Anisotropy of the electron spin-lattice relaxation. PO32- radical in glycinium phosphite gly·H3PO3 crystal.

Electron spin-lattice relaxation has been measured for PO32- radical in glycinium phosphite gly·H3PO3 crystal and its deuterated analogue in temperature range 40-300 K. Angular dependence of the relaxation rate was measured in three crystal planes at room temperature. The Debye cut-off temperature has been calculated as ΘD = 97 K, which indicates that the temperature dependence of 1/T1 is governed by radical local vibrations localized in optical phonon modes range. Theories of possible 1/T1 angular dependence are reviewed. The anisotropy of spin-lattice relaxation rate in our crystal is assumed to be a result of local magnetic field fluctuations due to electron-proton dipolar coupling. Theoretical evaluation of 1/T1 were performed for coupling with protons at distances up to 0.46 nm. Not perfect agreement was found with motion correlation time τc = 2·10-9 s.

Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K.

ESR spectra and electron spin relaxation of nitroxide radical in 4-oxo-TEMPO-d16-(15)N in propylene glycol were studied at X-band in the temperature range 10-295K. The spin-lattice relaxation in the liquid viscous state determined from the resonance line shape is governed by three mechanisms occurring during isotropic molecular reorientations. In the glassy state below 200K the spin-lattice relaxation, phase relaxation and electron spin echo envelope modulations (ESEEM) were studied by pulse spin echo technique using 2-pulse and 3-pulse induced signals. Electron spin-lattice relaxation is governed by a single non-phonon relaxation process produced by localized oscillators of energy 76cm(-1). Electron spin dephasing is dominated by a molecular motion producing a resonance-type peak in the temperature dependence of the dephasing rate around 120K. The origin of the peak is discussed and a simple method for the peak shape analysis is proposed, which gives the activation energy of a thermally activated motion Ea=7.8kJ/mol and correlation time τ0=10(-8)s. The spin echo amplitude is strongly modulated and FT spectrum contains a doublet of lines centered around the (2)D nuclei Zeeman frequency. The splitting into the doublet is discussed as due to a weak hyperfine coupling of nitroxide unpaired electron with deuterium of reorienting CD3 groups.

Phonon spectrum, electron spin-lattice relaxation and spin-phonon coupling of Cu2+ ions in BaF2 crystal.

Electron spin-lattice relaxation rate is determined by electron spin echo method in temperature range 4-60K. The Raman relaxation processes dominate and its theory is outlined in a form suitable for calculations of relaxation rate using real phonon spectrum. A few approximations have been considered: when phonon spectrum and Debye temperature are not available; when Debye temperature is available but phonon spectrum is not; and when spin-phonon coupling is known. All these approximations use the Debye model of phonons and give a non-satisfactory description the temperature dependence of the relaxation rate. A perfect description of experimental results is obtained when real phonon spectrum is considered. The value of the spin-phonon coupling parameter was determined as G=〈a|V|b〉=1362cm(-1). This value is discussed by a comparison with G-values published for various ions and crystals.

Electron spin echo and spin relaxation of low-symmetry Mn(2+)-complexes in ammonium oxalate monohydrate single crystal.

Pulse EPR experiments were performed on low concentration Mn(2+) ions in ammonium oxalate monohydrate single crystals at X-band, in the temperature range 4.2-60K at crystal orientation close to the D-tensor z-axis. Hyperfine lines of the resolved spin transitions were selectively excited by short nanosecond pulses. Electron spin echo signal was not observed for the low spin transition (+5/2↔+3/2) suggesting a magnetic field threshold for the echo excitation. Echo appears for higher spin transitions with amplitude, which grows with magnetic field. Opposite behavior displays amplitude of echo decay modulations, which is maximal at low field and negligible for high field spin transitions. Electron spin-lattice relaxation was measured by the pulse saturation method. After the critical analysis of possible relaxation processes it was concluded that the relaxation is governed by Raman T(7)-process. The relaxation is the same for all spin transitions except the lowest temperatures (below 20K) where the high field transitions (-3/2↔-1/2) and (-5/2↔-3/2) have a slower relaxation rate. Electron spin echo dephasing is produced by electron spectral diffusion mainly, with a small contribution from instantaneous diffusion for all spin transitions. For the highest field transition (-5/2↔-3/2) an additional contribution from nuclear spectral diffusion appears with resonance type enhancement at low temperatures.

EPR and ESE of CuS4 complex in Cu(dmit)2: g-factor and hyperfine splitting correlation in tetrahedral Cu-sulfur complexes.

