EPR of a fullerene-molecule-derived paramagnetic center as a mesoscopic conducting object

Discussion of theg-factor value of fullerene is based on the model of itinerant electrons restricted to the surface of the fullerene molecule C₆₀. The Ag shift, i.e., the difference between the experimentalg-factor and theg-factor of free electron Δg = g ? 2.0023 for C ₆₀ ^?1 is negative as for a very small metallic conducting particle.g-factor value is proportional to the interaction between itinerant electrons in the conduction band, thus the Δg is negative for C ₆₀ ^?1 and C ₆₀ ^?3 having less than half filled conduction band, while Δg is positive for C ₆₀ ⁺ where the conduction band is almost filled.     

Electronic properties of charged fullerenes characterized by EPR and Vis-NIR spectroscopy

EPR and Vis-NIR absorption spectra have both been measured for clarification of contradictory statements about the para-and diamagnetic states of fullerenes. Thereby identification of one sharp EPR signal in solution at room temperature to C ₆₀ ^? (g=2.000±4.0001; ΔB=0.07±0.01 mT) could be made upon using different fullerene sources (TechnoCarbo, Hoechst) and methods of anion generation (chemically, electrochemically, and photochemically). This fact is also supported by the similar observation for a monosubstituted derivative (g=1.9999; ΔB=0.10 mT), in which a small broadening of this sharp signal is found originating from additional¹H hyperfine interactions. Furthermore theg-values of the radical anions of C₆₀ increase with charge (g(C ₆₀ ^? <g(C ₆₀ ^2? ) < <g(C ₆₀ ^3? ) <g(C ₆₀ ^5? )) indicating largest contributions from spin-orbit coupling for the monoanion. No diamagnetic states for the dianions of [60]- and [70]- fullerenes could be found so far but biradical species with largest zero field splittingsD=2.7 mT (C ₆₀ ^2? ), andD=3.1 mT (C ₇₀ ^2? ), respectively. The cation formation of C₆₀ (g=2.0023-2.0029; ΔB=0.15-0.20 mT) with antimony pentachloride was controlled by mass spectrometry. Stable cations were found only in methylenechloride. In other solvents like toluene addition reactions seem to occur.     

EPR spectra of C60 anions

The broad and sharp EPR signals observed in spectra of C₆₀ and C ₆₀ ^3? are summarized and proposed assignments are discussed. The sharp signals in samples of C ₆₀ ^2? are reviewed.     

Potentiodynamic studies on corrosion of copper by chloride ions and its inhibition by inorganic and organic ions in aqueous buffer solution

Electrochemical oxidation of copper in aqueous phosphate buffer solution leads to the formation of passive films, which breakdown in the presence of aggressive chloride (Cl^?) ions. The addition of Cl^? ions in the medium causes shift in current and potential for both cathodic and anodic peaks. At low Cl^? ion concentrations, the corrosion current is greatly enhanced and continues to increase up to a certain concentration, after which it decreases. The effect of inorganic ions, chromate (CrO ₄ ^2? ) and tungstate (WO ₄ ^2? ), and organic ions, acetate (CH₃COO^?) and oxalate (C₂O ₄ ^2? ), on the formation and breakdown of passive films in aqueous buffer solution containing 0.10 M Cl^? ions have also been investigated. All of the ions inhibit corrosion of copper in the presence of aggressive Cl^? ions following the order of overall inhibition efficiency of CrO ₄ ^2? > C₂O ₄ ^2? > WO ₄ ^2? > CH₃COO^? under the experimental conditions.     

Statistical mechanism of the formation of the energy spectra of sputtered molecular clusters

The energy spectra of Si _n O _m ^? clusters sputtered from Si targets by Xe⁺ ions with O₂ pumping onto a bombarded surface, as well as by O ₂ ⁺ ions, are studied. It is shown that the form of the Si _n O _2n+1 ^? energy distributions does not depend on the experimental conditions. Significant differences in the energy spectra of O and Si monomers as compared to Si _n O _2n+1 ^? clusters are revealed. The mentioned features of the energy distribution of Si _n O _m ^? clusters are explained within the framework of the statistical recombination mechanism of their formation in combinatorial synthesis processes.     

