applicable, mechanistically based structure-reactivity approach to create a quantitative model for the oxidative addition of (hetero)aryl electrophiles to palladium(0), which is a key step in myriad catalytic processes. This model links simple molecular descriptors to relative rates of oxidative addition for 79 substrates, including chloride, bromide and triflate leaving groups. Because oxidative addition often controls the rate and/or selectivity of palladium-catalyzed reactions, this model can be used to make quantitative predictions about catalytic reaction outcomes. Demonstrated applications include a multivariate linear model for the initial rate of Sonogashira coupling reactions, and successful site-selectivity predictions for a series of multihalogenated substrates relevant to the synthesis of pharmaceuticals and natural products

Role of adatom defects in the adsorption of polyaromatic hydrocarbons on metallic substrates

The adsorption of polyaromatic hydrocarbons (PAHs) on metallic substrates is of interest in the field of optoelectronics, due to the possibility of designing complex materials with tunable properties through surface functionalization with organic molecules. Much of the modelling research in this field has focused on perfectly symmetrical (pristine) substrates. There is limited information on the effect of substrate irregularities, such as adatoms, on the binding of PAHs onto substrates. Here, we examine how the presence of substrate-bound adatoms affects the binding of coronene and hexahelicene monomers and dimers on Au(111) and Cu(111) substrates, using a density functional theory approach. We found that helicene monomers were more effectively able to adapt to the presence of the adatoms than coronene, by coiling around the adatoms. Whereas upon adsorption on a pristine (111) surface, coronene can establish significantly stronger dispersive interactions than helicenes, adatom defects reverse the trend. For helicenes, the extent of flattening near the surface and molecular coiling are strongly influenced by the size of the defect, as a result of the interplay between the molecule’s drive to maximize overlap with the underlying surface and the enhanced reactivity of the low-coordinated adatoms

Trichloro(Dinitrogen)platinate(II)

Zeise’s salt, [PtCl3(H2C=CH2)]–, is the oldest known organometallic complex, featuring ethylene strongly bound to a platinum salt. Many derivatives are known, but none involving dinitrogen, and indeed dinitrogen complexes are unknown for both platinum and palladium. Electrospray ionization mass spectrometry of K2[PtCl4] solutions generate strong ions corresponding to [PtCl3(N2)]–, whose identity was confirmed through ion mobility spectroscopy and MS/MS experiments that proved it to be distinct from its isobaric counterparts [PtCl3(C2H4)]– and [PtCl3(CO)]–. Computational analysis established a gas-phase platinum-dinitrogen bond strength of 116 kJ mol-1, substantially weaker than the ethylene and carbon monoxide analogues but stronger than for polar solvents such as water, methanol and dimethylformamide, and strong enough that the calculated N-N bond length of 1.119 Å represents weakening to a degree typical of isolated dinitrogen complexes. </p

Orbital Shaped Standing Waves Using Chladni Plates

Chemistry students are often introduced to the concept of atomic orbitals with a representation of a one-dimensional standing wave. The classic example is the harmonic frequencies which produce standing waves on a guitar string; a concept which is easily replicated in class with a length of rope. From here, students are typically exposed to a more realistic three-dimensional model, which can often be difficult to visualize. Extrapolation from a two-dimensional model, such as the vibrational modes of a drumhead, can be used to convey the standing wave concept to students more easily. We have opted to use Chladni plates which may be tuned to give a two-dimensional standing wave which serves as a cross-sectional representation of atomic orbitals. The demonstration, intended for first year chemistry students, facilitates the examination of nodal and anti-nodal regions of a Chladni figure which students can then connect to the concept of quantum mechanical parameters and their relationship to atomic orbital shape

Gas-Phase Oxidation of Reactive Organometallic Ions

Analysis of highly reactive compounds at very low concentration in solution using electrospray ionization mass spectrometry requires the use of exhaustively purified solvents. It has generally been assumed that desolvation gas purity needs to be similarly high, and so most chemists working in this space have relied upon high purity gas. However, the increasingly competitiveness of nitrogen generators, which provide gas purity levels that vary inversely with flow rate, prompted an investigation of the effect of gas-phase oxygen on the speciation of ions. For moderately oxygen sensitive species such as phosphines, no gas-phase oxidation was observed. Even the most reactive species studied, the reduced titanium complex [Cp2Ti(NCMe)2]+[ZnCl3]– and the olefin polymerization precatalyst [Cp2Zr(µ-Me)2AlMe2]+ [B(C6F5)4]–, only exhibited detectable oxidation when they were rendered coordinatively unsaturated through in-source fragmentation. Computational chemistry allowed us to find the most plausible pathways for the observed chemistry in the absence of observed intermediates. The results provide insight into the gas-phase oxidation of reactive species and should assure experimentalists that evidence of significant oxidation is likely a solution rather than a gas-phase process, even when relatively low-purity nitrogen is used for desolvation