A. A. Shubin

Modulation effects in the electron spin echo resulting from hyperfine interaction with a nucleus of an arbitrary spin

Abstract Exact analytic expressions for the modulation effects in two- and three-pulse electron spin echoes resulting from the hyperfine interaction of an electron and a nucleus with an arbitrary spin are derived. The two-pulse envelope modulation is calculated numerically for some nuclei for which modulation effects are most often observed experimentally. Applicability requirements of the conventional approximate formula for analysis of the ESE modulation effects are considered.

DFT study of oxygen-bridged Zn2+ ion pairs in Zn/ZSM-5 zeolites

A cluster model for Zn/ZSM-5 zeolite is proposed, which consists of an oxygen-bridged [Zn–O–Zn]2+ moiety attached to two framework aluminum ions of two adjacent ZSM-5 5-rings. Its stability and catalytic activity in ethane dehydrogenation were considered using the DFT method and compared with those for single Zn2+ ions in the same rings. It is shown that the oxygen-bridged Zn2+ pair is rather reactive towards ethane dissociation and that the rate-limiting step is release of hydrogen.

Composition of the active component of supported vanadium-magnesium catalysts according to 51 NMR data

Abstract Vanadium-magnesium supported catalysts and model compounds (magnesium vanadates) have been studied with use of 51V NMR. It has been established that on the catalyst surface there exist isolated tetrahedral and octahedral vanadium species, their associated forms and magnesium orthovanadate. Their relative concentrations were determined by both the surface concentration of vanadium and the nature of modifying agents (such as SO42−). The influence of CO2 and H2O adsorption and catalyst reduction with butene-1 on the vanadium coordination state has been studied.

The metallocene/methylaluminoxane catalysts formation : EPR spin probe study of Lewis acidic sites of methylaluminoxane

Abstract Using stable nitroxyl radical 2,2,6,6-tetramethylpiperidine- N -oxyl (TEMPO) as a spin probe, Lewis acidic sites of methylaluminoxane (MAO) were identified. It was found that MAO contains two types of acidic sites. TEMPO, coordinated to the sites I and II, exhibits in the EPR spectra triplet ( g o =2.0047, a N =18.6 G) and triplet of sextets ( g o =2.0045, a N =19.6 G and a Al =1.7 G), respectively ( a N and a Al are constants of hyperfine structure from the corresponding nucleus). According to EPR measurements, concentration of sites I is close to that of sites II, and MAO contains one site of each type per 100±30 aluminium atoms. The adducts of TEMPO with sites I are less stable than those with sites II. Based on the values of a Al and relative stabilities of the adducts with TEMPO, the acidic sites I and II were attributed to coordinatively unsaturated aluminium atoms in AlOMe 2 and AlO 2 Me environment, respectively. From the EPR spectra of coordinated TEMPO, the average radius of MAO oligomers (AlOMe) n was evaluated to be 5.8 A at 20°C, which corresponds to the value of n =15–20. Thus, the major part of MAO contains not more than one Lewis acidic site per one oligomeric (AlOMe) n molecule.

The influence of nuclear quadrupole interactions upon electron spin-echo modulation induced by weak hyperfine interactions

Abstract The influence of nuclear quadrupole interactions on the electron spin-echo modulation induced by weak hyperfine interaction was analyzed theoretically in terms of the perturbation theory as related to the nuclear Zeeman interaction. The possibility of applying the results obtained to investigation of modulations from different nuclei was assessed. The quadrupole interaction effects in the modulation damping in disordered systems were estimated quantitatively.

