@article {ISI:000227764700020,
	title = {Spectroscopy of non-heme iron thiolate complexes: insight into the electronic structure of the low-spin active site of nitrile hydratase.},
	journal = {Inorg. Chem.},
	volume = {44},
	number = {6},
	year = {2005},
	month = {mar},
	pages = {1826{\textendash}36},
	abstract = {Detailed spectroscopic and computational studies of the low-spin iron complexes [Fe(III)(S2(Me2)N3 (Pr,Pr))(N3)] (1) and [Fe(III)(S2(Me2)N3 (Pr,Pr))]1+ (2) were performed to investigate the unique electronic features of these species and their relation to the low-spin ferric active sites of nitrile hydratases. Low-temperature UV/vis/NIR and MCD spectra of 1 and 2 reflect electronic structures that are dominated by antibonding interactions of the Fe 3d manifold and the equatorial thiolate S 3p orbitals. The six-coordinate complex 1 exhibits a low-energy S(pi) {\textendash}> Fe 3d(xy) (approximately 13,000 cm(-1)) charge-transfer transition that results predominantly from the low energy of the singly occupied Fe 3d(xy) orbital, due to pure pi interactions between this acceptor orbital and both thiolate donor ligands in the equatorial plane. The 3d(pi) {\textendash}> 3d(sigma) ligand-field transitions in this species occur at higher energies (>15,000 cm(-1)), reflecting its near-octahedral symmetry. The Fe 3d(xz,yz) {\textendash}> Fe 3d(xy) (d(pi) {\textendash}> d(pi)) transition occurs in the near-IR and probes the Fe(III)-S pi-donor bond; this transition reveals vibronic structure that reflects the strength of this bond (nu(e) approximately 340 cm(-1)). In contrast, the ligand-field transitions of the five-coordinate complex 2 are generally at low energy, and the S(pi) {\textendash}> Fe charge-transfer transitions occur at much higher energies relative to those in 1. This reflects changes in thiolate bonding in the equatorial plane involving the Fe 3d(xy) and Fe 3d(x2-y2) orbitals. The spectroscopic data lead to a simple bonding model that focuses on the sigma and pi interactions between the ferric ion and the equatorial thiolate ligands, which depend on the S-Fe-S bond angle in each of the complexes. These electronic descriptions provide insight into the unusual S = 1/2 ground spin state of these complexes: the orientation of the thiolate ligands in these complexes restricts their pi-donor interactions to the equatorial plane and enforces a low-spin state. These anisotropic orbital considerations provide some intriguing insights into the possible electronic interactions at the active site of nitrile hydratases and form the foundation for further studies into these low-spin ferric enzymes.},
	keywords = {BINDING SITES, Computational Biology, ELECTROCHEMISTRY, Ferric Compounds, Ferric Compounds: chemical synthesis, Ferric Compounds: chemistry, Hydro-Lyases, Hydro-Lyases: chemistry, iron, Iron: chemistry, models, MOLECULAR, Molecular Conformation, molecular structure, PhD, SPECTROPHOTOMETRY, Sulfhydryl Compounds, Sulfhydryl Compounds: chemistry},
	issn = {0020-1669},
	doi = {10.1021/ic0487068},
	url = {http://www.ncbi.nlm.nih.gov/pubmed/15762709},
	author = {Kennepohl, Pierre and Neese, Frank and Schweitzer, Dirk and Jackson, Henry L and Kovacs, Julie A and Solomon, Edward I}
}
@article {ISI:000226477400012,
	title = {Variable energy photoelectron spectroscopy: electronic structure and electronic relaxation},
	journal = {Coord. Chem. Rev.},
	volume = {249},
	number = {1-2},
	year = {2005},
	month = {jan},
	pages = {229{\textendash}253},
	abstract = {Use of variable photon energy with polarization available from synchrotron radiation has revolutionized the use of photoelectron spectroscopy (PES) in the study of transition metal complexes. The intensity dependence of PES peaks allows their definite assignment, a quantitation of the covalent mixing of the metal and ligand valence orbitals over the entire valence band, and an experimental determination of electronic relaxation. The last effect is the change in electronic structure with ionization, which is found to be large for transition metal complexes. Variable energy PES (VEPES) provides a powerful method to study bonding and its change with ionization and this provides fundamental insight into redox processes in inorganic and bioinorganic chemistry. (C) 2004 Elsevier B.V. All rights reserved.},
