Total Spin Concentration Research Articles

Classes of Nanomagnets Created from Alkanethiol‐Coated Pt or Pd Nanoparticles and Their Alloys with Co

AbstractAlkanethiol‐coated metal nanoparticles are fascinating materials for designing unconventional electronic and magnetic functions. Novel classes of magnetic nanosystems can be created using Pd and Pt nanoparticles and their alloys with a 3d transition metal using quantum size, interface and exchange enhancement effects. The charge transfer at the metal–organic interface and the quantum size effect render Pt and Pd nanoparticles magnetic with localised moments. The substitution of capping molecules with a TTF derivative having a long alkanethiol substituent decreases the totalspin concentration due to partial oxidation of the TTF units. Pd nanoparticles containing Co atoms as magnetic centres have significantly enhanced magnetic moments because of the exchange enhancement. Interestingly, in Pd nanoparticles of 2 nm in diameter containing only one Co atom per particle, the Co atom magnetises all of the Pd atoms in the particle. Therefore, the nanoparticles behave as a single nanoparticle magnet that shows blocking behaviour.

Heavy-Ion-Induced Sucrose and L-α-Alanine Radicals Investigated by EPR

We investigated the details of sucrose and L-α-alanine radicals produced by heavy-ion irradiation with various linear energy transfer (LET) values. The impact of the heavy ions on samples produced stable free radicals, which were analyzed by electron paramagnetic resonance (EPR). The spectral patterns obtained were similar among those for the irradiation of carbon (C), neon (Ne), silicon (Si), and argon (Ar) ions. The radical production of irradiated sucrose and alanine samples showed a linear relation with the absorbed dose. The total spin concentration obtained by heavy-ion irradiation decreases as logarithmic function of the LET. Quantitative EPR analysis showed that the production of sucrose and alanine radicals depended on both the ion species and the LET values under the same dose. Moreover, the radical-production cross sections for sucrose and alanine irradiated with C ions (290 MeV/u) were about 1.3 · 10−8 and about 3.0 · 10−9 µm2, respectively. The results of the cross section suggest that each molecule becomes a stable radical with the interaction of several particles.

Optically Induced Conversion of Ge Centers in Quartz

An electron spin resonance (ESR) study of natural single crystalline quartz was performed to clarify the effect of ultraviolet rays (UV) exposure on electron traps at germanium impurities. ESR spectra of artificially γ-irradiated single crystalline samples from two different natural sites mainly showed a common Ge center, [Ge(C)e-/Li+], which is an electron trapped center at a Si-substituting Ge ion with a charge compensator of Li+. UV light exposure using a low-pressure mercury lamp converted [Ge(C)e-/Li+] into a different paramagnetic center. The observed ESR parameters of this center did not coincide with those of other known centers. This signal gradually decayed at room temperature and simultaneously produced a third center, Ge E2', which is a germanium dangling bond neighboring an oxygen vacancy with the form ≡Si–H •Ge≡. The experimental results indicated a two-step conversion of Ge centers, which was caused by UV exposure and aging. The total spin concentration was kept constant throughout the experiments.

An electron paramagnetic resonance study of PP and PP/SBS blends irradiated with gamma rays

Electron paramagnetic resonance (EPR) measurements of Polypropylene (PP) and its blends with a Styrene–Butadiene–Styrene (SBS) copolymer in 10, 20, 30 and 40 wt.% of SBS were carried out. The samples were irradiated in the 0⩽D⩽100 kGy range at a dose rate of 4.8 kGy/h. Typical spectra indicative of the formation of peroxy and alkyl radicals were obtained. The dynamics of formation and recombination of radicals in these samples could be explained using a mixed zero and first order generation–recombination fit of the total spin concentration as a function of the integral dose. The time dependence of the spin concentration was studied assuming a mixed first order fit in the decay process. The parameters obtained from the fitting are interpreted in terms of the rate of formation of free radicals at the irradiation time and in terms of the decay time of the total free radical concentration.

ESR investigation of sucrose radicals produced by particle irradiation

We investigated sucrose radicals produced by heavy-ion irradiation with various linear energy transfer (LETs) and the possibility for a sucrose electron spin resonance (ESR) dosimeter. The impact of heavy ions on sucrose produced sucrose radicals, which were measured by ESR. The obtained spectral pattern was the same as that for helium (He) ions, carbon (C) ions, neon (Ne) ions, iron (Fe) ions, and γ-ray irradiation. Identical spectra were measured after 1 year, but the initial intensities decreased by a few percent when the samples were kept in ESR tubes with the caps at ambient temperature. The total spin concentration obtained by heavy-ion irradiation had a linear relation with the absorbed dose, and correlated logarithmically with the LET. Qualitative ESR analyses showed that the production of sucrose radicals depended on both the particle identity and the LET at the same dose. The production of spin concentration by He ions was the most sensitive to LET. Empirical relations between the LET and the spin yield for various particles imply that the LET at a certain dose can be estimated by the spin concentration. Therefore, the present ESR results imply that sucrose can be used to monitor the absorbed dose and the LET of particle irradiation.

