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Item Electron Spin Resonance Studies of the Formation and Thermal Stability of Oxygen Radicals on CaO and some other Oxides(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (04), 1978) Cordischi, Dante; Indovina, Valerio; Occhiuzzi, ManlioThe formation of surface radicals by adsorption of oxygen on CaO, MgO, y-Al2O3 and ZnO has been studied by e.s.r. spectroscopy. The oxides, after activation by heating under vacuum, were exposed to oxygen at 298 K. In these conditions (O;)s and (O3~)s radicals, at concentrations of about 5.5 x 1015 and 1.5 x 1015 spins in-2, respectively, were observed on CaO thermally activated at 1173 K. By contrast the MgO sample requires pre-exposure to H2 at 298 K before O2 admission in order to produce (O2)s. Pre- exposurc to H2 also increases the (O2)s concentration on CaO (from 5.5 to 14.0 x 1015 spins m-2), whereas it destroys the activity of this oxide for (O3-)s formation. Neither treatment (thermal activation in vacuo with or without exposure to H2) is efficient in the case of the y-Al2O3 surface, which, however, produces (O2)s if irradiated by ultraviolet. The electron donor power of oxides, measured by the (O2)s concentration, is found to decrease in the order CaO > MgO >y-Al2O3. Surface centres for (O2)s formation arc believed to be O2- ions in surface sites of low coordination. Centres for (O3”)s formation are O_-ions, possibly in clusters, present on the CaO surface. The results on ZnO, which also adsorbs oxygen as (O;)s, indicate that the electron donor centres in this oxide are different in nature from those active in MgO, CaO and y-Al2O3.Item X-ray Photoelectron Spectroscopy Study of Pd and Pt Ions in Type Y-Zeolite: Electron Transfer between Metal Aggregates and the Support as Evidenced by X-ray Photoelectron Spectroscopy and Electron Spin Resonance(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (02), 1978) Vedrine, Jacques C.; Dufaux, Michel; Naccache, Claude; Imelik, BorisSpectroscopic study by ESCA and e.s.r. techniques was performed for Pt and Pd ions in type Y-zeolites. Pt and Pd ions were found to be ionically bonded to lattice oxygen ions. Atomically dispersed PtO and Pd° were shown to give significant X.P.S. positive chemical shifts: +1.3 and + 1.4 eV for Pt 4/and Pd 3d lines, respectively. These shifts were assigned to smaller electron relaxation energy. Metal aggregates of small diameter < 20 A) and narrow size distribution were prepared in zeolite or on silica. The X.P.S. chemical shift was quite significant (—}-0.7eV) for Pt on zeolite support with respect to those (< +0.3 eV) for Pd on zeolite or Pt or Pd on silica support and did not depend on the particle size. Electron donor and electron acceptor properties of the materials were studied by following the e.s.r. formation of charge transfer complexes (radical ion) as a function of Pt or Pd loading. Pt aggregates were shown to form charge transfer complexes with Lewis acid sites and to increase electron donor properties of the zeolite no matter what the particle size. This increase was shown to arise from electron donation from the metal to the zeolite lattice. However, Pd aggregates were shown by e.s.r. not to give rise to significant electron transfer.Item Electron Spin Resonance Studies of Isotopically Labelled Oxygen Species Adsorbed on Supported Molybdenum(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Che, Michel; Tench, Anthony J.The molecular ion (O2)5 has been observed on the surface of the moIybdenum/SiO2 and molyb- denum/Al2O3 systems by e.s.r. Using oxygen isotopically enriched in 17O, two different hyperfine splittings were observed in the Mo/Si system corresponding to A^ = 85 and 72 G respectively; the Ayy and Az: values of the tensor were too small to be resolved. The observed values for A^ are shown to arise from an unequal interaction of the unpaired electron with two oxygen nuclei in the same molecular ion. This result is contrary to the previous work for (Ops on other oxides where the oxygen nuclei have been found to be equivalent; the difference is attributed either to a more