Matching Items (5)

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Computational Characterization of a Ni Catalyst

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Industrial interest in electrocatalytic production of hydrogen has stimulated considerable research in understanding hydrogenases, the biological catalysts for proton reduction, and related synthetic mimics. Structurally closely related complexes are often

Industrial interest in electrocatalytic production of hydrogen has stimulated considerable research in understanding hydrogenases, the biological catalysts for proton reduction, and related synthetic mimics. Structurally closely related complexes are often synthesized to define structure-function relationships and optimize catalysis. However, this process can also lead to drastic and unpredictable changes in the catalytic behavior. In this paper, we use density functional theory calculations to identify changes in the electronic structure of [Ni(bdt)(dppf)] (bdt = 1,2-benzenedithiolate, dppf = 1,1ʹ-bis(diphenylphosphino)ferrocene) relative to [Ni(tdt)(dppf)] (tdt = toluene-3,4-dithiol) as a means to explain the substantially reduced electrocatalytic activity of the tdt complex. An increased likelihood of protonation at the sulfur sites of the tdt complex relative to the Ni is revealed. This decreased propensity of metal protonation may lead to less efficient metal-hydride production and subsequently catalysis.

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Date Created
  • 2018-05

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Simple and accurate correlation of experimental redox potentials and DFT-calculated HOMO/LUMO energies of polycyclic aromatic hydrocarbons

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The ability to accurately predict the oxidation and reduction potentials of molecules is very useful in various fields and applications. Quantum mechanical calculations can be used to access this information,

The ability to accurately predict the oxidation and reduction potentials of molecules is very useful in various fields and applications. Quantum mechanical calculations can be used to access this information, yet sometimes the usefulness of these calculations can be limited because of the computational requirements for large systems. Methodologies that yield strong linear correlations between calculations and experimental data have been reported, however the balance between accuracy and computational cost is always a major issue. In this work, linear correlations (with an R-2 value of up to 0.9990) between DFT-calculated HOMO/LUMO energies and 70 redox potentials from a series of 51 polycyclic aromatic hydrocarbons (obtained from the literature) are presented. The results are compared to previously reported linear correlations that were obtained with a more expensive computational methodology based on a Born-Haber thermodynamic cycle. It is shown in this article that similar or better correlations can be obtained with a simple and cheaper calculation.

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Date Created
  • 2013-10-28

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Dopamine Adsorption on TiO2 Anatase Surfaces

Description

The dopamine-TiO[subscript 2] system shows a specific spectroscopic response, surface enhanced Raman scattering (SERS), whose mechanism is not fully understood. In this study, the goal is to reveal the key

The dopamine-TiO[subscript 2] system shows a specific spectroscopic response, surface enhanced Raman scattering (SERS), whose mechanism is not fully understood. In this study, the goal is to reveal the key role of the molecule–nanoparticle interface in the electronic structure by means of ab initio modeling. The dopamine adsorption energy on anatase surfaces is computed and related to changes in the electronic structure. Two features are observed: the appearance of a state in the material band gap, and charge transfer between molecule and surface upon electronic excitation. The analysis of the energetics of the systems would point to a selective adsorption of dopamine on the (001) and (100) terminations, with much less affinity for the (101) plane.

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Date Created
  • 2014-09-04

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Catalytic Hydrogen Evolution by Fe(II) Carbonyls Featuring a Dithiolate and a Chelating Phosphine

Description

Two pentacoordinate mononuclear iron carbonyls of the form (bdt)Fe(CO)P[subscript 2] [bdt = benzene-1,2-dithiolate; P[subscript 2] = 1,1′-diphenylphosphinoferrocene (1) or methyl-2-{bis(diphenylphosphinomethyl)amino}acetate (2)] were prepared as functional, biomimetic models for the distal

Two pentacoordinate mononuclear iron carbonyls of the form (bdt)Fe(CO)P[subscript 2] [bdt = benzene-1,2-dithiolate; P[subscript 2] = 1,1′-diphenylphosphinoferrocene (1) or methyl-2-{bis(diphenylphosphinomethyl)amino}acetate (2)] were prepared as functional, biomimetic models for the distal iron (Fe[subscript d]) of the active site of [FeFe]-hydrogenase. X-ray crystal structures of the complexes reveal that, despite similar ν(CO) stretching band frequencies, the two complexes have different coordination geometries. In X-ray crystal structures, the iron center of 1 is in a distorted trigonal bipyramidal arrangement, and that of 2 is in a distorted square pyramidal geometry. Electrochemical investigation shows that both complexes catalyze electrochemical proton reduction from acetic acid at mild overpotential, 0.17 and 0.38 V for 1 and 2, respectively. Although coordinatively unsaturated, the complexes display only weak, reversible binding affinity toward CO (1 bar). However, ligand centered protonation by the strong acid, HBF[subscript 4]·OEt[subscript 2], triggers quantitative CO uptake by 1 to form a dicarbonyl analogue [1(H)-CO][superscript +] that can be reversibly converted back to 1 by deprotonation using NEt[subscript 3]. Both crystallographically determined distances within the bdt ligand and density functional theory calculations suggest that the iron centers in both 1 and 2 are partially reduced at the expense of partial oxidation of the bdt ligand. Ligand protonation interrupts this extensive electronic delocalization between the Fe and bdt making 1(H)[superscript +] susceptible to external CO binding.

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Date Created
  • 2014-09-01

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Molecular models for conductance in junctions and electrochemical electron transfer

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This thesis develops molecular models for electron transport in molecular junctions and intra-molecular electron transfer. The goal is to identify molecular descriptors that afford a substantial simplification of these electronic

This thesis develops molecular models for electron transport in molecular junctions and intra-molecular electron transfer. The goal is to identify molecular descriptors that afford a substantial simplification of these electronic processes.

First, the connection between static molecular polarizability and the molecular conductance is examined. A correlation emerges whereby the measured conductance of a tunneling junction decreases as a function of the calculated molecular polarizability for several systems, a result consistent with the idea of a molecule as a polarizable dielectric. A model based on a macroscopic extension of the Clausius-Mossotti equation to the molecular domain and Simmon’s tunneling model is developed to explain this correlation. Despite the simplicity of the theory, it paves the way for further experimental, conceptual and theoretical developments in the use of molecular descriptors to describe both conductance and electron transfer.

Second, the conductance of several biologically relevant, weakly bonded, hydrogen-bonded systems is systematically investigated. While there is no correlation between hydrogen bond strength and conductance, the results indicate a relation between the conductance and atomic polarizability of the hydrogen bond acceptor atom. The relevance of these results to electron transfer in biological systems is discussed.

Hydrogen production and oxidation using catalysts inspired by hydrogenases provides a more sustainable alternative to the use of precious metals. To understand electrochemical and spectroscopic properties of a collection of Fe and Ni mimics of hydrogenases, high-level density functional theory calculations are described. The results, based on a detailed analysis of the energies, charges and molecular orbitals of these metal complexes, indicate the importance of geometric constraints imposed by the ligand on molecular properties such as acidity and electrocatalytic activity. Based on model calculations of several intermediates in the catalytic cycle of a model NiFe complex, a hypothetical reaction mechanism, which very well agrees with the observed experimental results, is proffered.

Future work related to this thesis may involve the systematic analysis of chemical reactivity in constrained geometries, a subject of importance if the context of enzymatic activity. Another, more intriguing direction is related to the fundamental issue of reformulating Marcus theory in terms of the molecular dielectric response function.

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Created

Date Created
  • 2015