33 research outputs found
Investigation of the Electronic Structure and Reactivity of Non-Heme Iron Nitrosyl and Nitroxyl Complexes.
High-spin non-heme ferrous nitroxyl (NO-) complexes ({FeNO}8 in the Enemark-Feltham notation) have been proposed as important intermediates in bacterial nitric oxide reductases. Despite their significance, model compounds for these species have remained elusive and prior to the studies described here, little was known about their spectroscopic properties and reactivity. The work presented in this dissertation provides, for the first time, detailed insight into the properties of high-spin {FeNO}8 complexes.
The first high-spin non-heme {FeNO}8 complex has been synthesized via chemical or electrochemical reduction of a ferrous nitrosyl ({FeNO}7) precursor. The use of a sterically encumbering ligand prevents the disproportionation typically observed for {FeNO}8 complexes. A rare high-spin ferric nitrosyl ({FeNO}6) species has also been generated using the same ligand. This system constitutes the first complete high-spin {FeNO}6-8 series. Detailed spectroscopic investigations coupled to DFT calculations show that the {FeNO}6, {FeNO}7, and {FeNO}8 complexes have Fe(IV)-NO-, Fe(III)-NO-, and Fe(II)-NO- electronic structures, respectively. Importantly, the covalency of the Fe-NO bond decreases along this series. This has implications for the reactivity of these species. For example, only the {FeNO}8 complex, in which the Fe-NO bond is weakest, is basic. Protonation of the {FeNO}8 yields a highly unstable species which, based on spectroscopic investigations, is suggested to be the first high-spin Fe(II)-HNO complex.
The decomposition of other {FeNO}8 compounds has also been investigated. Our group previously showed that rapid and efficient N2O production can be achieved by reduction of [{FeNO}7]2 dimers with adjacent NO moieties. Here, it is demonstrated that N2O production is slow and substoichiometric when the NO units are not in close proximity to each other. Additionally, it is shown that for monomeric compounds, one prominent decomposition pathway involves disproportionation, leading to formation of a dinitrosyl iron complex (DNIC).
Finally, in a separate study, the electronic structure of DNICs at the {Fe(NO)2}9 and {Fe(NO)2}10 redox levels has been investigated using Mossbauer and vibrational spectroscopy. By coupling the findings from these techniques to DFT calculations, the bonding in these species is shown to be extremely covalent which explains their high stability.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133281/1/alspeelm_1.pd
Expanding and Testing a Computational Method for Predicting the Ground State Reduction Potentials of Organic Molecules on the Basis of Empirical Correlation to Experiment
A method for predicting the ground state reduction potentials of organic molecules on the basis of the correlation of computed energy differences between the starting S-0 and one-electron-reduced D-0 species with experimental reduction potentials in acetonitrile has been expanded to cover 3.5 V of potential range and 74 compounds across 6 broad families of molecules. Utilizing the conductor-like polarizable continuum model of implicit solvent allows a global correlation that is computationally efficient and has improved accuracy, with r(2) \u3e 0.98 in all cases and root mean square deviation errors of(mean absolute deviationsmV) for either B3LYP/6-311+G(d,p) or B3LYP//6-31G(d) with an appropriate choice of radii (UAKS or UA0). The correlations are proven to be robust across a wide range of structures and potentials, including four larger (27-28 heavy atoms) and more conformationally flexible photochromic molecules not used in calibrating the correlation. The method is also proven to be robust to a number of minor student mistakes or methodological inconsistencies
Synthesis and Structural Investigation of an \u27Oxazinoquinolinespirohexadienone\u27 That Only Exists as Its Long-Wavelength Ring-Opened Quinonimine Isomer
The spirocyclic oxazinoquinolinespirohexadienone (OSHD) photochromes are computationally predicted to be an attractive target as electron deficient analogues of the perimidinespirohexadienone (PSHD) photochromes, for eventual application as photochromic photooxidants. We have found the literature method for their preparation unsuitable and present an alternative synthesis. Unfortunately the product of this synthesis is the long wavelength (LW) ring-opened quinonimine isomer of the OSHD. We have found this isomer does not close to the spirocyclic short wavelength isomer (SW) upon prolonged standing in the dark, unlike other PSHD photochromes. The structure of this long wavelength isomer was found by NMR and X-ray crystallography to be exclusively the quinolinone (keto) tautomer, though experimental cyclic voltammetry supported by our computational methodology indicates that the quinolinol (enol) tautomer (not detected by other means) may be accessible through a fast equilibrium lying far toward the keto tautomer. Computations also support the relative stability order of keto LW over enol LW over SW
Efficient Computational Methods for Accurately Predicting Reduction Potentials of Organic Molecules
A simple computational approach for predicting ground-state reduction potentials based upon gas phase geometry optimizations at a moderate level of density functional theory followed by single-point energy calculations at higher levels of theory in the gas phase or with polarizable continuum solvent models is described. Energies of the gas phase optimized geometries of the S0 and one-electron-reduced D0 states of 35 planar aromatic organic molecules spanning three distinct families of organic photooxidants are computed in the gas phase as well as well in implicit solvent with IPCM and CPCM solvent models. Correlation of the D0 â S0 energy difference (essentially an electron affinity) with experimental reduction potentials from the literature (in acetonitrile vs SCE) within a single family, or across families when solvent models are used, yield correlations with r2 values in excess of 0.97 and residuals of about 100 mV or less, without resorting to computationally expensive vibrational calculations or thermodynamic cycles
