Douglas J. Doren

Band gap tailoring of Nd3+-doped TiO2 nanoparticles

Undoped and Nd3+-doped TiO2 nanoparticles were synthesized by chemical vapor deposition in order to tailor the band gap of TiO2. The doping reduced the band gap. The band gap was measured by ultraviolet-visible light absorption experiments and by near-edge x-ray absorption fine structure. The maximum band gap reduction was 0.55 eV for 1.5 at. % Nd-doped TiO2 nanoparticles. Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method were used to interpret the band gap narrowing. The band gap narrowing was primarily attributed to the substitutional Nd3+ ions which introduced electron states into the band gap of TiO2 to form the new lowest unoccupied molecular orbital.

Neural network models of potential energy surfaces

Neural networks provide an efficient, general interpolation method for nonlinear functions of several variables. This paper describes the use of feed‐forward neural networks to model global properties of potential energy surfaces from information available at a limited number of configurations. As an initial demonstration of the method, several fits are made to data derived from an empirical potential model of CO adsorbed on Ni(111). The data are error‐free and geometries are selected from uniform grids of two and three dimensions. The neural network model predicts the potential to within a few hundredths of a kcal/mole at arbitrary geometries. The accuracy and efficiency of the neural network in practical calculations are demonstrated in quantum transition state theory rate calculations for surface diffusion of CO/Ni(111) using a Monte Carlo/path integral method. The network model is much faster to evaluate than the original potential from which it is derived. As a more complex test of the method, the in...

Adsorption of water on Si(100)-(2×1): A study with density functional theory

Adsorption of water on the Si(100)-(2×1) surface has been investigated using density functional theory and cluster models of the surface. The reaction pathway and geometries of the product, the transition state and a molecular precursor state are described. There is no energy barrier to dissociative chemisorption. Adsorbed H and OH fragments are most stable when bonded to the same surface dimer with the hydroxyl oriented away from the surface dimer bond. The orbital and electrostatic interactions that determine the adsorbate and transition state geometries are analyzed. Surface distortion (dimer buckling) is a recurring theme in this analysis. Interactions of adsorbed molecular fragments with each other and with dangling bonds have significant effects, modifying the adsorbate geometry and leading to adsorbate islanding. Calculated vibrational frequencies of adsorbed H2O on Si(100)-(2×1) are discussed. The theoretical results are consistent with most available experimental results, and provide a microscopi...

Cycloaddition reactions of unsaturated hydrocarbons on the Si(100)-(2 × 1) surface : theoretical predictions

First-principles electronic structure calculations have been used to study the structure, energetics and vibrational spectra of the chemisorption products of several unsaturated hydrocarbons on the Si(100)-(2×1) surface. The calculations use a hybrid non-local density functional theory and a cluster model of the surface. Ethylene and acetylene react by a [2s+2s] cycloaddition mechanism. Conjugated dienes (1,3-cyclohexadiene, 1,3-butadiene, 2,3-dimethyl-1,3-butadiene) and benzene can also react by a novel [4s+2s] cycloaddition, or Diels–Alder mechanism. For each diene, the Diels–Alder product is energetically favored over the more strained [2s+2s] product. The reaction mechanism for Diels–Alder addition, other competing reactions, and the effects of post-hydrogenation are all discussed. Comparisons to experimental observations are made throughout.

CO diffusion on Pt(111) with time‐resolved infrared‐pulsed molecular beam methods: Critical tests and analysis

The microscopic diffusion of CO on stepped Pt(111) crystal surfaces has been investigated with pulsed molecular beam–time‐resolved surface infrared methods. Following a rapid exposure to CO, we record the time evolution of the CO surface vibrational spectra as the CO diffuse from the initial random distribution to the thermodynamically favored step sites. The data are simulated with a model that describes the kinetics of diffusion across the terraces, as well as the filling of step sites. We critically evaluate this model and the general experimental approach by extending our previous measurements of CO diffusion on Pt(28(111)–(110)) to a surface with higher step density, Pt(l2(111)–(110)), with varying coverages. The model describes both sets of data with the same parameters, confirming the original determination of the diffusion barrier (ΔET) and prefactor (AT) for microscopic surface hopping of CO/Pt(111). This further provides a quantitative means to estimate systematic errors. We report ΔET=4.0±0.7...

Development of a ReaxFF reactive force field for glycine and application to solvent effect and tautomerization.

