Guest inclusion in cucurbiturils studied by ESR and DFT: the case of nitroxide radicals and spin adducts of DMPO and MNP.

We present an ESR and DFT study of the interaction of cucurbiturils CB[6], CB[7], and CB[8] with di-tert-butyl nitroxide ((CH(3))(3)C)(2)NO (DTBN) and with spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2-methyl-2-nitrosopropane (MNP). The primary goal was to understand the structural parameters that determine the inclusion mechanism in the CBs using DTBN, a nitroxide with great sensitivity to the local environment. In addition, we focused on the interactions with CBs of the spin adducts DMPO/OH and MNP/CH(2)COOH generated in aqueous CH(3)COOH.

In-Depth Profiling of Degradation Processes in a Fuel Cell: 2D Spectral-Spatial FTIR Spectra of Nafion Membranes.

We present in-depth profiling by micro FTIR of cross sections for Nafion 115 membranes in membrane-electrode assemblies (MEAs) degraded during 52 or 180 h at open circuit voltage (OCV) conditions, 90 °C and 30% relative humidity. Analysis of optical images showed highly degraded zones in both MEAs. Corresponding 2D FTIR spectral-spatial maps indicated that C-H and C═O groups are generated during degradation.

Fragmentation of perfluorinated membranes used in fuel cells: detecting very early events by selective encapsulation of short-lived fragments in β-cyclodextrin.

The fragmentation of perfluorinated ionomeric membranes during fuel cell (FC) operation is studied in our laboratory by direct electron spin resonance (ESR) and by spin trapping ESR, and interpretation of the results is facilitated by the study of model compounds (MCs). The advantage of this approach is the ability to detect and identify "early events" in the fragmentation process, before the appearance of stable species that can be detected by NMR and other methods. We report a spin trapping ESR study of the fragmentation of Nafion, Aquivion, and 3M membranes in their water dispersions and of the corresponding model compounds in the presence of HO•, using 2-methyl-2-nitrosopropane (MNP) as a spin trap.

Using cyclodextrins to encapsulate oxygen-centered and carbon-centered radical adducts: the case of DMPO, PBN, and MNP spin traps.

We present electron spin resonance (ESR) experiments that describe the interaction of beta-cyclodextrin (beta-CD) with spin adducts of three spin traps: 5,5-dimethyl-1-pyrroline N-oxide (DMPO), N-tert-butyl-alpha-phenylnitrone (PBN), and 2-methyl-2-nitrosopropane (MNP). The focus was on spin adducts of oxygen-centered radicals trapped by DMPO and PBN and on carbon-centered radical adducts trapped by MNP. The radicals were generated by reaction with hydroxyl radicals and the spin adducts studied were DMPO/OH and PBN/OH, MNP/CH(2)COOH generated in CH(3)COOH, and MNP/CF(2)COOH in CF(2)HCOOH.

Visualizing chemical reactions and crossover processes in a fuel cell inserted in the ESR resonator: detection by spin trapping of oxygen radicals, nafion-derived fragments, and hydrogen and deuterium atoms.

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that offered the ability to observe separately processes at anode and cathode sides and to monitor the formation of HO and HOO radicals, H and D atoms, and radical fragments derived from the Nafion membrane. The presence of the radicals was determined by spin-trapping electron spin resonance (ESR) with 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. The in situ FC was operated at 300 K with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as catalyst, at closed and open circuit voltage conditions, CCV and OCV, respectively.

Visualizing the dose distribution and linear energy transfer by 1D and 2D ESR imaging: a potassium dithionate dosimeter irradiated with C6+ and N7+ ions.

We report the application of one- and two-dimensional (1D and 2D) spectral-spatial electron spin resonance imaging (ESRI) for visualizing the dose distribution and linear energy transfer (LET) in a potassium dithionate, K2S2O6 (PDT), dosimeter irradiated with the heavy ions C(6+) and N(7+). The ESR spectrum in the irradiated PDT consists of a superposition of two isotropic signals assigned to two *SO3(-) radicals, R1 and R2, with no hyperfine splittings and slightly different g values. The 1D ESRI profiles clearly indicate the spatial penetration of the beams and the location of the sharp maximum dose, the "Bragg peak", detected for each beam.