covalent bonding with the surface ions or to adsorption at an unusual site at the surface. This is thought to be a sensitive function of the surface environment since the oxygen nuclei appear to be nearly equivalent for the Mo/Al system although the (/-tensors for both systems are almost identical.Item Electron spin resonance study of the 1 : 1 hydrogen-bonded complex of di-t-butylnitroxide with methanol. An analysis of an acceptor : self-associated donor equilibrium(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (03), 1977) Bullock, Anthony T.; Howard, Christopher B.The formation constant Kf for the 1:1 complex of di-t-butylnitroxide with methanol has been determined over the temperature range –23°C to + 65°C by measuring the variation of aN with methanol concentration. The solvent was carbon tetrachloride. The Newton–Raphson (N.R.) method was used in treating the results. Proper account was taken of the self-associated nature of the methanol. With suitable modifications the analysis should be applicable to other spectroscopic techniques used to study complex formation between two species, one of which is self-associated. Evidence has been obtained which favours the existence of linear rather than cyclic tetramers of methanol in equilibrium with monomers. The enthalpy of formation of the hydrogen-bonded complex was low, namely –6.7 ± 1.8 kJ mol–1. The temperature coefficients of aN in the free and complexed nitroxide are of opposite sign. For the complex, this is shown to be the consequence of proton hopping in an asymmetric double-minimum potential with an energy difference between the minima of 9.4 ± 0.8 kJ mol–1.Item Electron Spin Resonance Study of the Reaction of Hydrogen Atoms with Hydrogen Sulphide(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (2), 1973) Bradley, John N.; Trueman, Susan P.; Whytock, David A.; Zaleski, Thomas A.The reaction between H atoms and hydrogen sulphide, with and without added nitric oxide, has been studied by following the H atom, S atom and SH radical concentrations with time using electron spin resonance detection. The results are fully explained by the mechanism (1)–(4) H + H2S→H2+ SH, k1= 5.0 × 108 l. mol–1 s–1(1), H + SH → H2+ S, k2= 2.5 × 1010 l. mol–1 s–1(2), SH + NO → stable species, k3= 6.3 × 108 l. mol–1 s–1(3), SH + SH → H2S + S, k4= 7.8 × 109 l. mol–1 s–1. (4) with no evidence for significant surface effects (the walls were coated with boric acid). The results demonstrate conclusively that reaction (2) is very important in this system and that previous measurements of k1 which neglect this reaction may require correction.Item Electron Spin Resonance Study of Intermolecular Cation Exchange Between Sodium Iodide and the Ion Pairs of Sodium Benzophenone and Sodium Naphthalene in Tetrahydrofuran(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (3), 1973) Bailey, J. C.; Colau, P. R.; Warhurst, E.Intermolecular cation exchange between sodium iodide and sodium benzophenone and sodium naphthalene ion pairs in THF has been studied by e.s.r. linewidth measurements. For sodium benzophenone the plot of the reciprocal lifetime of a sodium spin state against sodium iodide concentration is markedly curved and not linear as would be expected if the exchange takes place in a single bimolecular reaction step. A mechanism which explains the curvature has been suggested in which the exchange involves an equilibrium between the two ion pairs and a dipole-dipole association ion quadruplet. The change in spin state of the sodium coupled to the benzophenone anion takes place by an internal re-arrangement in the ion quadruplet. Values for the equilibrium constant and for the velocity constant of the re-arrangement have been obtained. The reciprocal lifetime plot for sodium naphthalene is approximately linear and a value for the velocity constant for the bimolecular exchange reaction step has been obtained.Item Electron Spin Resonance Studies of Photo-Oxidation by Metal Ions in Rigid Media at Low Temperatures: Part 3.