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The photochemistry of tetraphenylacetone (1) adsorbed on the external surface of a MFI zeolite (the sodium form of LZ-105) has been investigated in combination with computational chemistry, surface area measurements, EPR analysis, and classical adsorption isotherms. All of the methods are consistent with a supramolecular structural model in which 1 is first adsorbed strongly through intercalation of a single benzene ring into a hole on the LZ-105 external surface (site I) followed by a weaker binding to the external framework between the holes (site II) until a monolayer of 1 is formed. From both computational and surface area measurements, it is estimated that the site I holes on the external surface will be filled at ca. 0.3-0.5 wt %/wt loading of 1/LZ-105, which corresponds to 6.5 x 10(18) (ca. 10(-)(5) mol) of holes or molecules of 1 adsorbed in holes per gram of zeolite. The supramolecular composition of ca. 0.3-0.5% of 1 on LZ-105 characterizes a "break point" for the photochemistry and the EPR measurements, since it represents the value for saturation of the site I holes with 1. These conclusions are supported quantitatively by experimental isotherms of the adsorption of 1 on LZ-105. Photolysis of 1 intercalated in the site I holes causes fragmentation into two isomeric supramolecular diphenylmethyl (DPM) radicals, one (DMP)(in) which is adsorbed into the internal surface and becomes strongly persistent (half-life of many weeks) and the other (DMP)(ex) which diffuses on the external surface and rapidly dimerizes (less than a few minutes) to produce the radical-radical combination product tetraphenylethane (2). Photolysis of 1 adsorbed on the solid external surface produces two supramolecularly equivalent DPM radicals (DMP)(ex) that diffuse on the external surface and rapidly dimerize to produce 2, and do not produce persistent DPM radicals.

A computer aided analysis of both cw-and pulsed-Electron Paramagnetic Resonance (EPR) spectra of 5doxylstearic acid (5DXSA) as a probe was carried out to compare the aggregation process of Sodium Dodecyl Sulfate (SDS) surfactants in the absence and in the presence of Starburst Dendrimers (SBDs), and to provide information on the interactions between SDS and SBDs. Mobility and polarity parameters were extracted from the cw-EPR analysis whereas the analysis of the Electron Spin Echo Envelope Modulation (ESEEM) signal provided details about the doxyl environment in the SDS micelles. In the absence of SBDs, the formation of SDS micelles was revealed by the decrease in mobility of the probes inserted in the micelles. The high packing of SDS chains in the micelles prevented the water permeability at the doxyl site. In the presence of the dendrimers, the analysis of the EPR spectra suggested the formation of SDS aggregates at the dendrimer surface (cooperative interaction). The larger the size of the dendrimers and the protonation of their surface, the stronger the interactions resulted between the SDS surfactants and the SBD surface. The analysis of the ESEEM pattern indicated that the cooperative interaction of the surfactant with the SBD surface led to a less packed structure of the aggregates. A schematic view was proposed to describe the local structure of the doxyl group and its environment in the absence and in the presence of the dendrimers.

The behavior of 2-(docosylamino)-5-nitropyridine (DCANP) spreading monolayers was investigated as a function of temperature, pH, and subphase composition. The interactions of DCANP monolayers with copper ions dissolved into the subphase at different concentrations were also studied. The investigations were carried out by measuring surface pressure and surface potential-area isotherms. Langmuir-Blodgett (LB) films transferred from pure water and copper aqueous subphases were also studied by Fourier transform infrared (FTIR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, X-ray photoelectron spectroscopy (XPS), ellipsometry, and refractive index measurements. The interactions between copper ions and DCANP molecules were investigated by means of various techniques in the bulk phase, in monolayers, and in LB films. Clear evidence ofinteractions between DCANP and copper ions was provided only from EPR spectroscopy in the LB films, because of the very small molar ratio of bound/free DCANP in the LB films. The refractive index of Cu-doped LB films was shown to be increased by the presence of metal ions.

The oxygen-enhanced photoconductivity observed in crystalline rubrene is investigated using electron paramagnetic resonance (EPR) spectroscopy and steady-state and time dependent photoconductivity (PC) measurements. The EPR data indicate the presence of rubrene radical cation and oxygen radical anion pairs formed within the crystalline structure when rubrene is irradiated in the presence of oxygen. Radical lifetimes determined using EPR spectroscopy correlate well with transient PC data and provide strong evidence that the rubrene radical cation is the charge carrier responsible for enhanced conduction. This process is reversible, although photodegradation is also observed. The oxygen-enhanced PC of rubrene is thus explained by an electron transfer mechanism that generates radical cation “hole” carriers within the crystal via the oxygen acceptor