An electron paramagnetic resonance study of defects in PECVD silicon oxides

Electron paramagnetic resonance (EPR) has been used to determine how the type and population of paramagnetic defects are altered firstly by changes to the composition of oxides grown on silicon by plasma enhanced chemical vapor deposition (PECVD) and secondly by subsequent anneal treatments of the various oxide films. Silicon oxide films with refractive index, n, of 1.464, 1.487, 1.508 and 1.536 and thickness of 1 μm to 3 μm were studied; those of higher index have a higher silicon content. Only E′ centers are detected in films with n=1.464 and 1.487. Raising n to 1.508 leads to the introduction of other types of paramagnetic centers and a large increase in the average total spin concentration. Increasing n further to 1.536 produces a further small increase in average spin concentration. Identities are proposed for the additional types of defect and the dependence of their population on anneal temperature is also reported.

Electron paramagnetic resonance investigation of sucrose irradiated with heavy ions.

We investigated the potential use of sucrose to estimate linear energy transfer (LET) for heavy-ion irradiation. We also made a quantitative comparison between heavy-ion and gamma irradiation in terms of spin concentration. Heavy-ion irradiation of sucrose produces stable free radicals. Based on the electron paramagnetic resonance (EPR) spectra obtained, the stable sucrose radicals are the same among helium ions, carbon ions and gamma rays. The EPR spectrum was approximately 70 G wide and was composed of several hyperfine structures. The total spin concentration obtained after the heavy-ion irradiation increased linearly as the absorbed dose increased and decreased logarithmically as LET increased. Production of the spin concentration of helium ions was two times more dependent on LET than that for carbon-ion irradiation. The empirical relationships obtained imply that LET at a certain dose can be determined by the spin concentration. Furthermore, the results of gamma irradiation of deuterated sucrose suggest that one of the persistent radicals is a carbon-centered radical.

Gemcitabine 5'-triphosphate is a stoichiometric mechanism-based inhibitor of Lactobacillus leichmannii ribonucleoside triphosphate reductase: evidence for thiyl radical-mediated nucleotide radical formation.

Ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii utilizes adenosylcobalamin and catalyzes the conversion of nucleoside triphosphates to deoxynucleoside triphosphates. One equivalent of 2',2'-difluoro-2'-deoxycytidine 5'-triphosphate, F2dCTP, rapidly inactivates RTPR. Analysis of the reaction products reveals that inactivation is accompanied by release of two fluoride ions and 0.84 equiv of 5'-deoxyadenosine and attachment of 1 equiv of corrin covalently to an active-site cysteine residue of RTPR. No cytosine release was detected. Proteolysis of corrin-labeled RTPR with endoproteinase Glu-C and peptide mapping at pH 5.8 revealed that C419 was predominantly modified. The kinetics of the inactivation have been examined by stopped-flow (SF) UV-vis spectroscopy and rapid freeze quench (RFQ) electron paramagnetic resonance (EPR) spectroscopy. Monitoring DeltaA525 nm shows that cob(II)alamin is formed with an apparent kobs of 50 s-1, only 2. 5-fold slower than a similar experiment carried out with cytidine 5'-triphosphate (CTP). The same reaction mixture was thus quenched at times from 22 ms to 30 s and examined by EPR spectroscopy. At early time points the EPR spectrum resembled a thiyl radical exchange coupled to cob(II)alamin. From 22 to 255 ms the total spin concentration remained unchanged at 1.4 spins/RTPR, twice that predicted by the amount of cob(II)alamin determined by SF. However, with time the signal attributed to the thiyl radical-cob(II)alamin disappears and new signal(s) with broad feature(s) at g = 2.33 and a sharp feature at g = 2.00 appeared, suggesting formation of cob(II)alamin and a nucleotide-based radical with only dipolar interactions. These studies have been interpreted to support the proposal that an RTPR-based thiyl radical can give rise to a nucleotide-based radical.

Dynamic in situ 9 GHz c.w.-e.p.r. low-temperature oxidation study of selected Alberta coals: 4. Influence of moisture on hv bituminous coal

The results are reported of a dynamic in situ 9 GHz c.w.-e.p.r. study in the temperature interval 20 to 250 °C of virgin, dried and moisture-saturated hv bituminous coal samples subjected to dry air flow. The c.w.-e.p.r. spectral parameters and the relative total spin concentration were found to be sensitive indicators of the role of moisture in determining the free radical content of a coal sample. The relative total spin concentration in the dried sample was twice that of the other two samples at 20 °C and was essentially temperature-independent, whereas it increased very rapidly between 20 and 100 °C and attained a maximum near 100 °C in both the virgin and moisture-saturated samples. The lineshape changes were most pronounced in the moisture saturated sample and occurred between 20 and 100 °C. This lineshape may be the composite of lineshapes due to three resonances, but only one of the free radical resonances is moisture-sensitive.

