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. A range of interactions between DTBN and CBs was identified for pH 3.2, 7, and 10. No complexation of DTBN with CB[6] was deduced in this pH range. The interaction between DTBN and CB[7] is evident at all pH values: "in" and "out" nitroxides, with (14)N hyperfine splitting, a(N), values of 15.5 and 17.1 G, respectively, were detected by ESR. Interaction of DTBN with CB[8] was also detected for all pH values, and the only species had a(N) = 16.4 G, a result that can be rationalized by an "in" nitroxide in a less hydrophobic environment compared to CB[7]. Computational studies indicated that the DTBN complex with CB[7] is thermodynamically favored compared to that in CB[8]; the orientations of the NO group are parallel to the CB[7] plane and perpendicular to the CB[8] plane (pointing toward the annulus). Addition of sodium ions led to the ESR detection of a three-component complex between CB[7], DTBN, and the cations; the ternary complex was not detected for CB[8]. The DMPO/OH spin adduct was stabilized in the presence of CB[7], but the effect on a(N) was negligible, indicating that the N-O group is located outside the CB cavity. Computational studies indicated more favorable energetics of complexation for DMPO/OH in CB[7] compared to DTBN. An increase of a(N) was detected in the presence of CB[7] for the MNP/CH(2)COOH adduct generated in CH(3)COOH, a result that was assigned to the generation of the three-component radical between the spin adduct, sodium cations, and CB[7].

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. The highest band intensities for both groups appeared at a depth of 82 µm from the cathode in the MEA degraded for 180 h; the same bands were present but less intense at a depth of 22 µm from the cathode. Degradation at these depths is most likely associated with the location of the Pt band formed from Pt dissolution and migration into the membrane. The two degradation bands, C═O and C-H, appeared at the same depths from the cathode, 82 and 22 µm, suggesting that they are generated by a common mechanism or intermediate. This result is rationalized by a very important first reaction: Abstraction of a fluorine atom from the polymer main chain and side chain by hydrogen atoms, H•. This step is expected to cause main chain and side chain scission and to generate RF-CF2• radicals that can react with H2O2, H2O, and H2 to produce both -COOH and RCF2H groups.

Fragmentation of perfluorinated membranes used in fuel cells: detecting very early events by selective encapsulation of short-lived fragments in ß-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. Hydroxyl radicals were generated by UV irradiation of hydrogen peroxide. In the MCs the presence of both oxygen-centered radicals (OCRs) and carbon-centered radicals (CCRs) adducts as well as di-tert-butyl nitroxide radicals (DTBN, from spin trap decomposition) were detected. The presence of both OCR and CCR adducts is rationalized by the initial generation of OCRs with low stability and their transformation into the more stable CCRs. Addition of ß-cyclodextrin (ß-CD) led to a significant increase of the intensity of the MNP/OCR adducts and in one system also to the complete disappearance of the MNP/CCR adduct, results that we assign to the fast selective encapsulation of OCR adducts in the hydrophobic ß-CD host. In the membrane dispersions the presence of oxygen-centered radical (OCR) adducts and DTBN radicals have been detected; this result is rationalized by the slower rate of transformation of OCR adducts to CCR adducts in the membrane systems.

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. Di-tert-butyl nitroxide ((CH(3))(3)C)(2)NO (DTBN) was also detected in experiments with MNP as the spin trap. A range of interactions of the spin adducts and DTBN with beta-CD was identified. The presence of beta-CD led to significant stabilization of DMPO/OH and PBN/OH but to a negligible effect on the (14)N hyperfine splitting of the adducts, a(N), indicating that the N-O group is outside the beta-CD cavity. An increase of a(N) was detected for DTBN and MNP/CH(2)COOH in CH(3)COOH in the presence of beta-CD, a result we assigned to bonding at the rim of the host. Experiments with methylated beta-CD (Me beta-CD) provided support for this conclusion. A different type of complexation was detected for DTBN and MNP/CF(2)COOH in CF(2)HCOOH: for specific host concentrations both "in" and "out" species were detected. We suggest that the hydrophobicity of the fluorinated adduct leads to insertion of the adduct inside the host cavity. Calculation of the association constant K(a) indicated the competition between DTBN and the adduct for inclusion in the host. For MNP as spin trap, the two nitroxide radicals (adduct and DTBN) have the same type of interaction with the host: at the rim in acetic acid, and inside the host cavity in CF(2)HCOOH. Experiments with DTBN in the absence of the spin trap and of adducts illuminated the effect of the local polarity and of the pH on the hyperfine splittings and indicated that the presence of acetic acid encourages rim complexation.

