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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 synthesized to define structure-function relationships and optimize catalysis. However, this process can also lead to drastic and unpredictable changes in

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.
ContributorsHerringer, Nicholas Stephen (Author) / Jones, Anne (Thesis director) / Mujica, Vladimiro (Committee member) / Pilarisetty, Tarakeshwar (Committee member) / School of Molecular Sciences (Contributor) / Barrett, The Honors College (Contributor)
Created2018-05
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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 processes.

First, the connection between static molecular polarizability and the molecular conductance is examined. A correlation emerges whereby the measured

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.
ContributorsKhezr Seddigh Mazinani, Shobeir (Author) / Mujica, Vladimiro (Thesis advisor) / Pilarisetty, Tarakeshwar (Committee member) / Angell, Charles A (Committee member) / Jones, Anne K (Committee member) / Arizona State University (Publisher)
Created2015