Zachary D. Pozun

A Systematic Investigation of p-Nitrophenol Reduction by Bimetallic Dendrimer Encapsulated Nanoparticles

We demonstrate that the reduction of p-nitrophenol to p-aminophenol by NaBH4 is catalyzed by both monometallic and bimetallic nanoparticles (NPs). We also demonstrate a straightforward and precise method for the synthesis of bimetallic nanoparticles using poly(amido)amine dendrimers. The resulting dendrimer encapsulated nanoparticles (DENs) are monodisperse, and the size distribution does not vary with different elemental combinations. Random alloys of Pt/Cu, Pd/Cu, Pd/Au, Pt/Au, and Au/Cu DENs were synthesized and evaluated as catalysts for p-nitrophenol reduction. These combinations are chosen in order to selectively tune the binding energy of the p-nitrophenol adsorbate to the nanoparticle surface. Following the Brønsted–Evans–Polanyi (BEP) relation, we show that the binding energy can reasonably predict the reaction rates of p-nitrophenol reduction. We demonstrate that the measured reaction rate constants of the bimetallic DENs is not always a simple average of the properties of the constituent metals. In particular, DENs containing metals with similar lattice constants produce a binding energy close to the average of the two constituents, whereas DENs containing metals with a lattice mismatch show a bimodal distribution of binding energies. Overall, in this work we present a uniform method for synthesizing pure and bimetallic DENs and demonstrate that their catalytic properties are dependent on the adsorbate’s binding energy.

Hybrid density functional theory band structure engineering in hematite

We present a hybrid density functional theory (DFT) study of doping effects in α-Fe(2)O(3), hematite. Standard DFT underestimates the band gap by roughly 75% and incorrectly identifies hematite as a Mott-Hubbard insulator. Hybrid DFT accurately predicts the proper structural, magnetic, and electronic properties of hematite and, unlike the DFT+U method, does not contain d-electron specific empirical parameters. We find that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties. We then show that the antiferromagnetic symmetry in the pure α-Fe(2)O(3) crystal is broken by all dopants and that the ligand field theory correctly predicts local magnetic moments on the dopants. We characterize the resulting band gaps for hematite doped by transition metals and the p-block post-transition metals. The specific case of Pd doping is investigated in order to correlate calculated doping energies and optical properties with experimentally observed photocatalytic behavior.

Amino Acids from Ion-Irradiated Nitrile-Containing Ices

Solid CH(3)CN and solid H(2)O + CH(3)CN were ion irradiated near 10 K to initiate chemical reactions thought to occur in extraterrestrial ices. The infrared spectra of these samples after irradiation revealed the synthesis of new molecules. After the irradiated ices were warmed to remove volatiles, the resulting residual material was extracted and analyzed. Both unhydrolyzed and acid-hydrolyzed residues were examined by both liquid and gas chromatographic-mass spectral methods and found to contain a rich mixture of products. The unhydrolyzed samples showed HCN, NH(3), acetaldehyde (formed by reaction with background and atmospheric H(2)O), alkyamines, and numerous other compounds, but no amino acids. However, reaction products in hydrolyzed residues contained a suite of amino acids that included some found in carbonaceous chondrite meteorites. Equal amounts of D- and L-enantiomers were found for each chiral amino acid detected. Extensive use was made of (13)C-labeled CH(3)CN to confirm amino acid identifications and discriminate against possible terrestrial contaminants. The results reported here show that ices exposed to cosmic rays can yield products that, after hydrolysis, form a set of primary amino acids equal in richness to those made by other methods, such as photochemistry.

Optimizing transition states via kernel-based machine learning

We present a method for optimizing transition state theory dividing surfaces with support vector machines. The resulting dividing surfaces require no a priori information or intuition about reaction mechanisms. To generate optimal dividing surfaces, we apply a cycle of machine-learning and refinement of the surface by molecular dynamics sampling. We demonstrate that the machine-learned surfaces contain the relevant low-energy saddle points. The mechanisms of reactions may be extracted from the machine-learned surfaces in order to identify unexpected chemically relevant processes. Furthermore, we show that the machine-learned surfaces significantly increase the transmission coefficient for an adatom exchange involving many coupled degrees of freedom on a (100) surface when compared to a distance-based dividing surface.

Structure Revealing H/D Exchange with Co-Adsorbed Hydrogen and Water on Gold.

