Jon R. Schoonover

Flash Photolysis Studies of Roussin's Black Salt Anion: Fe4S3(NO)7-.

A time-resolved optical (UV-vis) spectroscopic study of Roussins black salt anion, Fe(4)S(3)(NO)(7)(-), revealed two separate intermediates, X and Y, following laser flash photolysis. Both intermediates react with nitric oxide with second-order kinetics to re-form the parent complex (k(NO)(X)() = 1.3 x 10(7) M(-)(1) s(-)(1); k(NO)(Y)() = 7.0 x 10(5) M(-)(1) s(-)(1) in aqueous solutions). The shorter-lived intermediate X was observed in time resolved infrared spectroscopic studies. Isotopic labeling experiments involving the exchange of black salt nitrosyls with (15)NO(2)(-) or (15)N(18)O were used to probe the correlation between nu(NO) bands and the anion structures. The identities of the intermediates are interpreted in terms of photolytic loss of chemically distinct nitrosyls found on the Fe(4)S(3)(NO)(7)(-) anion.

Application of transient vibrational spectroscopies to the excited states of metal polypyridyl complexes

Abstract Time-resolved resonance Raman and infrared spectroscopies have been applied to a variety of problems arising in excited states and molecular assemblies based on polypyridyl complexes of Re(I), Ru(II) and Os(II). Application of transient resonance Raman spectroscopy has been used to assign the acceptor ligand in heteroleptic Ru(II) complexes and to characterize molecular structure in acceptor ligands. It has been applied to intramolecular electron transfer in chromophore-quencher complexes and molecular assemblies and to intramolecular energy transfer in polynuclear complexes. Time-resolved infrared spectroscopy has opened new avenues for the study of excited-state electronic structure and dynamics by the observation of ν(CO) and ν(CN) bands in transient absorption difference spectra. This technique has been applied to distinguishing between metal-to-ligand charge transfer and ligand-based ππ ∗ states in Re(I) complexes and to intramolecular energy transfer in cyano-bridged oligomers.

Resonance Raman studies of Rieske-type proteins

Resonance Raman (RR) spectra are reported for the [2Fe-2S] Rieske protein from Thermus thermophilus (TRP) and phthalate dioxygenase from Pseudomonas cepacia (PDO) as a function of pH and excitation wavelength. Depolarization ratio measurements are presented for the RR spectra of spinach ferredoxin (SFD), TRP, and PDO at 74 K. By comparison with previously published RR spectra of SFD, we suggest reasonable assignments for the spectra of TRP and PDO. The spectra of PDO exhibit virtually no pH dependence, while significant changes are observed in TRP spectra upon raising the pH from 7.3 to 10.1. One band near 270 cm-1, which consists of components at 266 cm-1 and 274 cm-1, is attributed to Fe(III)-N(His) stretching motions. We suggest that these two components arise from conformers having a protonated-hydrogen-bonded imidazole (266 cm-1) and deprotonated-hydrogen-bonded imidazolate (274 cm-1) coordinated to the Fe/S cluster and that the relative populations of the two species are pH-dependent; a simple structural model is proposed to account for this behavior in the respiratory-type Rieske proteins. In addition, we have identified RR peaks associated with the bridging and terminal sulfur atoms of the Fe-S-N cluster. The RR excitation profiles of peaks associated with these atoms are indistinguishable from each other in TRP (pH 7.3) and PDO and differ greatly from those of [2Fe-2S] ferrodoxins. The profiles are bimodal with maxima near 490 nm and > approx. 550 nm. By contrast, bands associated with the Fe-N stretch show a somewhat different enhancement profile. Upon reduction, RR peaks assigned to Fe-N vibrations are no longer observed, with the resulting spectrum being remarkably similar to that reported for reduced adrenodoxin. This indicates that only modes associated with Fe-S bonds are observed and supports the idea that the reducing electron resides on the iron atom coordinated to the two histidine residues. Taken as a whole, the data are consistent with an St2FeSb2Fe[N(His)]t2 structure for the Rieske-type cluster.

Infrared linear dichroism study of a hydrolytically degraded poly(ester urethane)

Static and dynamic infrared linear dichroism data have been used to supply additional insight into changes in tensile properties as a consequence of hydrolytic degradation of a segmented poly(ester urethane). Unaged material responds to tensile deformation with the soft (polyester) segments supplying the elasticity and the hard (polyurethane) segments supplying strength. Upon hydrolytic degradation, the static and dynamic data indicate altered orientational responses at the molecular-level, which are interpreted as resulting from cleavage of the soft segment chains and altered hydrogen-bonding interactions for both segments.

Photosensitization of wide bandgap semiconductors with antenna molecules

Abstract Polynuclear metal complexes, supporting efficient intramolecular energy transfer processes, can be used to increase the light harvesting efficiency of sensitized wide bandgap semiconductors. Experimental studies are discussed to emphasize: (i) how structural changes at the molecular level may affect the performances of photoelectrochemical cells based on antenna-sensitizer molecular assemblies, (ii) the availability of fast time-resolved resonance Raman and infrared spectroscopies for monitoring intercomponent energy transfer processes, and (iii) the possibility to design extended antenna units acting as molecular conduits for long-range energy transfer.