Supramolecular complexation behavior of cucurbiturils with paramagnetic nitroxide spin probes was examined by 1H NMR, X-ray diffraction studies of crystals, computation, and EPR. Both cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) form a 1:1 complex with 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide (CAT1). The structure of the complex in the solid state was inferred by X-ray diffraction studies and in the gas phase by computation (B3LYP/6-31G(d)). Whereas ESI-MS data provided evidence for the existence of the complex in solution, indirect evidence was obtained through 1HNMRstudieswith a structural diamagnetic analogue, 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethyl- N-methylpiperidine iodide (DCAT1). The EPR spectrum of the CAT1@CB7 complex consisting of three lines suggested that probe CAT1 is associated with host CB7 such that the nitroxide part is exposed to water. The spectral pattern was independent of the concentration of the complex and the presence of salt such as NaCl. The most interesting observation was made with CB8 as the host. In this case, in addition to the expected three-line spectrum, an additional spectrum consisting of seven lines was recorded. The contribution of the seven-line spectrum to the total spectrumwas dependent on the concentration of the complex and added salt (NaCl) to the aqueous solution. The coupling constant for the seven-line spectrum for 14N-substituted CAT1 is 5 G, and that for the four-line spectrum for 15Nsubstituted CAT1 is 7.15 G. The only manner by which we could reproduce the observed spectra by simulation for both 14N- and 15N-substituted CAT1@CB8 was by assuming a spin exchange among three nitroxide radicals. To account for this observation, we hypothesize that three CAT1 molecules included within CB8 interact in such a way that there is an association of three supramolecules of CAT1@CB8 (i.e., [CAT1@CB8]3) in a triangular geometry that leads to spin exchange between the three radical centers. We have established, with the help of 13 additional examples, that this is a general phenomenon. We are in the process of understanding this unusual phenomenon.

The low volatility and thermal instability made the photoelectron (PE), electron transmission (ET), and dissociative electron attachment (DEA) spectroscopy measurements on curcumin (a potent chemopreventive agent) unsuccessful. The filled and empty electronic structure of curcumin was therefore investigated by exploiting the PES, ETS, and DEAS results for representative fragment molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron attachment energies of curcumin was proposed. The π frontier molecular orbitals (MOs) are characterized by sizable interaction between the two phenol rings transmitted through the dicarbonyl chain and are associated with a remarkably low ionization energy and a negative electron attachment energy (i.e., a largely positive electron affinity), diagnostic of a stable anion state not observable in ETS. The lowest-energy electronic transitions of half-curcumin and curcumin and their color change by alkalization were interpreted with time-dependent density functional theory (DFT) calculations. For curcumin, it is shown that a loss of a phenolic proton occurs in alkaline ethanolic solution.

The proton hyperfine tensors of five nitroxide radicals have been obtained by ENDOR spectroscopy in frozen solution. The spectra are interpreted by computing the dipolar hyperfine interaction and simulating the spectra. EPR spectra in solution of the same radicals have been simulated by taking into account the effects of the proton hyperfine tensors. We have been able to reproduce accurately the line broadening effects of the proton hyperfine structures inside each nitrogen hyperfine component and we have determined the correlation times for the rotational motion. In the case of the radical Tempol, our analysis allows discrimination between the effects due to the protons of the axial and equatorial methyl groups. On the basis of experimental evidence we can attribute the larger isotropic hyperfine coupling constant to the axial methyl protons. The possible use of the present results for interpreting the spectra of other nitroxide radicals is discussed.

The ESR spectra of ethanol and pyridine solutions of the spin probes 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol) and 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-oxyl (Tempyo) were analyzed to obtain information on the dynamic behavior of the radicals in solution. In order to evaluate the true line widths, the dependence of the proton and carbon-13 hyperfine coupling constants on temperature and on solvent polarity were analyzed as a function of a/sub N/. Tempol and Tempyo showed an opposite dependence of a/sub H/ on a/sub N/. Furthermore the structure of Tempol was affected more than that of Tempyo by variations of solvent polarity. From the line widths the correlation times for motion were evaluated. A weak hydrogen bond was formed by Tempol and Tempyo with ethanol. However, only Tempol in ethanol gave rise to an anisotropic rotational reorientational motion, with the fastest rotation about the x axis. All samples, except for Tempol in pyridine, showed a deviation from the Debye spectral densities. The hydrodynamic rotational radii were about 3 times lower than the effective geometric spherical radii. However, Tempyo exerted slightly greater torques than Tempol during its rotational motion. At the highest temperatures (T > 298 K), the dynamic behavior was as expected on the basis of the free volumemore » model, whereas at lower temperatures an increase in the H-bonding strength was found between radical and ethanol molecules.« less