A fundamental understanding of the interactions between coadsorbed water and hydrogen on metallic surfaces is critical to many chemical processes including catalysis and electrochemistry. Here, we report on the strong and intricate interactions between coadsorbed H/D and water on the close-packed (111) surface of gold. Deuterium isotopic labeling shows H/D exchange in H-D2O and D-H2O systems, indicating water dissociation and suggesting a nonrandom scrambling process by revealing the origin of hydrogen evolution (from surface H atoms or from water molecules) during annealing. In this reaction, the protonation of the H-bonding ice network (i.e., the formation of (H2O)nH(+)) is energetically favorable and is responsible for water dissociation. Density functional theory (DFT) modeling suggests that the thermodynamics and structure of the protonated clusters are predominant factors for yielding the traceable H2 desorption features from the surface interaction with H atoms, providing insights into reaction mechanisms.

Establishing the Ground State of the Disjoint Diradical Tetramethyleneethane with Quantum Monte Carlo

The nature of the electronic ground state of the tetramethyleneethane (TME) diradical has proven to be a challenge for both experiment and theory. Through the use of quantum Monte Carlo (QMC) methods and multireference perturbation theory, we demonstrate that the lowest singlet state of TME is energetically lower than the lowest triplet state at all values of the torsional angle between the allyl subunits. Moreover, we find that the maximum in the potential energy curve for the singlet state occurs at a torsional angle near 45°, in contrast to previous calculations that placed the planar structure of the singlet state as the highest in energy. We also show that the CASPT2 method when used with a sufficiently large reference space and a sufficiently flexible basis set gives potential energy curves very close to those from the QMC calculations. Our calculations have converged the singlet-triplet gap of TME as a function of methodology and basis set. These results provide insight into the level of theory required to properly model diradicals, in particular disjoint diradicals, and provide guidelines for future studies on more complicated diradical systems.

Low-Temperature Chemoselective Gold-Surface-Mediated Hydrogenation of Acetone and Propionaldehyde

Since nanoscale gold was first discovered to be catalytically active, gold-based catalysts have been studied both theoretically and experimentally in a wide range of reactions. These catalysts exhibit high activity for hydrogenation processes, in particular showing enhanced selectivity. However, there is a lack of relevant fundamental studies into these processes. Conducting hydrogenation reactions on model gold surfaces is useful for obtaining mechanistic insight and for further enhancing our understanding of the catalytic properties of supported-gold catalysts. Herein, we report the chemoselective hydrogenation of aldehydes over ketones on gold surfaces. The hydrogenation chemistry of oxygenated hydrocarbons has been studied on transition-metal surfaces for a variety of reactions that are important to the pharmaceuticaland chemical industries. For example, C=O bond hydrogenation is a key step in the catalytic conversion of cellulosic biomass. In addition, gold-based catalysts have also shown exceptional activity for the selective hydrogenation of a,b-unsaturated carbonyl compounds. Claus found that, in the production of allyl alcohol from the hydrogenation of acrolein, gold catalysts yielded about 10-times-higher selectivity for C=O bond hydrogenation than traditional platinum-based catalysts. Therefore, exploring the individual reactivity of carbonyl-hydrogenation could provide useful information for a better holistic understanding of these important catalytic reactions. By employing propionaldehyde and acetone as representative probe molecules of aldehydes and ketones, respectively, we investigated the hydrogenation of C=O bonds on a model pre-atomic-hydrogen-covered Au(111) catalyst. H atoms were used herein owing to the high energetic barrier of H2-dissociation on gold. Temperature-programmed-desorption (TPD) measurements indicated different activities for hydrogenation on gold: propionaldehyde underwent hydrogenation to afford 1-propanol on H-covered gold but acetone did not form 2propanol. Density functional theory (DFT) calculations revealed different activation energies for the reactions between a single carbonyl moiety and a H atom. First, the hydrogenation of acetone was investigated on a Au(111) surface. In a control experiment (Figure 1 a), 1.62 ML (ML = monolayer) of acetone (m/z = 43, the most-abundant mass fragment of acetone) was adsorbed onto a clean Au(111)

Excited Electronic State Mixing in 7-Azaindole. Quantitative Measurements Using the Stark Effect

Stark effect measurements of the +280 cm(-1) vibronic band at ∼286 nm in the high resolution S1-S0 fluorescence excitation spectrum of 7-azaindole (7AI) in a molecular beam show that the permanent (electric) dipole moment (PDM) of the upper state vibrational level reached in this transition is 4.6 D, twice as large as the PDM of the zero-point level of the S1 state. This large difference is attributed to state mixing with a more polar state. EOM-CSSD calculations suggest that this more polar state is σπ* in nature and that it crosses the ππ* state in energy along the coordinate connecting the two potential energy minima. Such state mixing apparently provides more facile access to conical intersections with the ground state, and subsequent hydrogen atom detachment reactions, since independent studies by Sakota and Sekiya have shown that the N-H stretching frequency of 7AI is significantly reduced when it is excited to the +280 cm(-1) vibrational level of the S1 state.