—Ce(lV) Photo-Oxidations of Aldehydes, Ketones, Esters and Amides(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (1), 1972) Greatorex, D.; Kemp, T. J.A survey is presented of the primary processes in the photo-oxidation by Ce(IV) ions at 77 K of a variety of organic molecules classified by functional group; these processes have been determined by characterization of the resulting organic free radical by electron spin resonance spectroscopy. Aldehydes RCHO and aldehyde hydrates RCH(OH)2 are oxidized to RĊO and RĊC(OH)2 respectively, although C2H5ĊO disappears at 120 K, producing instead CH3ĊHCHO. Ketones produce either the alkyl radical R from RCOR′ or the radical derived by abstraction of a hydrogen atom from the methylene or methine groups adjacent to the carbonyl group; in some cases both types of radical are discernible. Esters also undergo C—H or C—C fission processes depending on the degree of substitution of the R fragment of RCO2R′, whilst formamide yields a spectrum of ·CONH2. Methyl methacrylate yields a vinylic monomer radical at 77 K which induces polymerization at 175 K to give the polymethyl methacrylate radical.Item Electron Spin Resonance Studies of Photo-oxidation by Metal Ions in Rigid Media at Low Temperatures: Part 4.—Survey of Photo-oxidation by the Uranyl Ion(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (11), 1972) Greatorex, D.; Hill, R. J.; Kemp, T. J.; Stone, T. J.The long-standing problem of the mechanism of photo-oxidation of organic and inorganic molecules by the UO2+2 ion has been explored in some detail by the matrix isolation method, the detection of trapped substrate-derived radicals being effected by electron spin resonance spectroscopy, mostly using organic matrices at 77 K. A variety of mechanisms is apparent even within a given homologous series, but the most common feature is abstraction of a hydrogen atom from a carbon atom adjacent to an activating group such as —CO—, —CN, —CO2H, —CHO, —OH, —CONH2, —CO2R or from a phosphorus atom; carbon-carbon cleavage is found (sometimes exclusively) with secondary and tertiary alcohols, ketones, some carboxylic acids and diethyl ether. These results are discussed in the light of recent steady-state studies involving comparison of quantum yield and fluorescence quenching data.Item Electron Spin Resonance Studies of Elementary Processes in Radiation- and Photo-chemistry: Part 11.—Radical Reactions of α-Aminobutyric Acid between 26 and 440 K(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (9), 1972) Ayscougfi, P. B.; Olsen, K. J.Irradiation of α-aminobutyric acid single crystals with 60Co γ-rays at 26 to 77 K results in the trapping of the protonated molecular-anion CH3CH2CH(NH3+)CO2H–(R1). During thermal annealing to 440 K, six further radicals are formed, including CH3CH2ĊHCO2H(R4), ĊH2CH2CH(NH+3)CO–2(R2), CH3ĊHCH(NH+3)CO–2(R3) and (CH3)2ĊCO2H(R7). The conversion of R1 to R4 and then to R2 and R3 occurs between 100 and 145 K by processes involving only 8–17 kJ mol–1“activation” energy and determined largely by steric considerations. The conversion of R3 to R7 has been followed isothermally between 330 and 365 K and the decay of R7 between 390 and 430 K. These processes have “activation” energies of 71 ± 7 and 83 ± 5 kJ mol–1 respectively and are likely to involve some translational motion of the radicals. A possible mechanism for the radiolysis is discussed.Item Electron Spin Resonance Studies of Elementary Processes in Radiation- and Photo-chemistry: Part 10.—Carboxylic Acid Anhydrides at Cryogenic Temperatures(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (7), 1972) Ayscough, P. B.; Oversby, J. P.The four carboxylic acid anhydrides, acetic, propionic, n-butyric and iso-butyric, -(-irradiated at 77 K show esr spectra indicating the presence of acyl radicals and radicals formed by loss ol H from the anhydride The latter radicals are derived in part from simple alkyl radical precursors as mav be adduced from experiments on thermal annealing of the irradiated samples. No evidence is found for the presence of trapped radical-anions or cations. Hydrogen atoms react with anhydrides at 77 K to give the same types of radicals that arc formed during y-radtolysis, though in different relative yields.