Deducing 1D concentration profiles from EPR imaging: a new approach based on the concept of virtual components and optimization with the genetic algorithm.

Application of the genetic algorithm (GA) in conjunction with the concept of virtual components (VC) to determine 1D concentration profiles from EPRI spectra (images) is described. In this approach the concentration profile is expressed as the superposition of virtual components described by analytical functions of the Gaussian and Boltzmann type. The method was implemented in the computer program ACon, which allows for fully automated profile extraction via the nonlinear least-squares fitting of experimental images.

Determining the geometry and magnetic parameters of fluorinated radicals by simulation of powder ESR spectra and DFT calculations: the case of the radical RCF2CF2* in nafion perfluorinated ionomers.

The ESR spectrum of the chain-end radical RCF2CF2* detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F alpha and two F beta nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two F alpha nuclei, indicating a nonplanar structure about the C alpha radical center. The fitted isotropic hyperfine splittings for the two F beta nuclei in the Nafion radical, 24.

ESR study of aqueous micellar solutions of perfluoropolyether surfactants with the use of fluorinated spin probes.

Fluoroalkyl esters of 3-carboxy pyrroline nitroxide, FPn (n=8 and 12), containing (n-2) CF(2) groups in the side-chain, were used as novel ESR spin probes of fluorinated micellar systems. The method was applied to study aqueous solutions of perfluoropolyether surfactants of the general formula Cl(C(3)F(6)O)(2)CF(2)COOX, consisting of two perfluoroisopropoxy units and the counterion X = Na(+) or NH(+)(4). By measuring the change of (14)N hyperfine splitting with surfactant concentration the critical micellar concentration of the ammonium salt was determined at temperatures of 297, 313 and 333 K.

Spatial distribution of stabilizer-derived nitroxide radicals during thermal degradation of poly(acrylonitrile-butadiene-styrene) copolymers: a unified picture from pulsed ELDOR and ESR imaging.

Double Electron-Electron Resonance (DEER) provides information on the spatial distribution of radicals on the length scale of a few nanometres, while Electron Spin Resonance Imaging (ESRI) provides information on a length scale of millimetres with a resolution of about 100 micrometres. Despite the gap between these length scales, results from the two techniques are found to complement and support each other in the characterization of the identity and distribution of nitroxide radicals derived from the Hindered Amine Stabilizer (HAS) Tinuvin 770 in poly(acrylonitrile-butadiene-styrene) (ABS) copolymers. DEER measurements demonstrate that there is no significant formation of biradicals from the bifunctional HAS, and provide the distributions of local radical concentrations.

Membrane-derived fluorinated radicals detected by electron spin resonance in UV-irradiated Nafion and Dow ionomers: effect of counterions and H2O2.

Electron spin resonance (ESR) spectroscopy was used to detect and identify radicals formed by UV irradiation of Nafion and Dow perfluorinated membranes partially or fully neutralized by Cu(II), Fe(II), and Fe(III). This method allowed the monitoring of ESR signals from the paramagnetic counterions together with the appearance of membrane-derived radical species. The most surprising aspect of this study was the formation of membrane-derived radical species only in the neutralized membranes, and even in the absence of H2O2 in the case of Nafion/Cu(II) and Nafion/Fe(III).

Spin trapping by 5,5-dimethylpyrroline-N-oxide in Fenton media in the presence of Nafion perfluorinated membranes: limitations and potential.

Spin trapping by 5,5-dimethylpyrroline-N-oxide (DMPO) was used for the detection of radicals in Fenton media in the presence and absence of Nafion perfluorinated ionomers. For ethanol as solvent, the same types of spin adducts were detected in the presence or absence of Nafion. Solvent-derived adducts, DMPO/*OC2H5 and DMPO/*CH(OH)CH3, were identified, and their presence was rationalized by Fe(III)-catalyzed nucleophilic addition of ethanol to the spin trap and hydrogen abstraction by *OH radicals; oxygen radical adducts, DMPO/*O2(-) and DMPO/*OOH, were also detected.