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. Experiments with H(2) or D(2) at the anode and O(2) at the cathode were performed. The DMPO/OH adduct was detected only at the cathode for CCV operation, suggesting generation of hydroxyl radicals from H(2)O(2) formed electrochemically via the two-electron reduction of oxygen. The DMPO/OOH adduct, detected in this study for the first time in a FC, appeared at the cathode and anode for OCV operation, and at the cathode after CCV FC operation of >or=2 h. These results were interpreted in terms of electrochemical generation of HOO at the cathode (HO + H(2)O(2) --> H(2)O + HOO) and its chemical generation at the anode from hydrogen atoms and crossover oxygen (H + O(2) --> HOO). DMPO/H and DMPO/D adducts were detected at the anode and cathode sides, for CCV and OCV operation; H and D are aggressive radicals capable of abstracting fluorine from the tertiary carbon in the polymer membrane chain and of leading to chain fragmentation. Carbon-centered radical (CCR) adducts were detected at the cathode after CCV FC operation; weak CCR signals were also detected at the anode. CCRs can originate only from the Nafion membranes, and their presence indicates membrane fragmentation. Taken together, this study has demonstrated that FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H or D that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments.

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. The depth penetrations are different: approximately 2.3 mm for C(6+) and approximately 1.8 mm for N(7+) beams, +/-0.1 mm; beyond these limits, no radicals were detected. 2D spectral-spatial ESRI images reflect both the dose distribution and the spatial dependence of the relative intensities of radicals R1 and R2, an effect that is assigned to the depth variation of the LET. This study has demonstrated that ESRI is a promising new method for dose and LET determination. Of particular interest are applications in the field of radiotherapy with heavy ions, because in this case the Bragg peak is pronounced and the dose can be focused at specific depths while the surrounding areas are protected.

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. The parametric sensitivity of the GA internal parameters such as population size, probabilities of the crossover, mutation and elitist retention to the search space was investigated in detail in order to find their optimal settings. The customized genetic algorithm was evaluated using simulated and experimental test data sets and its performance was compared with the Monte Carlo approach.

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, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF2CF2CF2* or CF3OCF2CF2* radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two F beta nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2*, which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2* in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions.

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. The ESR line shape was also examined as a function of surfactant concentration and of temperature in the range 120-360 K. The results are discussed in terms of solubilization and local environment of the probes in micelles of different size and shape, depending on the surfactant concentration and the kind of the counterion.

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. These distributions are poorly resolved for model-free analysis of the DEER data by the Tikhonov regularization; the resolution was significantly improved by utilizing information obtained by ESRI. DEER data can be fitted with only one adjustable parameter, namely the average radical concentration, if 1D and 2D spectral--spatial ESRI results on both the spatial distribution of nitroxides and their distribution between the acrylonitrile--styrene-rich (SAN) and butadiene-rich (B) microphases are considered.

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. In Fenton media with methanol as solvent (and no Nafion), the DMPO/*O2(-) adduct dominated immediately after sample preparation, and a mixture consisting of DMPO/*OCH3, DMPO/*CH3, DMPO/*O2(-), and DMPO/*OOH adducts was detected after 30 min. In the presence of Nafion, only the adduct DMPO/*OH was detected. For water as solvent, only the DMPO/*OH adduct was detected, in both the absence and the presence of Nafion. The full hyperfine tensor components of this adduct were determined in Fenton media in the presence of Nafion with water and methanol as solvents. In Nafion/water exposed to the Fenton reagent at 358 K for 3 h, a DMPO adduct of a carbon-centered radical was also identified and assigned to a Nafion-derived fragment; its exact nature is under investigation. Variations of the 14N and Hbeta hyperfine splittings of a given adduct with the local polarity were key to the identification of some DMPO adducts, in particular DMPO/*O2(-). Both *OOH and O2*- adducts, with different 14N and Hbeta splittings, were detected simultaneously in some samples, for the first time in the spin trapping literature. Comparison with the results of a direct electron spin resonance study of Nafion exposed to the Fenton reagent indicated that spin trapping by DMPO can provide complementary information on the type of radicals present during Nafion degradation. The spin trapping approach described in this paper is limited, however, to systems that do not contain organic solvents.

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). In Nafion/Cu(II), ESR spectra from radicals exhibiting hyperfine interactions with three equivalent 19F nuclei (the "quartet") and with four equivalent 19F nuclei (the "quintet") were detected. In Nafion/Fe(II) exposed to H2O2 solutions, the formation of Fe(III) was detected. Upon UV irradiation, strong signals from the chain-end radical ROCF2CF2* were detected first, followed by the appearance, upon annealing above 200 K, of the quartet signal observed in Nafion/Cu(II). In subsequent experiments with Nafion and Dow membranes neutralized by Fe(III), the ROCF2CF2* radicals were formed even in the absence of H2O2, indicating that the role of H2O2 is oxidation of Fe(II) to Fe(III); moreover, in these systems small amounts of the chain-end radicals were detected even without UV irradiation. This result validates the method used to form the radicals: the role of UV irradiation is to accelerate the formation of a signal that is produced, albeit slowly, even in the dark, and possibly during fuel cell operation. The major conclusion is that cations are involved in degradation processes; the point of attack appears to be at or near the pendant chain of the ionomer. Therefore when studying membrane stability, it is important to consider not only the formation of oxygen radicals, such as HO*, HOO*, and O2*-, that can attack the membrane but also the specific reactivity of counterions.