New electron spin resonance (ESR) technologies have been developed, which have led to new and improved applications. (a) The development of two-dimensional Fourier transform (FT) ESR required spectrometers providing intense π/2 microwave pulses of very short (3–5 ns) duration, wide bandwidths, and very short dead times. It has enabled studies that resolve sophisticated details of molecular dynamics in complex fluids. (b) Methods that produce multiple quantum coherences by pulsed ESR now enable accurate measurements of large distances (>12A). (c) One of the most important advances has been the extension of ESR to high magnetic fields and high frequencies. This has benefited from the utilization of quasi-optical methods, especially above 150 GHz. The greatly improved orientational resolution and the faster “snapshot” of motions that are provided by ESR at high frequencies enhance studies of molecular dynamics. The use of both high and lower frequencies enables one to unravel faster and slower modes from the...

New technologies in electron spin resonance.

We describe the preparation and characterization of unusual pentavalent molybdenum oxide stabilized by water molecules, Mo2O5·2H2O. Ultrasound irradiation of a slurry of molybdenum hexacarbonyl, Mo(CO)6, in Decalin for 3 h under ambient air yields blue-colored Mo2O5·2H2O. Infrared (FT-IR) spectrum analysis of the resulting blue product reveals that the Mo ions possess molybdenyl bonds (MoO) and Mo−O character and also shows the presence of hydrogen-bonded, as well as coordinated, water molecules. The DSC profile of the blue oxide shows the presence of two endothermic peaks at around 100 °C and 140 °C, corresponding to the elimination of hydrogen-bonded and coordinated water molecules, respectively. The amount of water molecules was determined by thermogravimetric analysis (TGA). Characterization using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) with selected area electron diffraction (SAED) shows the amorphous nature of the blue product. The TEM picture shows that the blue ox...

Characterization of sonochemically prepared unsupported and silica-supported nanostructured pentavalent molybdenum oxide

ESR spectroscopy for the study of polymer heterogeneity

We introduce the novel fluoroalkoxy molybdenum(V) reagent 1 which has superior reactivity and selectivity in comparison to MoCl5 or the MoCl5/TiCl4 reagent mixture in the oxidative coupling reactions of aryls. Common side reactions, such as chlorination and/or oligomer formation, are drastically diminished creating a powerful and useful reagent for oxidative coupling. Theoretical treatment of the reagent interaction with 1,2-dimethoxybenzene-type substrates indicates an inner-sphere electron transfer followed by a radical cationic reaction pathway for the oxidative-coupling process. EPR spectroscopic and electrochemical studies, X-ray analyses, computational investigations, and the experimental scope provide a highly consistent picture. The substitution of chlorido ligands by hexafluoroisopropoxido moieties seems to boost both the reactivity and selectivity of the metal center which might be applied to other reagents as well.

Powerful fluoroalkoxy molybdenum(V) reagent for selective oxidative arene coupling reaction.

Microwave Engineering Fundamentals and Spectrometer Design.- EPR Spectrometers at Frequencies Below X-band.- Frequency Dependence of EPR Sensitivity.- ENDOR Coils and Related Radiofrequency Circuits.- The Generation and Detection of Electron Spin Echoes.- Convolution-Based Algorithm: from Analysis of Rotational Dynamics to EPR Oximetry and Protein Distance Measurements.- ID and 2D Electron Spin Resonance Imaging (ESRI) of Transport and Degradation Processes in Polymers.- Peptide-Aggregation and Conformation Properties as Studied By Pulsed Electron-Electron Double Resonance.

EPR : instrumental methods

The translational diffusion coefficients of a small paramagnetic molecule (the deuteriated nitroxide PDTEMPONE) as tracer were measured in solutions of polystyrene (PS) in toluene and dimethylformamide (DMF) and in cross-linked polystyrene (cPS) networks swollen by the same solvents, using two-dimensional spatial−spectral electron spin resonance imaging (2D ESRI). PS solutions containing 30, 40, and 50 w/w % polymer were examined in toluene at 300, 305, 310, and 315 K and in DMF at 300 K. PS networks prepared with 0.5, 1.0, 2.5, and 5.0 mol % divinylbenzene as cross-linker were swollen in toluene or DMF and studied at 300 K. Two-dimensional images describing the distribution of the diffusant in the sample were reconstructed from a complete set of projections obtained as a function of the magnetic field gradient applied along the symmetry axis of the sample. The concentration profile of the diffusant at a given time was obtained by integrating absorption ESR spectra in slices perpendicular to the spatial a...

ESR Imaging of Tracer Translational Diffusion in Polystyrene Solutions and Swollen Networks

EPR spectra of MoV(as MoCl5) in ethanol solutions and in perfluorinated ionomers (PFI) with long pendant chains (Nafion) have been measured as a function of MoCl5 concentration in the solutions (in the range 3.60 × 10–6 to 1.15 × 10–4M) and degree of neutralization of the sulfonic groups in the ionomers. Two MoV sites were characterized in the ethanol solutions, and their relative intensity was found to depend on MoCl5 concentration. Both sites, however, show evidence for chlorine ligands around the paramagnetic cation. The MoV site in PFI swollen by ethanol is identical to the site detected in neat ethanol solutions at higher MoCl5 content, and was assigned to a molybdenum site with five chlorine ligands and an oxygen ligand from the solvent. The diffusion coefficients of this species were determined by 2D spatial–spectral electron paramagnetic resonance imaging (2D EPRI) in Nafion–ethanol, by simulating the concentration profiles along the spatial coordinate measured during the diffusion process. The diffusion coefficients in Nafion–ethanol are 5.0 × 10–7, 7.1 × 10–7, 10.4 × 10–7 and 13.3 × 10–7 cm2 s–1 at 280, 300, 315 and 330 K, respectively. The activation energy for diffusion is 15.4 kJ mol–1.

Binding and transport of mov in perfluorinated ionomers by epr and epr imaging

Electron spin resonance (ESR) spectra of Mo5+ in perfluorinated membranes (Nafion) neutralized by MoCl5 suggest that the ionomer acts as an ion-selective medium and replaces the chlorine ligands of the molybdenum center by oxygen ligands from the sulfonic groups. This conclusion is based on an analysis of the ESR parameters that characterize the paramagnetic center and is supported by the detection of only one major paramagnetic molybdenum site, Mo(A) in the bulk ionomer, in the absence of solvents. Ligand design via the ionomer was thus achieved. We propose that the main molybdenum species is MoO(SO3-)5, where the unique oxygen ligand is from an SO group of the sulfonic moiety. Exposure of Mo/Nafion to water (up to 5 Torr) has no significant effect on the ESR parameters and on the intensity of the ESR signal. Exposure to 25 Torr of water leads to the reversible disappearance of the ESR signal. Brief exposure of Mo/Nafion to gaseous acetonitrile (as CD3CN) results in the appearance of two new Mo species, ...

Zhan Gao

Papers

ESR Imaging of Tracer Translational Diffusion in Polystyrene Solutions and Swollen Networks

Binding and transport of mov in perfluorinated ionomers by epr and epr imaging

Ligand Design by Ionomers. ESR of MoV in Perfluorinated Ionomer Supports