	keywords = {PhD},
	issn = {00108545},
	doi = {10.1016/j.ccr.2004.02.016},
	url = {http://linkinghub.elsevier.com/retrieve/pii/S0010854504000426},
	author = {Solomon, Edward I and Basumallick, Lipika and Chen, Peng and Kennepohl, Pierre}
}
@article {ISI:000180870300010,
	title = {Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 1. Photoelectron spectroscopic determination of electronic relaxation.},
	journal = {Inorg. Chem.},
	volume = {42},
	number = {3},
	year = {2003},
	pages = {679{\textendash}88},
	abstract = {Electronic relaxation, the change in molecular electronic structure as a response to oxidation, is investigated in [FeX(4)](2)(-)(,1)(-) (X = Cl, SR) model complexes. Photoelectron spectroscopy, in conjunction with density functional methods, is used to define and evaluate the core and valence electronic relaxation upon ionization of [FeX(4)](2)(-). The presence of intense yet formally forbidden charge-transfer satellite peaks in the PES data is a direct reflection of electronic relaxation. The phenomenon is evaluated as a function of charge redistribution at the metal center (Deltaq(rlx)) resulting from changes in the electronic structure. This charge redistribution is calculated from experimental core and valence PES data using a valence bond configuration interaction (VBCI) model. It is found that electronic relaxation is very large for both core (Fe 2p) and valence (Fe 3d) ionization processes and that it is greater in [Fe(SR)(4)](2)(-) than in [FeCl(4)](2)(-). Similar results are obtained from DFT calculations. The results suggest that, although the lowest-energy valence ionization (from the redox-active molecular orbital) is metal-based, electronic relaxation causes a dramatic redistribution of electron density ( approximately 0.7e) from the ligands to the metal center corresponding to a generalized increase in covalency over all M-L bonds. The more covalent tetrathiolate achieves a larger Deltaq(rlx) because the LMCT states responsible for relaxation are significantly lower in energy than those in the tetrachloride. The large observed electronic relaxation can make significant contributions to the thermodynamics and kinetics of electron transfer in inorganic systems.},
	keywords = {PhD},
	issn = {0020-1669},
	doi = {10.1021/ic020330f},
	url = {http://www.ncbi.nlm.nih.gov/pubmed/12562181},
	author = {Kennepohl, Pierre and Solomon, Edward I}
}
@article {ISI:000180870300011,
	title = {Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 2. Reduction potentials.},
	journal = {Inorg. Chem.},
	volume = {42},
	number = {3},
	year = {2003},
	month = {feb},
	pages = {689{\textendash}95},
	abstract = {This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl(4)](2)(-)(,1)(-) and [Fe(SR)(4)](2)(-)(,1)(-) (DeltaE(0) approximately 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (DeltaI(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to DeltaI(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to DeltaI(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I(vert) relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge () of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe)( )(3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases and significantly lowers its redox potential.},
	keywords = {PhD},
	issn = {0020-1669},
	doi = {10.1021/ic0203318},
	url = {http://www.ncbi.nlm.nih.gov/pubmed/12562182},
	author = {Kennepohl, Pierre and Solomon, Edward I}
}
@article {ISI:000180870300012,
	title = {Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 3. Kinetics of electron transfer.},
	journal = {Inorg. Chem.},
	volume = {42},
	number = {3},
	year = {2003},
	month = {feb},
	pages = {696{\textendash}708},