Pseudotetrahedral CuS4 complexes of Cu(dmit)2 compound in DMF solution were studied by EPR, UV-Vis and electron spin echo methods. After rapid freezing at 77 K a good glassy state is formed and the CuS4 complex has a D(2d) symmetry of a compressed tetrahedron with xy ground state and spin-Hamiltonian parameters g||=2.089, g⊥=2.026, A||=146×10(-4) cm(-1) and A⊥=30×10(-4) cm(-1). The complex is not deformed in the glassy state and is very rigid as indicated by the echo detected spectrum and by electron spin relaxation which is governed by reorientations of methyl groups of surrounding DMF molecules as shown by electron spin echo envelope modulation (ESEEM) spectrum. The g|| and A|| of Cu(dmit)2 and other CuS4 complexes collected in Peisach-Blumberg correlation diagram were analyzed using extended Molecular Orbital theory. We explain why the correlation line for copper-sulfur complexes has larger slope compared to the CuO4 and CuN4 tetrahedra. Along the correlation line the delocalization of unpaired electron density onto ligand is constant and varies from ß=0.78-0.83 for g|| in the range 2.06-2.10 of correlation diagram. The slope of the line is determined by the product of MO-coefficients αc1, where α is a parameter characterizing delocalization of unpaired electron in x(2)-y(2) and c1<1 is a mixing parameter decreasing when 4p contribution grows. We found, unexpectedly, that αc1≈0.7 for all CuS4 complexes suggesting a correlation between degree of tetrahedral deformation and MO-parameters. MO-coefficients for Cu(dmit)2 are α=0.753, ß=0.752 and c1=0.930 confirming a strong delocalization of unpaired electron in xy and x(2)-y(2) orbitals.

Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution.

Co2+ binding to the nicotinamide adenine dinucleotide (NAD+) molecule in water solution was studied by electron paramagnetic resonance (EPR) and electron spin echo at low temperatures. Cobalt is coordinated by NAD+ when the metal is in excess only, but even in such conditions, the Co/NAD+ complexes coexist with Co(H2O)6 complexes. EPR spin-Hamiltonian parameters of the Co/NAD+ complex at 6 K are gz  = 2.01, gx  = 2.38, gy  = 3.06, Az  = 94 × 10-4 cm-1, Ax  = 33 × 10-4 cm-1 and Ay  = 71 × 10-4 cm-1. They indicate the low-spin Co2+ configuration with S = 1/2. Electron spin echo envelope modulation spectroscopy with Fourier transform of the modulated spin echo decay shows a strong coordination by nitrogen atoms and excludes the coordination by phosphate and/or amide groups. Thus, Co2+ ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of adenine rings of two NAD+ molecules.

Raman electron spin-lattice relaxation with the Debye-type and with real phonon spectra in crystals.

Electron spin-lattice relaxation temperature dependence was measured for Ti(2+) (S=1) and for Cu(2+) (S=1/2) ions in SrF(2) single crystal by electron spin echo method in temperature range 4-109K. The spin relaxation was governed by the two-phonon Raman processes. The relaxation theory is outlined and presented in a form suitable for applying with real phonon spectra. The experimental relaxation results were described using Debye-type phonon spectrum and the real phonon spectrum of SrF(2) crystal. The Debye approximation does not fit well the results for SrF(2) both at low and at high temperature. The relaxation rate is faster than that predicted by Debye-type phonon spectrum at low temperatures where excess of lattice vibrations over the Debye model exists but is slower at higher temperatures (above 50K) where density of phonon states continuously decreases when approaching to the maximal acoustic phonon frequency. The expected deviation from Debye approximation was analyzed also for Cu(2+) in NaCl and MgSiO(3) crystals for which phonon spectra are available. The fitting with the real phonon spectrum allowed us to calculate spin-phonon coupling parameter as 267 cm(-1) for Ti(2+) and 1285 cm(-1) for Cu(2+) in SrF(2).

Electron spin relaxation governed by Raman processes both for Cu²âº ions and carbonate radicals in KHCO3 crystals: EPR and electron spin echo studies.