Raman Spectroscopy and Factor Analysis of Tumorigenic and Non-Tumorigenic Cells:

Raman spectra, measured for viable suspensions of M1 and MR1 cells, demonstrate spectral differences correlating with an increased protein and phospholipid content relative to DNA in the MR1 cells. The M1 and MR1 cell lines consist of rat fibroblast cells that have been transfected with a gene causing immortality, and the MR1 line has been further transfected with a gene causing tumor formation. The cell suspensions were prepared from monolayer cultures in the same phase of growth and remained viable throughout the experiment. The Raman data have been further examined by principal factor analysis with two approaches, (1) a constrained analysis to provide a relative contribution to the Raman data from cellular components, and (2) analysis of differences in the raw data files of the two cell lines. Utilizing the constrained principal factor analysis for the two cell lines analyzed separately provides a measure of the relative contribution of protein, lipid, DNA, RNA, and buffer to the cell Raman spectra. Factor analysis of M1 and MR1 Raman spectra analyzed together demonstrates that the two cell lines can be differentiated by Raman spectroscopy with no prior spectral processing.

INTEGRATION OF ELEMENTAL AND MOLECULAR IMAGING TO CHARACTERIZE HETEROGENEOUS INORGANIC MATERIALS

Spatially resolved elemental and molecular information has been obtained for complex inorganic mixed-phase systems, including plutonium-contaminated ash samples, with the use of micro-X-ray fluorescence (MXRF), micro-Raman, and micro-infrared imaging spectroscopies. Approaches for spatially guided analysis and the combination of multispectral data sets to improve assignment of chemical composition are discussed for two heterogeneous materials. This novel integrated experimental approach provides the advantages of a very small sample size and the ability to correlate elemental analysis with molecular functional group analysis for species identification, as well as insight into chemical interactions in complex matrices.

Static and Dynamic Infrared Linear Dichroic Study of a Polyester/Polyurethane Copolymer Using Step-Scan FT-IR and a Photoelastic Modulator:

Dynamic infrared linear dichroism (DIRLD) measurements of films of a thermo-plastic polyester/polyurethane random copolymer (Estane 5703, B. F. Goodrich) are reported. Step-scan Fourier transform infrared (FT-IR) is used for dynamic measurements, and a photoelastic modulator (PEM) is used to create broadband polarization modulation as the carrier frequency for the strain modulation. A novel modulation/demodulation strategy has been employed that simplifies the triple-modulation experiment into a double-modulation experiment; the theory is thoroughly discussed. Both static and dynamic dichroic absorption difference spectra have been measured on the prestretched polymer film. The results are of high signal-to-noise ratio (SNR) and clearly indicate the static and dynamic orientation of the transition dipole moments due to the tensile deformation. The dynamic orientation responses are primarily in phase with the perturbation. The orientation magnitudes of the infrared absorption bands are quantified and compared, and the orientations of the hard and soft domains are differentiated. To assist in the interpretation of the dynamically measured data, we also describe a static linear dichroic measurement using a wire-grid polarizer and FT-IR in the rapid-scan mode for a sample incrementally drawn until the point of breaking. The orientation functions of selected bands have been calculated, and the static results agree with the dynamic data, indicating the dependency of the dynamic orientation response on the preorientation state.

Time-Resolved, Step-Scan FTIR Spectroscopy of Excited States of Transition Metal Complexes

Abstract Time-resolved, step-scan Fourier transform infrared spectroscopy has been developed as a method of studying electronically excited states of transition metal complexes. The technique takes advantage of the unique properties of carbonyl and cyanide stretching vibrations, including high infrared oscillator strength and well established sensitivity of vibrational frequency, intensity, and bandwidth to electronic and molecular structure. Electronic excitation generally produces significant transient infrared absorption changes which are characteristic of the changes in electronic structure, such as oxidation of the metal. TRIR spectroscopy thus provides new insight on the nature of the excited state transition (e.g. charge transfer vs. ligand centered), the extent of charge transfer, communication between metal centers, and energy and electron transfer processes. The step-scan FTIR approach has significant advantages over conventional time-resolved techniques, including spectral multiplexing, increas...

Pressure-dependent Fourier transform infrared spectroscopy of a poly (ester urethane)

The effects of hydrostatic pressure upon (1) a segmented poly (ester urethane), (2) a hydrolytically degraded sample of the same polymer, and (3) models for the polyurethane and polyester segments in this polymer have been studied by Fourier transform infrared spectroscopy using high-pressure diamond anvil cells (DACs). The pressure responses of the vibrational frequencies of specific functional groups of the poly (ester urethane) in the 0-100-kbar range are compared with data for individual segment models and the partially degraded sample. The results indicated that the polymer is highly stable in this pressure regime, with no measurable degradation or phase changes. Differences in the pressure dependency of specific infrared bands between the poly (ester urethane) sample and the partially degraded sample are slight and consistent with changes in hydrogen-bonding interactions and shorter chain lengths in the degraded sample.