Characterisation of strongly bonded V(V) species in VOx/TiO2 catalyst by static and MAS solid-state 51V NMR spectroscopy

Abstract 51 V static and MAS NMR spectroscopy of central transition is shown to be an effective method for the characterisation of strongly bonded V(V) species in VO x /TiO 2 catalysts. Simultaneous determination of quadrupole and CSA tensors parameters (i.e., C Q , η Q , δ σ , η σ ) from both static and MAS NMR spectra of central transition permits the extraction of all NMR parameters with a reasonable accuracy. For the first time, a large 51 V quadrupolar constant (14–16 MHz) has been obtained for strongly bonded V(V) species in a supported vanadia catalyst, whereas CSA tensor principal components were found to be very close to those for bulk vanadium (V) oxide.

51V and 31P NMR studies of VOx/TiO2 catalysts modified by phosphorous

Phosphorous-doped VOx/TiO2 catalysts prepared by the spray-drying method and treated under catalytic reaction, as well as individual phases (αI-, αII-, β-) of VOPO4 were studied using modern high-resolution solid-state NMR techniques, including fast magic angle spinning (MAS), combined with the analysis of rotational satellites intensities by the satellite transition spectroscopy (SATRAS) method; 2D triple-quantum, quintuple-quantum MAS NMR, and spin mapping echo experiments. The simultaneous determination of chemical shielding anisotropy and quadrupole tensor parameters, as well as their distributions, permits to draw a conclusion on the local environment of vanadium sites in the catalysts. The formation of a triple V–P–Ti compound in phosphorous-doped VOx/TiO2 catalysts has been revealed. Only one type of slightly distorted tetrahedral vanadium atoms bound via oxygen to phosphorous was found in this compound. The very large distribution of the quadropole constant points to the irregular structure of this compound.

Study of nature of the factors determining activity, stability and selectivity of zeolite catalyst

Abstract The dependences of catalytic properties of zeolites of type Y, mordenite, highly siliceous zeolite (ZSM) in cracking of n-octane and synthesis of hydrocarbons from methanol (activity and stability of action) on the concentration and state of hydroxyl groups, aluminium atoms, and on the coordination environment of silicon atoms in zeolites have been investigated. The state of active centres was studied by 2Al and 29Si n.m.r., and i.r. spectroscopy. Activity, stability and selectivity of zeolites are concluded to be dependent on the concentration of groups having more than one aluminium atom in the second coordination sphere of silicon atom, one of which can be in the cation position. Concerted (associative) effect of the proton-donor and Lewis centres on reagent is proposed. Lewis centres may have aluminium atoms in the cation position outside the framework.

Exploring by Pulsed EPR the Electronic Structure of Ubisemiquinone Bound at the QH Site of Cytochrome bo3 from Escherichia coli with in Vivo 13C-Labeled Methyl and Methoxy Substituents

The cytochrome bo3 ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O2 to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. The semiquinone is also formed in the D75E mutant, where the mutation has little influence on the catalytic activity, and in the D75H mutant, which is virtually inactive. In this work, wild-type cytochrome bo3 as well as the D75E and D75H mutant proteins were prepared with ubiquinone-8 13C-labeled selectively at the methyl and two methoxy groups. This was accomplished by expressing the proteins in a methionine auxotroph in the presence of l-methionine with the side chain methyl group 13C-labeled. The 13C-labeled quinone isolated from cytochrome bo3 was also used for the generation of model anion radicals in alcohol. Two-dimensional pulsed EPR and ENDOR were used for the study of the 13C methyl and methoxy hyperfine couplings in the semiquinone generated in the three proteins indicated above and in the model system. The data were used to characterize the transferred unpaired spin densities on the methyl and methoxy substituents and the conformations of the methoxy groups. In the wild type and D75E mutant, the constraints on the configurations of the methoxy side chains are similar, but the D75H mutant appears to have altered methoxy configurations, which could be related to the perturbed electron distribution in the semiquinone and the loss of enzymatic activity.

Numerical analysis of the intluence of nuclear quadrupole interaction on modulation effects in electron spin echo from deuterium nuclei in disordered systems

Abstract Nuclear quadrupole-coupling effects on electron spin-echo modulation by weak hyperfine coupling with deuterium nuclei in disordered samples have been analyzed numerically.