EPR studies of Cu²âº and two free radicals formed by γ-radiation were performed for KHCO3 single crystal at room temperature. From the rotational EPR results we concluded that Cu²âº is chelated by two carbonate molecules in a square planar configuration with spin-Hamiltonian parameters g(||)=2.2349 and A(||)=18.2 mT. Free radicals were identified as neutral HOCO· with unpaired electron localized on the carbon atom and a radical anion CO3·â» with unpaired electron localized on two oxygen atoms. The hyperfine splitting of the EPR lines by an interaction with a single hydrogen atom of HOCO· was observed with isotropic coupling constants a0=0.31 mT. Two differently oriented radical sites were identified in the crystal unit cell. Electron spin-lattice relaxation measured by electron spin echo methods shows that both Cu²âº and free radicals relax via two-phonon Raman processes with almost the same relaxation rate. The temperature dependence of the relaxation rate 1/T1 is well described with the effective Debye temperature Θ(D)=175 K obtained from a fit to the Debye-type phonon spectrum. We calculated a more realistic Debye temperature value from available elastic constant values of the crystal as Θ(D)=246 K. This Θ(D)-value and the Debye phonon spectrum approximation give a much worse fit to the experimental results. Possible contributions from a local mode or an optical mode are considered and it is suggested that the real phonon spectrum should be used for the relaxation data interpretation. It is unusual that free radicals in KHCO3 relax similarly to the well localized Cu²âº ions, which suggests a small destruction of the host crystal lattice by the ionizing irradiation allowing well coupling between radical and lattice dynamics.

EPR and potentiometric studies of copper(II) binding to nicotinamide adenine dinucleotide (NAD+) in water solution.

Coordination of Cu(II) by nicotinamide adenine dinucleotide (NAD(+)) molecule has been studied in water solutions of various pH by potentiometry and electron paramagnetic resonance (EPR) and electron spin echo (ESE) spectroscopy. Potentiometric results indicate Cu(II) coordination by protonated NAD(+) at low pH and by deprotonated NAD(+) at high pH. At medium pH value (around pH=7) NAD(+) is not able to coordinate Cu(II) ions effectively and mainly the Cu(H(2)O)(6) complexes exist in the studied solution. This has been confirmed by EPR results. Electronic structure of Cu(II)-NAD complex and coordination sites is determined from EPR and ESE measurements in frozen solutions (at 77K and 6K). EPR spectra exclude coordination with nitrogen atoms. Detailed analysis of EPR parameters (g(||)=2.420, g(perpendicular)==2.080, A(||)=-131×10(-4)cm(-1) and A(perpendicular)=8×10(-4)cm(-1)) performed in terms of molecular orbital (MO) theory shows that Cu(II)NAD complex has elongated axial octahedral symmetry with a relatively strong delocalization of unpaired electron density on in-plane and axial ligands. The distortion of octahedron is analyzed using A(||) vs. g(||) diagram for various CuO(x) complexes. Electron spin echo decay modulation excludes the coordination by oxygen atoms of phosphate groups. We postulate a coordination of Cu(II) by two hydroxyl oxygen atoms of two ribose moieties of the NAD molecules and four solvated water molecules both at low and high pH values with larger elongation of the octahedron at higher pH.

Electronic structure and dynamics of low symmetry Cu2+ complexes in kainite-type crystal KZnClSO4.3H2O: EPR and ESE studies.

EPR measurements at X-band were performed in the temperature range 4.2-300 K with angular dependence measurements at 77 K for Cu(2+) in KZnClSO(4).3H(2)O. Rigid lattice spin-Hamiltonian parameters are: g(z) = 2.4247, g(y) = 2.0331, g(x) = 2.1535, A(z) = -103 x 10(-4) cm(-1), 63 x 10(-4) cm(-1), and -31 x 10(-4) cm(-1). The parameters were analyzed using MO-theory with the d(x(2)-y(2)) ground state containing admixture of the d(z(2))-state in the rhombic symmetry D(2h). The analysis consistently explained unusual g-factor sequence and relatively small hyperfine splitting anisotropy as the consequence of the mixing and spin density delocalization via excited orbital states. We assigned that Cu(2+) ions substituting host Zn(2+) prefer one of the four structurally different zinc sites where they are coordinated by four water molecules and two SO(4) groups in an distorted octahedron elongated along SO(4)-Cu-SO(4) direction. The distortion is due to the Jahn-Teller effect which is static at low temperatures but becomes dynamic above 20 K with jumps of the Cu(2+) complex between two lowest potential wells. The jumps produce continuous g-factor and hyperfine splitting averaging when temperature increases. This process is discussed in terms of two motional averaging theories: classical theory based on generalized Bloch equations and Silver-Getz model. Their limitations are discussed. Importance of the difference in the g-factors of the averaged line is explained and a new expression for calculation of jump frequency from the line shift is proposed. The jumps are described as phonon induced tunneling via excited vibrational level of energy 76 (+/-6) cm(-1). This process is not effective enough at low temperatures and Boltzmann population of the two lowest energy potential wells is reached above 110 K. From electron spin-lattice relaxation measurements by electron spin echo methods the Debye temperature was determined as Theta(D) = 172 K. Fourier Transform of strongly modulated spin echo decay gives pseudo-ENDOR spectrum with peaks from (1)H and (35)Cl nuclei. From splitting of the peaks into doublets we determined the distance to the modulating nuclei and confirmed the position of the site where Cu(2+) ion is located.