Structural and Spectroscopic Characterization of a HighâSpin {FeNO}6 Complex with an Iron(IV)âNOâ Electronic Structure
Although the interaction of lowâspin ferric complexes with nitric oxide has been well studied, examples of stable highâspin ferric nitrosyls (such as those that could be expected to form at typical nonâheme iron sites in biology) are extremely rare. Using the TMG3tren coâligand, we have prepared a highâspin ferric NO adduct ({FeNO}6 complex) via electrochemical or chemical oxidation of the corresponding highâspin ferrous NO {FeNO}7 complex. The {FeNO}6 compound is characterized by UV/Visible and IR spectroelectrochemistry, Mössbauer and NMR spectroscopy, Xâray crystallography, and DFT calculations. The data show that its electronic structure is best described as a highâspin iron(IV) center bound to a triplet NOâ ligand with a very covalent ironâNO bond. This finding demonstrates that this highâspin iron nitrosyl compound undergoes ironâcentered redox chemistry, leading to fundamentally different properties than corresponding lowâspin compounds, which undergo NOâcentered redox transformations.Die EinâElektronenâOxidation des HighâSpinâEisennitrosylkomplexes [Fe(TMG3tren)(NO)]2+ egibt ein seltenes HighâSpin(S=1)âEisenâNOâAddukt ({FeNO}6), das laut spektroskopischen Untersuchungen und DFTâRechnungen eine FeIVâNOââElektronenstruktur aufweist. Dieser Befund zeigt, dass HighâSpinâNichtâHĂ€mâEisennitrosylkomplexe ein fundamental anderes Redoxverhalten als entsprechende LowâSpinâHĂ€mâSysteme haben.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137344/1/ange201601742.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137344/2/ange201601742-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137344/3/ange201601742_am.pd
A Structural Model for the IronâNitrosyl Adduct of Gentisate Dioxygenase
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146641/1/ejic201800992_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146641/2/ejic201800992-sup-0001-SupMat.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146641/3/ejic201800992.pd
The Fe2(NO)2 Diamond Core: A Unique Structural Motif In NonâHeme IronâNO Chemistry
Nonâheme highâspin (hs) {FeNO}8 complexes have been proposed as important intermediates towards N2O formation in flavodiiron NO reductases (FNORs). Many hsâ{FeNO}8 complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of nonâheme iron nitrosyl complex that is stabilized by an unexpected spinâstate change. Upon reduction of the hsâ{FeNO}7 complex, [Fe(TPA)(NO)(OTf)](OTf) (1), the NâO stretching band vanishes, but no sign of DNIC or N2O formation is observed. Instead, the dimer, [Fe2(TPA)2(NO)2](OTf)2 (2) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hsâ{FeNO}8 intermediate, followed by a spin state change of the iron centers to lowâspin (ls), and speculate that 2 models intermediates in hsâ{FeNO}8 complexes that precede the disproportionation reaction.Eine seltene Raute: Der hsâ{FeNO}7âKomplex 1 wurde durch spektroskopische Methoden und Röntgenkristallographie charakterisiert. Unter reduzierenden Bedingungen wird das Dimer 2 mit rautenförmigem Kern durch Dimerisierung eines hsâ{FeNO}8âZwischenprodukts und anschlieĂende SpinzustandsĂ€nderung der Eisenzentren zu LowâSpin (ls) gebildet.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152586/1/ange201911968_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152586/2/ange201911968-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152586/3/ange201911968.pd
Modeling Fluorescence Observables, Particularly for FRET Experiments, using Markov Chain Analysis of Molecular Dynamics and Quantum Mechanics Simulations
We present a new method for simulating ïŹuorescence observables, particularly those related to bulk and single-molecule ïŹuorescence-detected resonance energy transfer (FRET) experiments. In this method, a molecular dynamics (MD) simulation is used to sample conïŹguration space and quantum mechanics (QM) calculations are used to estimate the electronic coupling between the donor and acceptor probes for snapshots along the MD trajectory. A Markov chain method is used to sample the resulting electronic coupling trajectory allowing accurate simulation of any desired ïŹuorescence observables, such as FRET efïŹciency histograms or time-resolved donor ïŹuorescence decays. The Markov chain results will be compared with the results of simple histogram and averaging schemes showing that the Markov chain is the only one that yields realistic results in well known examples such as the rapid diffusion limit. This combination of computational methods also avoids some pitfalls of traditional FRET analysis such as the kappa-squared and the ideal dipole approximations. Because the simulation results can be compared directly with experimental observables, this method may allow more detail to be derived from experiment than is traditionally possible.
3037-PosB80
The Fe2(NO)2 Diamond Core: A Unique Structural Motif In NonâHeme IronâNO Chemistry
Nonâheme highâspin (hs) {FeNO}8 complexes have been proposed as important intermediates towards N2O formation in flavodiiron NO reductases (FNORs). Many hsâ{FeNO}8 complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of nonâheme iron nitrosyl complex that is stabilized by an unexpected spinâstate change. Upon reduction of the hsâ{FeNO}7 complex, [Fe(TPA)(NO)(OTf)](OTf) (1), the NâO stretching band vanishes, but no sign of DNIC or N2O formation is observed. Instead, the dimer, [Fe2(TPA)2(NO)2](OTf)2 (2) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hsâ{FeNO}8 intermediate, followed by a spin state change of the iron centers to lowâspin (ls), and speculate that 2 models intermediates in hsâ{FeNO}8 complexes that precede the disproportionation reaction.A rare diamond: The hsâ{FeNO}7 complex 1, was characterized by spectroscopic methods and Xâray crystallography. Upon reduction, the diamondâcore dimer 2 is formed by dimerization of an hsâ{FeNO}8 intermediate, followed by a spin state change of the iron centers to lowâspin (ls).Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152873/1/anie201911968.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152873/2/anie201911968_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152873/3/anie201911968-sup-0001-misc_information.pd