Preparation and characterization of MgB2 superconductor

The MgB₂ superconductor, synthesized using solid-state and liquid-phase sintering methods, have been characterized for various properties. The upper critical field, irreversibility line and critical current density have been determined using magnetization data. The current-voltage characteristics recorded under an applied magnetic field revealed the existence of vortex glass transition. The surface analysis using X-ray photoelectron spectroscopy shows that MgB₂ is sensitive to atmospheric degradation.     

Rubidium and proton ENDOR on ion pairs in solution

The E.S.R. spectrum of the o-dimesitoylbenzene anion-alkali cation radical shows unusually large isotropic alkali hyperfine splitting constants. We report a solution ENDOR study of this radical in which both alkali (^85,87Rb) and proton ENDOR spectra were recorded. Both the alkali and proton intensities showed a strong dependence on the metal ion nuclear spin quantum number of the E.S.R. line being saturated. This dependence is attributed to strong flip-flop cross-relaxation induced by modulation of the isotropic alkali hyperfine splitting. The powder E.S.R. spectrum of the complex reveals a small anisotropy of the Rb hyperfine splitting tensor. This indicates a small metal non-s-contribution to the half-filled molecular orbital, which is consistent with the observed relaxation behaviour and the small g shift. The intensity variations in the alkali and proton ENDOR spectra were used to determine the relative signs of all hyperfine splitting constants, and the absolute signs of the hyperfine splitting constants are deduced from a model of the structure of the complex.     

Complexation of copper(II)-Chelidamate: A multifrequency-pulsed electron paramagnetic resonance and electron nuclear double resonance analysis

Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques were used to obtain structural information about the copper(II)-chelidamate complex. Well-resolved nitrogen ENDOR spectra could be recorded from solid solution samples by using selective excitation of spin packets. Evaluation of nuclear quadrupole and dipolar hyperfine interaction of the directly ligated nitrogen allowed for an identification of the bond direction to the copper ion within the eigen frame of the copper g-matrix. Invoking two-dimensional EPR techniques, additional hyperfine interaction with a "distant" nitrogen spin, identified as resulting from the solvent dimethylformamide (DMF), was observed. The experimental data are only consistent with formation of a stable pseudoplanar copper complex with single solvent ligation via its oxygen atom.     

Anion radicals of [60]fullerenes. An EPR study

Photochemically induced electron transfer in homogeneous systems (using triethylamine donor) and heterogeneous systems (using photoexcited TiO₂ suspension) was applied in in situ reduction of [60]fullerene. The anion radicals generated were characterized by means of EPR and VIS/near-IR spectroscopy. Narrow EPR lines were found. Radical A with g_A=2.0000 and peak-to-peak width, pp_A=0.09mT was observed as the primary product; followed by its consecutive product B with g_B=2.0006, pp_B=0.04mT, and in some cases product C with g_C=2.0009 and pp_c<0.1 mT. Radical A was assigned to [60]fullerene mono-anion, also characterized by a near-IR band at 1077 nm. B is presumably di-anion or a dimeric form of mono-anion. Identical results were also obtained using cathodic in situ reduction. Applying these generation techniques to [60]fullerene derivatives produced narrow EPR lines analogous to those described for pristine [60]fullerene. This was the case not only in organic solvents, but also in aqueous solutions. The results obtained present a contrast with the original ex situ EPR investigations describing [60]fullerene mono-anion with wide lines. According to the results presented here, the narrow and wide EPR lines do not represent contradictory phenomena, but are an integral part of the relatively complicated manifestations of various fullerene states and both will have to be seriously considered in the future.     

Electron paramagnetic resonance investigation of endohedral fullerenes N@C(60) and N@C(70) in a liquid crystal

Endohedral fullerenes N@C(60) and N@C(70) were dissolved in the liquid crystal 4-methoxybenzylidene-4'-n-butylaniline (MBBA) and investigated by electron paramagnetic resonance. In both cases well resolved EPR spectra give proof for molecular orientation in the nematic mesophase. Spectral features are dominated by a nonvanishing zero-field interaction, indicating a deviation from spherical spin density distribution at the encased nitrogen atom. In N@C(70), a maximum order parameter O(33) = 0.18(3), correlated with the long axis of the cage, and a zero-field-splitting parameter D = -2.6(4) MHz were determined. A persistent zero-field splitting is also observed in C(60) via the quartet spin of the encapsulated nitrogen, although no assignment of the director with respect to the molecular frame is possible. The observed line splitting is indicative of pseudo orientation of the rapidly rotating cage in this case.     

Evidence for Quantum Beats generated by a photo-induced charge transfer reaction

Photo-initiated charge transfer reactions in solution proceed via a radical pair intermediate which in most cases will separate on a time scale of several nanoseconds. The non-adiabicity of this process is the source of coherences in the spin system. A quantitative discussion of the observability of the resulting quantum beats using Fourier-transform EPR is presented. We also show experimental evidence for the predicted quantum beats using the photo-initiated charge transfer reaction of Zn-Tetraphenylporphyrin and benzoquinone in 2-propanol. The analysis of the beat frequency leads to an estimate of the exchamge couplingJ≈2.6·10⁶ s^?1 of the Coulomb-coupled radical pair.     

Exploring the morphotropic phase boundary in copper(II)-modified Pb[Zr0.54Ti0.46]O3 ferroelectrics

Copper(II)-doped Pb[Zr_0.54Ti_0.46]O₃ (PZT) ferroelectrics with a dopant concentration of 0.25 mol% were investigated at high magnetic fields, enabling an enhanced resolution of the structural distortion at the dopant site. The results obtained suggest that Cu²⁺ substitutes as an acceptor centre for [Zr,Ti]⁴⁺ in oxygen octahedra with tetragonal, monoclinic and rhombohedral distortion, confirming the model of mesoscopic mixing at the morphotropic phase boundary. PACS 61.72.Ji; 61.72.Ww; 76.30.Fc; 77.80.Bh; 77.84.Dy     

Influence of an identified dimer vibration on the emission spectrum of [2,2]paracyclophane

The emission spectrum of polycrystalline [2,2]paracylophane shows a resolved vibronic structure with a 241 cm^?1 progression at He temperatures. The dependence of the energy of this mode upon selective deuteration in combination with results from FIR and Raman spectra could be used to identify the mode as a torsional dimer vibration. The emission spectra could be simulated assuming a linear coupling of the torsional mode to the electronic transitions with coupling strengths of S = 10 (fluorescence) and S = 13 (phosphorescence). This corresponds to an equilibrium displacement of the benzene rings under electronic excitation by a torsional angle of 10.6° (S₁) and 12.1° (T₁), in addition to the small torsion in the ground state S₀ by about 3°.