Electron spin relaxation of exchange coupled pairs of transition metal ions in solids. Ti2+-Ti2+ pairs and single Ti2+ ions in SrF2 crystals.

EPR (X- and Q-band) and electron spin relaxation measured by electron spin echo method (X-band) were studied for Ti(2+)(S=1) and Ti(2+)-Ti(2+) pairs in SrF(2) crystal at room temperature and in the temperature range 4.2-115 K. EPR spectrum consists of a strong line from Ti(2+) and quartets 2:3:3:2 from titanium pairs (S=2). Spin-Hamiltonian parameters of the pairs are g( parallel)=1.883, g( perpendicular)=1.975 and D=0.036 cm(-1). Temperature behavior of the dimer spectrum indicates ferromagnetic coupling between Ti(2+). Spin-lattice relaxation of individuals Ti(2+) is dominated by the ordinary two-phonon Raman process involving the whole phonon spectrum up to the Debye temperature Theta(D)=380 K with spin-phonon coupling parameter equal to 215 cm(-1). Important contribution to the relaxation arises from local mode vibrations of energy 133 cm(-1). The pair relaxation is faster due to the exchange coupling modulation mechanism with the relaxation rate characteristic for ferromagnetic ground state of the pairs 1/T(1) is proportional to [exp(2J/kT)-1](-1) which allowed to estimate the exchange coupling J=36 cm(-1). The theories of electron-lattice relaxation governed by exchange interaction are outlined for extended spin systems, for clusters and for individual dimers. Electron spin echo decay is strongly modulated by coupling with surrounding (19)F nuclei. FT-spectrum of the modulations shows a dipolar splitting of the fluorine lines, which allows the evaluation of the off-center shift of Ti(2+) in pair as 0.132 nm. The electron spin echo dephasing is dominated by an instantaneous diffusion at low temperatures and by the spin-lattice relaxation processes above 18K.

Structure and dynamics of S3(-) radicals in ultramarine-type pigment based on zeolite A: electron spin resonance and electron spin echo studies.

X-band electron spin resonance (ESR) spectra of S(3)(-) radicals in ultramarine analog (pigment) prepared from zeolite A and maintaining the original structure of parent zeolite were recorded in the temperature range of 4.2-380 K. Electron spin echo experiments (echo detected ESR, electron spin-lattice relaxation, and spin echo dephasing) were performed in the temperature range of 4.2-50 K. The rigid lattice g factors are g(x) = 2.0016, g(y) = 2.0505, and g(z) = 2.0355, and they are gradually averaged with temperature to the final collapse into a single line with g = 2.028 above 300 K. This is due to reorientations of S(3)(-) molecule between 12 possible orientations in the sodalite cage through the energy barrier of 2.4 kJ/mol. The low-lying orbital states of the open form of S(3)(-) molecule having C(2v) symmetry are considered and molecular orbital (MO) theory of the g factors is presented. The orbital mixing coefficients were calculated from experimental g factors and available theoretical orbital splitting. They indicate that the unpaired electron spin density in the ground state is localized mainly (about 50%) on the central sulfur atom of S(3)(-) anion radical, whereas in the excited electronic state the density is localized mainly on the lateral sulfur atoms (90%). A strong broadening of the ESR lines in directions around the twofold symmetry axis of the radical S(3)(-) molecule (z-axis) is discovered below 10 K. It is due to a distribution of the S-S-S bond angle value influencing mainly the energy of the (2)B(2)-symmetry MO. This effect is smeared out by molecular dynamics at higher temperatures. A distribution of the g factors is confirmed by the recovery of the spin system magnetization during spin-lattice relaxation measurements, which is described by a stretched exponential function. Both the spin-lattice relaxation and electron spin echo dephasing are governed by localized phonon mode of energy of about 40 cm(-1). Thus, the anion-radical S(3)(-) molecules are weakly bonded to the zeolite framework, and they do not participate in the phonon motion of the host lattice because of their own local dynamics.