Une tres petite fraction de Mn(II), intervient dans la complexation avec Py ou le polymere. Dans ce dernier cas, on suppose que les complexes de Mn(II) sont pieges dans des cages polymeres

In this paper the results obtained by using TEMPOL, TM-TEMPAMINE+, Octanoyl-, Dodecanoyl-, and Hexadecanoyl -TEMPO as spin labels for micellar solutions of C8—SO4 Na, C12-SO4Na and C16-SO4Na are reported. The analysis of ESR lineshape, of reorientational correlation time, τ, and hyperfine coupling constant, , have been carried out to obtain details on these systems. TEMPOL, TM-TEMPAMINE+ and Octanoyl-TEMPO did not give micellar solutions at the concentrations (≃10−4M) used. The ESR spectra obtained with these probes were easily interpretable over a wide range of surfactant concentrations. On the contrary, Dodecanoyl and Hexadecanoyl -TEMPO give micellar solutions at concentration

Polynitroxides with varying numbers of nitroxide groups (one to four) derived from different aromatic core structures show intramolecular electron spin-spin coupling. The scope of this study is to establish an easy methodology for extracting structural, dynamical, and thermodynamical information from the EPR spectra of these polynitroxides which might find use as spin probes in complex systems, such as biological and host/guest systems, and as polarizing agents in dynamic nuclear polarization (DNP) applications. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level provided information on the structural details such as bond lengths and angles in the gas phase, which were compared with the single crystal X-ray diffraction data in the solid state. Polarizable continuum model (PCM) calculations were performed to account for solvent influences. The electron paramagnetic resonance (EPR) spectra of the polynitroxides in chloroform were analyzed in detail to extract information such as the percentages of different conformers, hyperfine coupling constants a, and rotational correlation times τ(c). The temperature dependence on the line shape of the EPR spectra gave thermodynamic parameters ΔH and ΔS for the conformational transitions. These parameters were found to depend on the number and relative positions of the nitroxide and other polar groups.

Cu(dibenzo[b,i][1,4,8,11]tetraaza[14]annulene), Cu(dbTAA), Cu(dinaphtho- [b,il[1,4,8,11]tetraaza[14]annulene), Cu(dnTAA), Cu(diphenanthro[b,i][1,4,8,11]tetraaza [14]annulene) and Cu(dpTAA) complexes have been used for synthesizing new conductive materials. Chemical oxidation of the three compounds in acetone slurries by variable amounts of iodine yielded microcrystalline conductive powders with a wide range of stoichiometries. Thermal stability of the oxidized materials was investigated by TGA and DTGA techniques showing a relationship between solid phase stability and iodine content and, together with IR data, suggests the presence of a single phase. Raman spectra show strong scatterings attributable to I3− (107 cm−1) and (160 cm−1) I5− polyiodides, the latter occurs only in highly iodinated complexes (I/Cu ratio larger than three) and the I2 presence can be excluded. Electrical conductivity measurements of powder samples of the oxidized compounds exhibit some dependence on the iodine content in the I/Cu ratio range 0–0.9. The temperature response of the conductivity of the oxidized compounds, in the range 70–220 K, is interpretable in terms of an activated formation of carriers. ESR spectra, static magnetic susceptibility and IR data as a function of the I/Cu ratio suggest that the oxidative process involves not only the copper unpaired electron, but also electrons associated with delocalized ligand orbitals on the macrocycle. Large enhancement of the conductivity is shown by Cu(dpTAA)I0.2 (1.5 S cm−1) and the temperature dependence of conductivity shows metallic behaviour starting from 200 K.

The accumulation of β-amyloid peptides into senile plaques is one of the hallmarks of Alzheimer's disease (AD). There is mounting evidence that the lipid matrix of neuronal cell membranes plays an important role in the β-sheet oligomerization process of β-amyloid. Aβ(25–35), the sequence of which is GSNKGAIIGLM, is a highly toxic segment of amyloid β (Aβ)-peptides, which forms fibrillary aggregates. In the present work, two spin-labelled Aβ(25–35) analogues containing the nitroxide group of the amino acid TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) as a paramagnetic probe at the N- or the C-terminus of the peptide sequence, respectively, were synthesized in order to investigate the peptide-membrane interaction. The orientation and associated changes of the peptide conformation in the presence of different artificial membrane models (micelles, liposomes) were evaluated by electron paramagnetic resonance and circular dichroism techniques. The results of this study allowed us to propose a model in which the C-terminal portion of the peptide is highly associated to the membrane, while the N-terminal part extends into the aqueous phase with occasional contacts with the lipid head-group region. Interestingly, the interaction of the C-terminal portion of the peptide is particularly enhanced in the presence of sodium dodecyl sulfate (SDS) molecules. Copyright © 2006 European Peptide Society and John Wiley & Sons, Ltd.