Tautomerization of amino acids between the neutral form (NF) and the zwitterionic form (ZW) in water has been extensively studied, often using glycine as a model to understand this fundamental process. In spite of many advanced studies, the tautomerization reaction remains poorly understood because of the intrinsic complexities of the system, including multiple accessible reaction pathways, charge transfer, and variations of solvation structure. To establish an accurate model that can be used for molecular dynamics simulations, a ReaxFF reactive force field has been developed for glycine. A training set for the ReaxFF hydrocarbon potential was augmented with several glycine conformers and glycine-water complexes. The force field parameters were optimized to reproduce the quantum mechanically derived energies of the species in the training set. The optimized potential could accurately describe the properties of gas-phase glycine. It was applied to investigate the effect of solvation on the conformational distribution of glycine. Molecular dynamics simulations indicated significant differences in the dominant conformers in the gas phase and in water. This suggests that the tautomerization of glycine occurs through a conformational isomerization followed by the proton transfer event. The direct reaction mechanism of the NF → ZW proton transfer reaction in water, as well as mechanisms mediated by one or two water molecules, were investigated using molecular dynamics simulations. The results suggest that the proton transfer reaction is most likely mediated by a single water molecule. The ReaxFF potential developed in this work provides an accurate description of proton transfer in glycine and thus provides a useful methodology for simulating proton transfer reactions in organic molecules in the aqueous environment.

Development and validation of a ReaxFF reactive force field for Cu-cation/water interactions and copper metal/metal oxide/metal hydroxide condensed phases

To enable large-scale reactive dynamic simulations of copper oxide/water and copper ion/water interactions we have extended the ReaxFF reactive force field framework to Cu/O/H interactions. To this end, we employed a multistage force field development strategy, where the initial training set (containing metal/metal oxide/metal hydroxide condensed phase data and [Cu(H(2)O)(n)](2+) cluster structures and energies) is augmented by single-point quantum mechanices (QM) energies from [Cu(H(2)O)(n)](2+) clusters abstracted from a ReaxFF molecular dynamics simulation. This provides a convenient strategy to both enrich the training set and to validate the final force field. To further validate the force field description we performed molecular dynamics simulations on Cu(2+)/water systems. We found good agreement between our results and earlier experimental and QM-based molecular dynamics work for the average Cu/water coordination, Jahn-Teller distortion, and inversion in [Cu(H(2)O)(6)](2+) clusters and first- and second-shell O-Cu-O angular distributions, indicating that this force field gives a satisfactory description of the Cu-cation/water interactions. We believe that this force field provides a computationally convenient method for studying the solution and surface chemistry of metal cations and metal oxides and, as such, has applications for studying protein/metal cation complexes, pH-dependent crystal growth/dissolution, and surface catalysis.

Dynamics of molecular surface diffusion: Origins and consequences of long jumps

The mechanics of molecular surface diffusion have been studied in a theoretical model of CO/Ni(111). Using molecular dynamics, diffusion rates have been calculated over a wide range of temperatures and interpreted using methods typically applied to experimental measurements. This interpretation is based on transition state theory and a model of uncorrelated hops between near neighbors. An Arrhenius plot of diffusion constants from the simulations is linear from 175 to 1000 K. However, the underlying dynamics do not conform to the model of uncorrelated hops. Instead, molecules that have been excited to a transition state tend to fly past several sites before settling onto a new one. These multiple site flights (‘‘long jumps’’) make the Arrhenius prefactor larger than the transition state theory prediction by more than an order of magnitude. Transition state recrossings have a small effect on the diffusion rate. Long jumps are typical of a ‘‘low friction’’ regime in which energy exchange is slow between lat...

Free energies of solvation with quantum mechanical interaction energies from classical mechanical simulations

A free energy perturbation technique is described in which configurations from a classical simulation (molecular dynamics or Monte Carlo) with empirical solute–solvent interactions are used to calculate free energies with quantum mechanically derived solute–solvent interactions. This approach is much less costly than simulations with forces derived from quantum mechanics at each time step, since it only requires quantum energies to be calculated at classically determined configurations. The method is not limited to free energies of solvation, and can potentially be applied to calculations of activation energies and other condensed phase chemical transformations. As a test, this method was used to calculate the free energy of hydration of water at ambient conditions. With a good classical model the method gives accurate results with only 50 quantum calculations. The method is self-correcting in the sense that it can be used to recognize a bad classical model, and improved classical models can be derived by...

High-Temperature Dehydrogenation of Brønsted Acid Sites in Zeolites

We present evidence for the dehydrogenation of zeolite Bronsted acid sites at temperatures above 500 °C forming hydrogen gas and a [AlO4]0 site that could participate in one-electron oxidations or react further to form extraframework aluminum.