Chemistry and Chemical Biology ETDs

Publication Date

7-10-2013

Abstract

Three molecular systems have been studied, which exhibit strong intramolecular electronic communication between constituent moieties. Experimental results for electronic absorption, resonance Raman, electron paramagnetic resonance (EPR), and magnetic circular dichroism (MCD) spectroscopies are presented. Density functional theory (DFT) electronic structure calculations were employed to augment the interpretation of experimental results. Using a valence bond configuration interaction (VBCI) model we show that the ground state properties of the molecules are influenced by excited state configurations. Donor ( = semiquinone)-bridge-acceptor ( = nitronylnitroxide) biradicals show strong ferromagnetic donor-acceptor coupling, and contributions to the ferromagnetic coupling from spin delocalization and spin polarization mechanisms are discussed. We explore effects of bridge substitutions on donor-acceptor coupling, where the bridge can be phenyl, thiophene, biphenyl, bithiophene, xylene, or other sterically hindered phenyl-derivatives. Meta connectivity of the Donor and Acceptor on a phenyl bridge results in antiferromagnetic donor-acceptor coupling, while para connectivity results in ferromagnetic exchange coupling. In this dissertation we discuss in detail the mechanisms for the observed exchange coupling. The heterobimetallic active site of the carbon monoxide dehydrogenase (CODH) enzyme catalyzes the oxidation of carbon monoxide to carbon dioxide in presence of water. Mechanisms presented in the literature differ in their description of how the [MoSCu] unit functions to catalyze CO oxidation. Computational and spectroscopic studies of synthetic CODH active site models show that electronic transitions below ~27000 cm-1 are Cu → Mo charge transfer in nature, and the observation of large isotropic Cu hyperfine coupling to the Mo(V) confirms the importance of Cu \u2192 Mo charge transfer excited states to covalency within the [MoSCu] unit. The large covalency across the [MoSCu] unit in both the enzyme and models points to a likely role of the [MoSCu] unit in the transfer of electrons between CO and Mo(VI). Calculated geometric and electronic changes for CO binding to the Cu(I) site are discussed. The Re2X4(PMe3)4PF6 compounds (X = Cl, Br) are highly symmetric Re dimers with a bond order of 3.5 that includes a partial δ bond. Spectroscopic and computational data of the δ→δ* band show that the contribution of the δ bond to the Re-Re bonding is small. Bonding calculations show that antibonding interactions of the halide and trimethylphosphine ligands with the Re d-orbital manifold dictate the energetic ordering of the Re d-orbitals.

Project Sponsors

National Science Foundation, National Institutes of Health

Language

English

Keywords

rhenium, valence bond configuration interaction, carbon monoxide dehydrogenase, donor-acceptor, donor, acceptor, biradical

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Atlas, Susan R.

Second Committee Member

Kemp, Richard A.

Third Committee Member

Guo, Hua

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