Boris Rakvin

Stabilisation of tetrabromo- and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals†

Crystal structures of alkali salts of tetrahaolgenosemiquinone anion radical acetone solvates and their solvent-free salts are determined. p-Semiquinone anion radical reveals enhanced aromaticity of the ring compared to the quinone. A pair of π-stacked radical anion (p-semiquinone) rings occurs in crystal structures of potassium and rubidium salts of tetrachlorosemiquinone anion acetone solvates and their potassium tetrabromo analogue. The ring centroid separation distances are about 3.2 A and carbon–carbon contacts between the contiguous rings are 0.3 A shorter than the sum of van der Waals radii. The spin-coupling of the two unpaired electrons between the two anion radical rings (forming a stacked dimer) correlates with the diamagnetic property of the crystals. Magnetic properties of alkali salts of tetrahaolgenosemiquinone anion radical acetone solvates were examined by electron paramagnetic resonance spectroscopy.

Electron spin-lattice relaxation in solid ethanol: Effect of nitroxyl radical hydrogen bonding and matrix disorder

The electron spin-lattice relaxation of TEMPO and TEMPONE was measured at temperatures between 5 and 80 K in crystalline and glassy ethanol using X-band electron paramagnetic resonance spectroscopy. The experimental data at the lowest temperatures studied were explained in terms of electron-nuclear dipolar interaction between the paramagnetic center and the localized excitations, whereas at higher temperatures low-frequency vibrational modes from the host matrix and Raman processes should be considered. The strong impact of hydrogen bonding between the dopant molecule and ethanol host on the spin relaxation was observed in ethanol glass whereas in crystalline ethanol both paramagnetic guest molecules behaved similarly.

Single crystals of DPPH grown from diethyl ether and carbon disulfide solutions - crystal structures, IR, EPR and magnetization studies.

Single crystals of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) obtained from diethyl ether (ether) and carbon disulfide (CS₂) were characterized by the X-ray diffraction, IR, EPR and SQUID magnetization techniques. The X-ray structural analysis and IR spectra showed that the DPPH form crystallized from ether (DPPH1) is solvent free, whereas that one obtained from CS₂ (DPPH2) is a solvate of the composition 4DPPH·CS₂. Principal values of the g-tensor were estimated by the X-band EPR spectroscopy at room and low (10 K) temperatures. Magnetization studies revealed the presence of antiferromagnetically coupled dimers in both types of crystals. However, the way of dimerization as well as the strength of exchange couplings are different in the two DPPH samples, which is in accord with their crystal structures. The obtained results improved parameters accuracy and enabled better understanding of properties of DPPH as a standard sample in the EPR spectrometry.

EPR and magnetization studies on single crystals of a heterometallic (CuII and CrIII) complex: Zero-field splitting determination

Magnetic properties of single crystals of the heterometallic complex [Cu(bpy)3]2[Cr(C2O4)3]NO3 9H2O (bpy = 2, 2-bipyridine)have been investigated. From the recorded EPR spectra, the spin-Hamiltonian parameters have been determined. The magnetization measurements have shown magnetic anisotropy at low temperatures, which has been analysed as a result of the zero-field splitting of the CrIII ion. By fitting the exactly derived magnetization expression to the measured magnetization data, the axial zero-field splitting parameter, D, has been calculated. Comparing to the EPR measurements, it has been confirmed that D can be determined from the measurements of the macroscopic magnetization on the single crystals.

Magnetic order in a novel 3D oxalate-based coordination polymer {[Cu(bpy)3][Mn2(C2O4)3]·H2O}n

A heterometallic coordination polymer {[Cu(bpy)3][Mn2(C2O4)3]·H2O}n (1; bpy = 2,2-bipyridine) was synthesized using a building-block approach and characterized by IR spectroscopy, single-crystal X-ray diffraction, magnetization measurement, and X-band ESR spectroscopy both on a single crystal and a polycrystalline sample. The molecular structure of 1 is made of a three-dimensional (3D) anionic network [Mn2(C2O4)3]n(2n-) and tris-chelated cations [Cu(bpy)3](2+) occupying the vacancies of the framework. In compound 1 magnetic order is confirmed below 12.8 K - magnetization measurements reveal an antiferromagnetic-like network of canted Mn(2+) spins with incorporated paramagnetic Cu(2+) centres. The ESR spectroscopy distinctly shows the phase transition; above T≈ 13 K, single isotropic Lorentzian lines of Mn(2+) ions in the high spin state S = 5/2 were observed, while below this temperature, only characteristic Cu(2+) signals from cations were detected. Thermal decomposition residues of 1 at different temperatures (800-1000 °C) were analyzed by powder X-ray diffraction; by heating the sample up to 1000 °C the spinel oxide CuMn2O4 [94.1(2) wt%] was formed. From the refined structural parameters, it could be seen that the obtained spinel is characterized by the inversion parameter δ∼ 0.8, and therefore the structural formula at room temperature can be written as (tet)[Cu0.17Mn0.83](oct)[Mn1.17Cu0.83]O4.