Simultaneous electrochemical and electron paramagnetic resonance studies of fullerene anion radicals C 60 1− and C 60 3−

Simultaneous electrochemical and electron paramagnetic resonance experiments have been carried out on the reduced C₆₀ fullerene to examine theg-factor assignment of the radical species. C ₆₀ ^1? and C ₆₀ ^3? show the following EPR characteristics at room temperature: C ₆₀ ^1? :g _1?=2.0002±0.0001, 2ΔB _1s=0.17 mT, and C ₆₀ ^3? :g _{3?=2.0008±0.0002}, 2ΔB _1s=0.07 mT. EPR linewidths are apparently narrower compared to those in most of the spectra previously reported. Variable temperature EPR study on solution containing C ₆₀ ^1? has shown thatg _1? value is not while the linewidth is only slightly temperature dependent.     

Optical and electrochemical properties of cyclometalated Pd(II) and Pt(II) complexes with a metal-metal chemical bond

Cyclometalated [M(C^N)(μ-(N-S))]₂ complexes ((M = Pd(II), Pt(II)), (C^N)^? are the deprotonated forms of 2-tolylpyridine and benzo[h]quinoline, and (N-S)^? are pyridine-2-thiolate and benzothiazole-2-thiolate ions) are studied by ¹H NMR, IR, electronic-absorption, and emission spectroscopy, as well as by voltammetry. It is shown that the formation of the metal-metal chemical bond and the σ _dz2 ^* orbital as a HOMO of complexes leads to the long-wavelength spin-allowed (410–512 nm) and spin-forbidden (595–673 nm) optical transitions σ _dz2 ^* ?π _(C^N) ^* in the absorption and phosphorescence spectra, as well as to the two-electron and successive one-electron oxidation with the formation of binuclear Pt(III) and Pd(III) complexes. The substitution of Pt(II) by Pd(II) is characterized by hypso- and bathochromic shifts of the spin-allowed and forbidden σ _dz2 ^* ?π _(C^N) ^* optical transitions in the absorption and phosphorescence spectra of complexes, by phosphorescence quenching of Pd(II) complexes in liquid solutions, and by an anodic shift of the oxidation potential of Pd(II) complexes compared with Pt(II) complexes.     

Paramagnetic relaxation kinetics of the cation radical C 60 + in C60 powder

The kinetics of spin-lattice and phase relaxation of C ₆₀ ⁺ radicals in C₆₀ powder has been studied at room temperature by pulsed EPR. It is found that the kinetics can be described by the relation exp $( - a\sqrt t )$ characteristic of the case in which the paramagnetic centers are distributed over relaxation times. It is concluded that the observed kinetics are due to the presence of oxygen molecules, which act as a fast-relaxing impurity and accelerate the relaxation of the C ₆₀ ⁺ radicals. The results obtained offer an explanation for a number of features of C ₆₀ ⁺ relaxation in fullerites discovered earlier.     

Resonant Raman spectroscopy of ethylene-acetylene copolymers doped with iodine

The relative contents of short and long conjugated chains in ethylene-acetylene copolymers (EAC) are determined by varying the lasing wavelength, as was done earlier for pure poly(acetylene). The Raman spectra of the copolymer samples doped with iodine contain the bands attributed to the iodine polyions I ₃ ^? and I ₅ ^? Unlike poly(acetylene), the ethylene-acetylene copolymer is characterized by the Raman spectra in which the intensities of the bands assigned to the I ₅ ^? polyions are higher than those of the I ₃ ^? polyions even at low degrees of doping. The structural features responsible for this difference are discussed.     

Pulsed ESR on the organic ion radical salt: 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-[TCNQ]2

We present firstT _1e ^?1 andT _2e ^?1 measurements on the organic ion radical salt 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)₂ as function of temperature and of orientation. The electronic spin diffusion constant could be determined directly by the electron spin echo field gradient technique:D (300 K)=0.03±0.02 cm²/sec. Pulsed ESR experiments have — in comparison to conventional cw-ESR — the advantage to monitor viaT _1e ^?1 andT _2e ^?1 the spectral density of dynamical processes at different frequencies. This is shown in a general manner on 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)₂. Between 300 and 60 K,T _1e ^?1 andT _2e ^?1 are close in amplitude and have a similar temperature dependence. At 60 K their degeneracy is lifted, yielding a quantitative value for the effective spin exchange between localized spinsτ _ex ^?1 sec^?1 and via the absolute value of the relaxation an average distance of the localized centers of about 12 Å. The dynamical data as evaluated above cannot be correlated with the conductivity, clearly indicating that the conduction electrons are a minority, not being monitored by the ESR-experiments.     