Dynamic in situ 9 GHz c.w.-e.p.r. low-temperature oxidation study of selected Alberta coals: 2. Influence of mineral matter in an hv bituminous coal

The effect of the presence of manganese and cobalt impurity ions on the formation of free radicals in the low-temperature oxidation of an Alberta hv bituminous coal has been investigated using dynamic in situ 9 GHz c.w.-e.p.r. spectroscopy. Changes in the effective g-factor and effective linewidth ΔB 1 2 e.p.r. spectral parameters as well as in the relative total spin concentration and the lineshape were monitored in both virgin and demineralized coal samples subjected to dry air flow whilst being heated from 20 to 250 °C. The virgin sample produced a larger number of free radicals than the demineralized sample when heated. This demonstrates that the presence of transition metal ions in the mineral matter component of the virgin coal plays an important role in promoting the low-temperature oxidation process. It is shown that two free radical species are present and that the presence of the transition metal ions promotes the preferential creation of the free radical with the larger g-factor.

Orientational distribution of spin-labeled actin oriented by flow.

Previous studies on spin-labeled F-actin (MSL-actin), using saturation transfer electron paramagnetic resonance (ST-EPR), have demonstrated that actin has submillisecond rotational flexibility and that this flexibility is affected by the binding of myosin and its subfragments. This rotational flexibility does not change during the active interaction of myosin heads, actin, and adenosine triphosphate. However, these ST-EPR studies, performed on randomly oriented actin, would not be sensitive to orientational changes on the millisecond time scale or slower. In the present study, we have clarified these results by performing conventional EPR experiments on MSL-actin oriented by flow to detect changes in the orientational distribution. We have determined the orientational distribution of the spin labels relative to the magnetic field (flow direction) by comparing experimental EPR spectra to simulated EPR spectra corresponding to known orientational distributions. Spectra acquired during flow indicate two populations of probes: a highly ordered population and a disordered population. For the ordered population (28% of the total spin concentration), the angle between the actin filament axis and the nitroxide z axis (theta) fits a Gaussian distribution centered at 32.0 +/- 0.9 degrees, with a full width at half maximum of 20.7 +/- 3.9 degrees. The angle between the nitroxide x axis and the projection of the field in the xy plane (phi) is centered at 37.5 +/- 9.2 degrees with a full width of 24.9 +/- 10.7 degrees. This orientational distribution is not significantly changed upon the binding of phalloidin or myosin subfragment 1 (S1), indicating that these proteins do not affect the axial orientation of actin subunits. Spectra of spin-labeled S1 (MSL-S1) bound to actin oriented by flow have about the same orientational distribution as MSL-S1 bound to actin in oriented fibers. Thus, the oriented fraction of flow-oriented actin filaments has nearly the same high degree of alignment as the actin filaments in muscle fibers.

Dynamic in situ 9 GHz c.w.-e.p.r. low-temperature oxidation study of selected Alberta coals: 3. Influence of exinite macerals and moisture in an hv bituminous coal

Dynamic in situ 9 GHz c.w.-e.p.r. has been used to study the low-temperature oxidation in the interval 20 to 250 °C of hv bituminous coal samples enriched and depleted in exinite, when dried or moisture-saturated. The changes in the e.p.r. spectral parameters and relative total spin concentration are correlated with the maceral composition and moisture content of the samples. These parameters exhibit maxima at ~100 °C. The percentage increase in the spin concentration for dried coal samples between 20 and 100 °C is greater in the exinite-depleted than in the exinite-enriched sample. The 20 °C free radical spin concentration is higher in dried than in moisture-saturated coals but the concentration changes more rapidly with temperature for moisture-saturated samples. This observation is in a good agreement with the interpretation of the lineshape distortion changes determined using radial difference a o d o -Argand diagrams. It is concluded that the moisture content associated with the vitrinite maceral is the most important factor in determining the rate at which the low-temperature oxidation processes occur in coal.

Ａ，Ｘ，Ｙ型ゼオライトにおけるスピンプローブ拡散のＥＳＲイメージングによる研究

Spectral-spatial two-dimensional ESR imaging was applied to studies on the behavior of di-t-butyl nitroxide (DTBN) adsorbed on A-, X-, and Y-type zeolites. The variation of spin distribution and total spin concentration was observed under annealing treatments. In X- and Y-type zeolites DTBN was adsorbed preferentially at the gas-introduced side of the sample. In consrast DTBN was adsorbed over the whole sample uniformly in A-type zeolite. A difference in the spatial distribution was noted also by desorption.