Low-temperature electron-spin relaxation in the crystalline and glassy states of solid ethanol

X-band electron paramagnetic resonance spectroscopy was used to study the spectral properties of a nitroxide spin probe in ethanol glass and crystalline ethanol, at 5 – 11.5 K. The different anisotropy of molecular packing in the two host matrices was evidenced by different rigid limit values for maximal hyperfine splitting in the signal of the spin probe. The significantly shorter phase memory time Tm for the spin probe dissolved in crystalline ethanol, as compared to ethanol glass, was discussed in terms of contribution from spectral diffusion. The effect of low-frequency dynamics was manifested in the temperature dependence of Tm and in the difference between the data measured at different spectral positions. This phenomenon was addressed within the framework of the slow-motional isotropic diffusion model S. Lee and S. Z. Tang, Phys. Rev. B 31, 1308 1985 predicting the spin probe dynamics within the millisecond range, at very low temperatures. The shorter spin-lattice relaxation time of the spin probe in ethanol glass was interpreted in terms of enhanced energy exchange between the spin system and the lattice in the glass matrix due to boson peak excitations.

A multi-frequency EPR spectroscopy approach in the detection of boson peak excitations

The influence of boson peak (BP) excitations on low-temperature spin-lattice relaxation rate of a paramagnetic center embedded in a glassy matrix is investigated in the context of multi-frequency electron paramagnetic resonance (EPR) detection. In the theoretical analysis, the transition rate of spin one-half in the presence of a phonon field is calculated within the approximation of Fermis golden rule. Several phonon densities of states are compared, among which one originating from a model of quasi-localized vibrations has been introduced into electron spin relaxation formalism for the first time. The respective frequency dependencies of spin-lattice relaxation rates are predicted which should lead to observable effects of BP modes if a multi-frequency study at very low temperatures is performed.

ENDOR study of the dynamic properties of stable paramagnetic centres in γ -irradiated L-alanine crystals

The dynamic properties of the stable L-alanine radicals SAR1 and SAR2 induced by γ-irradiation of a L-alanine crystal have been investigated using the electron nuclear double resonance technique (ENDOR). The study was focused on the dynamic properties of these centres in the temperature range from 180 to 320u2009K. In this region, the motion of the and CH3 groups exhibits slow and fast motional dynamics in comparison with the nuclear and electron Larmor frequencies, respectively. The correlation rotation times of the CH3 and groups of the SAR2 CH3 group and the neighbouring CH3 group of the SAR1 paramagnetic centre were estimated from the spectral line broadening and spectral intensity.

EPR probing of bonding and spin localization of the doublet-quartet states in a spin-frustrated equilateral triangular lattice: Cu3(O2C16H23)6·1.2 C6H12

We present high-frequency (34 and 95 GHz) EPR spectroscopic measurements of the magnetic parameters of both the ground state (spin-doublet) and the excited state (spin-quartet) of the model frustrated-spin triangular lattice of Cu3(O2C16H23)6·1.2 C6H12, containing the Cu36+ core. From 295 down to about 100 K, the EPR spectra from single crystals consist of a well-resolved triplet, but with the central component being overlapped by a single peak. At 4 K, the triplet is replaced by a singlet. The triplet is shown to arise from the quartet state, located at 324 K above the ground state. Its magnetic parameters are: D = –535 G, E = 0, g// = 2.209, g⊥ = 2.057 with the parallel direction being the three-fold axis of the Cu36+ core. The singlet is assigned to the S = 1/2 ground state, with gxx = 2.005, gyy = 2.050, and gzz = 2.282. Its hyperfine structure was that from a single Cu nucleus, with Azz = 157 G, and Axx = Ayy < 60 G, demonstrating that in the doublet state the unpaired electron is localized on only one of the three Cu2+ ions. We ascribe this localization to an antiferromagnetic exchange interaction between Cu36+ cores, with zJ′ = –0.15 K. These results serve as a basis for detailed theoretical calculations of spin dynamics and electronic bonding in a frustrated triangular magnetic lattice. To cite this article: B. Cage et al., C. R. Chimie 6 (2003) 000–000.

Investigation of the nitrogen hyperfine coupling of the second stable radical in γ-irradiated l-alanine crystals by 2D-HYSCORE spectroscopy.

The second stable radical, NH(3)(+)C()(CH(3))COO(-), R2, in the γ-irradiated single crystal of l-alanine and its fully (15)N-enriched analogue were studied by an advanced pulsed EPR technique, 2D-HYSCORE (two-dimensional hyperfine sublevel correlation) spectroscopy at 200K. The nitrogen hyperfine coupling tensor of the R2 radical was determined from the HYSCORE data and provides new experimental data for improved characterization of the R2 radical in the crystal lattice. The results obtained complement the experimental proton data available for the R2 radical and could lead to increased accuracy and reliability of EPR spectrum simulations.