Stoichiometric synthesis of fullerene compounds with lithium and sodium and analysis of their IR and EPR spectra

A modified method is proposed for preparing fullerene compounds with alkali metals in a solution. The compounds synthesized have the general formula Me _n C₆₀(THF)_x, where Me = Li or Na; n=1–4, 6, 8, or 12; and THF = tetrahydrofuran. The use of preliminarily synthesized additives MeC₁₀H₈ makes it possible to prepare fullerene compounds with an exact stoichiometric ratio between C ₆₀ ^n? and Me ⁺. The IR and EPR spectra of the compounds prepared are analyzed and compared with the spectra of their analogs available in the literature. The intramolecular modes T _u (1)-T _u (4) for the C ₆₀ ^n? anion are assigned. The splitting of the T _u (1) mode into a doublet at room temperature for Me _n C₆₀(THF)_x (n=1, 2, 4) compounds indicates that the fullerene anion has a distorted structure. An increase in the intensity of the T _u (2) mode, a noticeable shift of the T _u (4) mode toward the long-wavelength range, and an anomalous increase in the intensity of the latter mode for the Li₃C₆₀(THF)_x complex suggest that, in the fullerene anion, the coupling of vibrational modes occurs through the charge-phonon mechanism. The measured EPR spectra of lithium-and sodium-containing fullerene compounds are characteristic of C ₆₀ ^? anions. The g factors for these compounds are almost identical and do not depend on temperature. The g factor for the C ₆₀ ^n? anion depends on the nature of the metal and differs from the g factor for the C ₆₀ ^? anion.     

Formation of doubly charged negative ions under the conditions of the resonant electron capture by fluorofullerenes

Doubly charged negative ions formed when electrons with controlled energies interact with isolated fluorinated fullerene molecules C₆₀F _n (n = 36, 48) have been detected and investigated by resonant electron capture mass spectrometry. The dependence of the intensity of the formation of doubly charged negative ions of fluorofullerenes on the energy of attached electrons has been measured. An original method, which is based on the experimental data and does not require additional calibration quantities, has been developed for estimating the absolute cross section for the formation of doubly charged negative ions. The absolute cross sections for the formation of the most intensely formed ions C₆₀F ₃₆ ^2? and C₆₀F ₄₈ ^2? are estimated to be about 1.1 × 10^?24 and 1.5 × 10^?24 m² at their maximum-yield energies of 2.0 and 1.6 eV, respectively.     

EPR investigations of SiC and SiOC nanometric powders

EPR investigations are performed in SiC and SiOC nanometric powders annealed between 1200 and 1800°C. By using different EPR frequency bands and a suitable spectra analysis, three quite different paramagnetic defects with well defined $\tilde g_i $ (i=1, 2, 3) and hyperfine $\tilde A_i $ (i=1, 2) tensors account for the EPR signal in these materials. The defects are characterized by $\tilde g_1 $ (g ₁ ^‖ =2.0046(3), g ₁ ^? =2.0023(3)), $\tilde g_2 $ (g ₂ ^‖ =2.0037(3), g ₂ ^? =2.0028(3)) and isotropic $\tilde g_3 $ (2.0030(3)) tensors. In SiC powders, the defects assignment is discussed with respect to the different SiC forms, namely α-SiC and β-SiC polytypes as well as amorphous SiC and carbon present in minor concentration in the network. In SiOC powders, the above defects are evidenced only at high annealing temperature (T _a≥1200°C) when the oxygen contained is highly reduced.     

Elektronenspinresonanz von elektronenbestrahlten Ammoniakschichten

Ammonia, NH₃ or ND₃, is vaporized upon a cold substrate (20–100 K) and irradiated with electrons of energies from 0.2 to 6 keV. The different EPR spectra show hyperfinestructure and may be ascribed to centers of the type H ₂ ^? , D ₂ ^? , (NH ₂ ^? )^?, (ND ₂ ^? )^?. With respect to electrostatic charge a chemical decomposition of ammonia must be assumed. No electronic centers are found, which could be compared with the solvated electrons known in liquid metal-ammonia-solutions or quenched solid metal-ammonia-layers.     

