An NMR Spectroscopy and Density-Functional Theory Study of Organometallic Complexes and Heme Proteins With Carbonyl, Alkylisocyanide, Nitrosoarene and Olefin Ligands: Applications to Structure Determinations
McMahon, Michael Thomas
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https://hdl.handle.net/2142/84449
Description
Title
An NMR Spectroscopy and Density-Functional Theory Study of Organometallic Complexes and Heme Proteins With Carbonyl, Alkylisocyanide, Nitrosoarene and Olefin Ligands: Applications to Structure Determinations
Author(s)
McMahon, Michael Thomas
Issue Date
1999
Doctoral Committee Chair(s)
Oldfield, Eric
Department of Study
Chemical Physics
Discipline
Chemical Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Language
eng
Abstract
In this thesis I describe how density functional theory methods can be combined with NMR spectroscopy to determine ligand binding geometries in organometallic complexes and heme proteins. Results of density functional studies on compounds with well characterized structures are presented to test the accuracy of the calculations, and then these calculations are used to determine the structure of ligands bound to myoglobin and hemoglobin. The question of how CO ligands bind to iron in metalloproteins was investigated by using a combination of nuclear magnetic resonance (NMR), 57Fe Mossbauer and infra-red spectroscopic techniques, combined with the calculations to analyze the spectroscopic results. The 13C isotropic shift and shift anisotropy as well as the 17O chemical shift, and the 17O nuclear quadrupole coupling constant (NQCC) lead to most probable ligand tilt (tau) and bend (beta) angles of 0°,l° for the Ao-substate when using a Bayesian probability or Z-surface method for structure determination. Results for the A1-substate (including the 57Fe NMR chemical shift and Mossbauer quadrupole splitting) are also consistent with close to linear and untilted Fe-C-O geometries (tau = 4°, beta = 7°), with the small changes in ligand spectroscopic parameters being attributed to electrostatic field effects. In addition density functional calculations were carried out on alkyl isocyanide ligands bound to hernes. As seen from the calculations, a bend at the ligand nitrogen is energetically feasible and also gives the large deshielding seen experimentally for the proteins. These results represent the first detailed quantum chemical analysis of metal-ligand geometries using up to seven different spectroscopic observables from three types of spectroscopy, and suggest a generalized approach for structure determination.
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