	abstract = {The kinetics of electron transfer for rubredoxins are examined using density functional methods to determine the electronic structure characteristics that influence and allow for fast electron self-exchange in these electron-transport proteins. Potential energy surfaces for [FeX(4)](2-,1-) models confirm that the inner-sphere reorganization energy is inherently small for tetrathiolates ( approximately 0.1 eV), as evidenced by the only small changes in the equilibrium Fe-S bond distance during redox (Deltar(redox) approximately 0.05 A). It is concluded that electronic relaxation and covalency in the reduced state allow for this small in this case relative to other redox couples, such as the tetrachloride. Using a large computational model to include the protein medium surrounding the [Fe(SCys)(4)](2-,1-) active site in Desulfovibrio vulgaris Rubredoxin, the electronic coupling matrix element for electron self-exchange is defined for direct active-site contact (H0(DA)). Simple Beratan-Onuchic model is used to extend coupling over the complete surface of the protein to provide an understanding of probable electron-transfer pathways. Regions of similar coupling properties are grouped together to define a surface coupling map, which reveals that very efficient self-exchange occurs only within 4 sigma-bonds of the active site. Longer-range electron transfer cannot support the fast rates of electron self-exchange observed experimentally. Pathways directly through the two surface cysteinate ligands dominate, but surface-accessible amides hydrogen-bonded to the cysteinates also contribute significantly to the rate of electron self-exchange.},
	keywords = {PhD},
	issn = {0020-1669},
	doi = {10.1021/ic0203320},
	url = {http://www.ncbi.nlm.nih.gov/pubmed/12562183},
	author = {Kennepohl, Pierre and Solomon, Edward I}
}
@article {ISI:A1997XJ83300011,
	title = {A Multiplet Analysis of Fe K-Edge 1s {\textrightarrow} 3d Pre-Edge Features of Iron Complexes},
	journal = {J. Am. Chem. Soc.},
	volume = {119},
	number = {27},
	year = {1997},
	month = {jul},
	pages = {6297{\textendash}6314},
	abstract = {X-ray absorption Fe-K edge data on ferrous and ferric model complexes have been studied to establish a detailed understanding of the 1s {\textendash}> 3d pre-edge feature and its sensitivity to the electronic structure of the iron site. The energy position and splitting, and intensity distribution, of the pre-edge feature were found to vary systematically with spin state, oxidation state, geometry, and bridging ligation (for binuclear complexes). A methodology for interpreting the energy splitting and intensity distribution of the 1s {\textendash}> 3d pre-edge features was developed for high-spin ferrous and ferric complexes in octahedral, tetrahedral, and square pyramidal environments and low-spin ferrous and ferric complexes in octahedral environments. In each case, the allowable many-electron excited states were determined using ligand field theory. The energies of the excited states were calculated and compared to the energy splitting in the 1s {\textendash}> 3d pre-edge features. The relative intensities of electric quadrupole transitions into the many-electron excited states were obtained and compared to the intensity pattern of the pre-edge feature. The effects of distorting the octahedral iron site to tetrahedral and square pyramidal geometries were analyzed. The contributions to the pre-edge intensity from both electric quadrupole and electric dipole (from 3d-4p mixing) intensity mechanisms were established for these distorted cases; the amount of 4p character and its distribution over the many-electron final states were experimentally estimated and compared to theoretical predictions from density functional calculations. The methodology was also applied to binuclear complexes, and a clear marker for the presence of a mu-oxo Fe-O-Fe bridge was established. General trends in 3d-4p mixing are developed and discussed for a series of geometries and oxidation states of Fe complexes. The results presented should further aid in the interpretation of the Is - 3d pre-edge region of iron complexes and non-heme iron enzymes.},
	keywords = {PhD},
	issn = {0002-7863},
	doi = {10.1021/ja964352a},
	url = {http://pubs.acs.org/doi/abs/10.1021/ja964352a},
	author = {Westre, Tami E and Kennepohl, Pierre and DeWitt, Jane G and Hedman, Britt and Hodgson, Keith O and Solomon, Edward I}
}
@article {ISI:A1996VT05300002,
	title = {Structural and Functional Aspects of Metal Sites in Biology},
	journal = {Chem. Rev.},
	volume = {96},
	number = {7},
	year = {1996},
	month = {jan},
	pages = {2239{\textendash}2314},
	keywords = {PhD, review},
	issn = {0009-2665},
	doi = {10.1021/cr9500390},
	url = {http://pubs.acs.org/doi/abs/10.1021/cr9500390},
	author = {Holm, Richard H and Kennepohl, Pierre and Solomon, Edward I}
}