Characterization of rhodium species in Na-X zeolite by electron spin resonance and electron spin echo modulation analyses

Rh(II) species are formed in X zeolite by heating in flowing oxygen. The various species formed after activation at various temperatures, rehydration, and adsorption of various molecules are characterized by electron spin resonance (ESR) and electron spin echo modulation (ESEM) experiments. Two dominant species, termed A and C, are studied in detail. Species A is generated at lower activation temperature than is species C. At intermediate activation temperatures the total Rh spin concentration decreases which is attributed to Rh dimer formation. At higher activation temperatures the total Rh spin concentration increase is consistent with dimer decomposition. Species A is suggested to be located in the ..beta..-cage and species C in the hexagonal prisms of the zeolite X structure based on interaction distances with adsorbates and on the effect of O/sub 2/ on broadening the ESR lines. Rather large distances to water, methanol, and other adsorbates are obtained from the ESEM measurements which suggest that these Rh species are not directly coordinated to the adsorbates. Complete rehydration generates other Rh species which show shorter distances to water characteristic of direct coordination. 33 references, 8 figures, 4 tables.

Paramagnetic Relaxation Study of Spatial Distribution of Trapped Electrons in Gamma-Irradiated Organic Glasses

Paramagnetic relaxation characteristics of trapped electrons in γ-irradiated glassy 3-methylpentane (3-MP) at 71°K, and triethylamine (TEA) and methyltetrahydrofuran (MTHF) at 77°K were studied by power saturation techniques. For MTHF and TEA the characteristic relaxation time, (T1T2)1/2, is constant with radiation dose at low doses and decreases at higher doses. For 3-MP the relaxation time decreases even at low dose. Together with data on the total spin concentration the results are interpreted in terms of a ``spur'' model in which the radiation-produced trapped electrons and radicals are trapped in clusters at low dose and constitute a nonuniform spatial distribution. According to this model the spur radius is 〉130 Å in 3 MP, 101 Å in TEA, and 63 Å in MTHF. These radii compare with 58 Å for trapped electrons in glassy methanol and 41 Å in glassy alkaline ice (10M NAOH). For all of these matrices the radius within which the electrons are trapped appears to increase with decreasing polarity of the matrix, and consequently radiation-produced electrons appear to travel farther before being trapped in less polar matrices.

Paramagnetic Relaxation of Trapped Electrons in Gamma-Irradiated Organic Glasses

Abstract Paramagnetic relaxation characteristics of trapped electrons in γ-irradiated, glassy methanol, ethanol and 2-methyltetrahydrofuran (MTHF) at 73 °K were studied by power saturation techniques. The spin-spin relaxation time is constant with radiation dose at low doses and decreases at higher doses. Together with data on the total spin concentration the results are interpreted in terms of a “spur” model in which the initial spatial distribution of radiation-produced, trapped electrons and radicals is non-uniform. According to this model the spur radius is about 59 A in methanol and 72 A in MTHF. These radii compare with 42 A for trapped electrons in alkaline ice. The radius within which electrons are trapped appears to increase with decreasing polarity of the matrix. The ratio of local spin concentration to the sample average spin concentration is greater than one at low doses in all matrices.

Light absorption of flavosemiquinone metal chelates.

The light absorption of flavosemiquinone metal chelates (Zn2+, Cd2+, Mn2+, Co2+, Ni2+, Fe2+, Mo5+, and Cu+) in aprotic polar solvents has been studied. The light absorption of flavosemiquinone metal chelates, the uncomplexed cation and anion free radicals, and the corresponding, very unstable, flavoquinone metal chelates are compared. The visible spectra of the flavosemiquinone metal chelates, irrespective of the metal, are very similar to that of the free radical anion. In addition, they show a rather unique, sharp transition in the near infrared range (650 to 850 nm). The extinction coefficients for d1, d5, d6, d7, d8, and d10 metal centres are between 10 and 2 × 103 M−1cm−1. Solvents that act as quaternary ligands (pyridine, acetonitrile, but not 2,6-lutidine) strongly affect this near infrared transition, but not the visible spectrum. We therefore conclude that the near infrared absorption reflects the symmetry of the flavosemiquinone-metal-H2O-solvent complex. The molar extinction coeffcients in the near infrared are independent of chelate concentration, solvent polarity, and ionic strength, and the optical densites are proportional to total spin concentration as monitored by electron spin resonance (for systems of total spin 1/2, i. e. with d10 central ions). This excludes association phenomena. Altering the flavin moiety, even slightly, effects the light absorption spectrum in the visible and near infrared regions. This near infrared absorption may be a useful tool for detecting flavin-metal chelation in enzyme systems.