Theoretical Study For Predicting Electronic And Vibrational Spectra Of Compounds Used In The Renewable Energy Applications

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Laccase enzymes have been investigated for their potential as eco-friendly biofuel cell cathode catalysts. In these applications, electrons transfer from the electrode directly to the so-called Type 1 redox site within the enzyme. Over the years, resonance Raman (RR) spectroscopy has been utilized to probe the nature of ligand interactions with the Cu2+ center at the T1 site of the resting, oxidized form of laccases and related blue copper proteins. Spectral interpretation has been guided by parallel studies of small molecule T1 site mimics. The dissertation reports the results of time-dependent density functional theory (TDDFT) calculations performed on a series of experimentally well-studied T1 site mimics based on Cu2+ thiolate complexes stabilized by either a tridentate tris(pyrazolyl)hydroborate ligand or a bidentate β-diketiminate ligand. Vertical excitation energies were computed for the first 10 doublet states of each complex. The electronic absorption and RR spectra derived show excellent agreement with the experiment. RR spectra were dominated by a strong Cu−S stretching transition near 400 cm−1, which is sensitive to molecular conformation within the thiolate ligand. The results provide a foundation to build from in advancing models of laccase catalytic active sites in support of sustainable technologies. In follow on work, TDDFT was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, and chemical sensing and synthesis. MV2+ is easily reduced by single electron transfer to form a radical cation species (MV•+), which has an intense UV-visible absorption near ~600 nm. The redox properties of the MV2+ / MV•+ couple and light-sensitivity of MV•+ have made the system appealing for photo-electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo-induced charge transfer processes through electronic absorption and RR spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+. Using a conventional hybrid exchange functional (B3-LYP) and a long-range corrected hybrid exchange functional (ωB97X-D3), including with a conductor-like polarizable continuum model to account for solvation, the electronic absorption and RR spectra predicted are in good agreement with experiment. The findings and models developed provide groundwork for advancing understanding of viologen electronic properties and related organic redox mediators important in renewable energy applications.

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Resonance, Raman, Vibrational, Computational, Chemistry