A computational study of entropy rules for charged fullerenes

Thermochemical data on fullerenes are relatively scarce. However, some thermochemical information can be derived from gas-phase experiments using the Knudsen cell mass spectrometry method. The third-law treatment can be carried out on the observed data, though one has to make the crucial presumption that the change in the thermodynamic potential ΔΦ _T ^o in the course of the reactions considered is negligible: ΔΦ _T ^o =0. It would be difficult to check the presumption directly in the experiment, but it can be checked computationally. Model reactions like C₆₀+ ₇₀ ^? = ₆₀ ^? +₇₀ are selected. The change in the thermodynamic potential ΔΦ _T ^o and the change in the standard entropy ΔS _T ^o are computed. For example, at a temperature of T=1000 K, the standard changes for the reaction evaluated using the SAM1 method are ΔΦ _T ^o =1.513 cal/(mol K) and ΔS _T ^o =?0.054 cal/(mol K). Overall, the computations support the critical thermodynamic presumption.     

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

The secondary acceptor A₁ of the electron transport chain(s) of photosystem (PS) I is a phylloquinone (vitamin K₁, VK₁). Pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) experiments at X-band frequencies were performed on the photoaccumulated acceptor radical A ₁ ^·? and the radical pair state P ₇₀₀ ^·+ A ₁ ^·? in PS I ofThermosynechococcus elongatus. The data obtained were compared with data from the respective radical anion of VK₁ in organic solvents. The unusualg tensor magnitude of A ₁ ^·? is explained by the hydrophobic binding pocket of this radical. The hyperfine couplings and the spin (and charge) density distribution is very different for A ₁ ^.? in PS I and VK ₁ ^·? in frozen alcoholic solution. This is attributed to a rather strong one-sided hydrogen bond to A ₁ ^·? . The presence of a hydrogen bond to A ₁ ^·? has only a minor effect ong. The hyperfine coupling constants of A ₁ ^·? determined from the radical pair spectra deviate only slightly from those derived from photoaccumulated A ₁ ^·? in PS I treated with dithionite at high pH. ENDOR resonances of the proton in a H bond were detected and an estimate of the strength and geometry of this bond to A ₁ ^·? was obtained. The significance of the hydrogen bond and other (hydrophobic) interactions of A₁ with the surrounding are briefly discussed.     

Absorptionsspektren des Er3+-Ions in Scandiumvanadat und Scandiumphosphat bei verschiedenen Dotierungen

The optical absorption spectra of trivalent erbium in ScVO₄ single crystals and ScPO₄ powders with doping ratios between (500∶1) and (10∶1) have been obtained between 15,000 and 27,000 cm^?1. From the positions of the Stark components of (Sc, Er)VO₄ (100∶1) the five crystal field parametersB ₀ ² ,B ₀ ⁴ ,B ₀ ⁶ ,B ₄ ⁴ andB ₄ ⁶ have been determined. Moreover the paramagnetic resonance of (Sc, Er)VO₄ single crystals with doping ratios mentioned above have been examined. With these experimental results informations have been obtained about the doping with metal-ions of an essentially different ionic radius.     

ESR study on the structure of the ethylene radical cation stabilized in frozen SF6 and some fluoroalkanes: An evidence for a twisted structure

ESR spectra of the ethylene radical cation were detected at cryogenic temperatures in SF₆, C₂F₆ and C₃F₈. From the unusually small hyperfine couplings estimated for¹H and¹³C, it has been shown that the ethylene radical cation has a non-planar structure with a torsional angle in the range of 8°–23°. Upon annealing the sample at a temperature above 93 K, the ethylene radical cation in SF₆ changed into a monofluoroethyl radical through charge recombination with fluoride anion or SF ₆ ^? .     

Studying track electrons in frozen aqueous solutions by means of emission Mössbauer spectroscopy

It is shown by means of emission Mössbauer spectroscopy that the reactivity of NO ₃ ^? cations with quasi-free track electrons exceeds those of H₃O⁺, ClO ₄ ^? , and HSO ₄ ^? ions by a factor of only three. This is in drastic contradiction to published assertions that these values differ by two orders of magnitude. An explanation for this discrepancy is proposed.     

Near and far infrared vibrational absorption ofH i − andD i − centres in KBr

Near infrared absorption measurements by Gross and Bron led to the suggestion that the threefold degeneracy of the localized vibration ofH _i ^? andD _i ^? centres in KI is completely lifted in KBr. Using thermal annealing procedures,H _i ^? -D _i ^? substitution and far infrared techniques we found that the lines at 794, 840, and 896 cm^?1 cannot be interpreted as a splitting of the main line. Only the 794 cm^?1 line can be correlated with the direct excitation of theH _i ^? oscillator. The absence of any splitting in this line (its halfwidth at 9 °K is only 1.5 cm^?1) indicates that also in KBrH _i ^? centres have at most only slightly perturbedT _d symmetry.