The g-value of the 2E state of K3CR(CN)6 and the lower symmetry field

The ⁴A₂ → ²E transition of potassium chromicyanide has been studied under magnetic field. It has been found that the g-value is nearly isotropic, quite unlike the case of ruby. An attempt has been made to explain the zero-field splittings of ^2E, ²T₁ and ²T₂ states and the g-value of the ²E state on the assumption that the distant ions are mainly responsible for lowering of symmetry of the crystal field from O_h. A reasonable choice of parameters can make the calculated values agree with the observed ones.     

EPR Parameters of quantum mixed-spin ferric complexes with rhombic and tetragonal symmetry

Calculations are presented for the EPR g values of ferric iron in a crystal field with axial and rhombic symmetry components for a spin state which is a quantum mechanical mixture of intermediate-spin (S = 3/2) and high-spin (S = 5/2) components. The crystal field parameter μ/δ₁ is discussed as a more fundamental measure of rhombic distortion than the spin Hamiltonian ratio E/D.     

Electron paramagnetic resonance parameters of V2+ ions in both Cd2+ sites of CsCdCl3 crystal

From the perturbation formulas based on a two-spin-orbit-parameter model, the electron paramagnetic resonance (EPR) zero-field splitting (D), g-factors (g_//, g_⊥) and hyperfine structure constants (A_//, A_⊥) for V²⁺ in Cd²⁺(I) and Cd²⁺(II) sites of CsCdCl₃ crystal at room and liquid nitrogen temperatures are calculated. From the calculations, the signs of zero-field splittings and hyperfine structure constants are determined and so all of the EPR parameters are explained reasonably on the basis of the structure data of lattice.     

Electron paramagnetic resonance study on Cu[(C6H11O)2PS2]2 complex single crystal

X-band EPR studies on Cu[(C₆H₁₁O)₂PS₂]₂ complex single crystal at room temperature were reported. The EPR spectrum shows that the hyperfine structure is produced by ⁶³Cu nuclei, and the ligand hyperfine structure by ³¹P nuclei. The spin-Hamiltonian parameters were rigorously calculated using a least-squares-fitting technique, specially adapted to noncoincident g and A tensors systems. The principal values of g tensor imply that Cu²⁺ occupies the site consisting of four ligands S, and is tetragonal symmetric, but the principal values of A tensor in Cu—S₄ plane reveal a large anisotropy caused by the presence of two ligands P. One of principal axes for g and A tensors is coincident, and they are perpendicular to the Cu—S₄ plane. It was found that the interaction between electron spin and ligands ³¹P nuclear spin is isotropic and the hyperfine-coupling constant A^P related ligands was obtained.     

EPR and IR spectra of Ni(ClO4)2)·6H2O between 98 and 298 K

EPR measurements between 98 and 298 K on single crystal of Ni(ClO₄)₂·6H₂O have indicated the appearance of a rhombic component in the axial crystal field at Ni²⁺ sites at T_c = 224 K, confirming a phase transition first reported by Chaudhuri from magnetic susceptibility measurements. Temperature variations of g, D and E parameters were determined. IR spectra at room and liquid nitrogen temperatures are consistent with our EPR results.     

Local structure distortion and spin Hamiltonian parameters for Cr4+ ions in Cr4+:α-Al2O3 crystals

The relations between the spin Hamiltonian (SH) parameters and crystal structure of Cr⁴⁺:α-Al₂O₃ crystals have been established. On the basis of this, the SH parameters including zero-field splitting parameter D and Zeeman g-factors (g_|| and g_⊥) for Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals, taking into account the spin–spin (SS), spin-other-orbit (SOO) and orbit–orbit (OO) magnetic interactions in addition to the spin–orbit (SO) magnetic interaction, are theoretically investigated using complete diagonalization method (CDM). The theoretical results are in excellent agreement with the experimental ones when the upper three O^2? ions rotate 0.94° toward [1 1 1] axis and the lower three O^2? ions rotate 0.92° toward it. Hence, the local structure distortion effect plays an important role in explaining the spectroscopic properties of Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals. This study shows that for Cr⁴⁺:α-Al₂O₃ the contributions arising from SS, SOO, and OO interactions to the zero-field splitting (ZFS) parameter D are appreciable, whereas those to g_|| and g_⊥ are quite small.     

Magnetic and EPR Studies of α-, β-, and γ-Fe2WO6 Phases at Low Temperatures

The polymorphic modifications α-, β-, and γ-Fe₂WO₆ of the iron tungstate system were studied by means of magnetic susceptibility and EPR measurements at low temperatures. Both methods revealed a significant paramagnetic contribution, probably resulting from local distortions of the antiferromagnetic bulk structure induced by a disturbed cation ordering or the presence of Fe²⁺ ions. The magnetic susceptibility revealed a peak at 260 K for all samples which can be related with an AF phase transition. The EPR spectra comprised the contribution of various isolated paramagnetic iron centers, one arising from high-spin Fe³⁺ ions in rhombic crystal field symmetry with E/D ≈ 1/3 and D ≈ 0.22 cm^-1, an anisotropic EPR signal consistent with an S= 3/2 ground state with large zero-field splitting, and a dominant component in the g ≈ 2 region presumably arising from an S = 1/2; spin state. The latter spectra were tentatively attributed to the formation of multi-iron clusters, one of them invoking the presence of Fe²⁺ ions as well. For the βFe₂WO₆ phase an additional EPR spectrum was observed, which probably results from high-spin Fe³⁺ ions in a weak crystal field.     

Optical and ODMR investigation of the low lying excited states of neat α-oxalic acid dihydrate

The lowest excited singlet and triplet states of neat α-oxalic acid dihydrate have been investigated by optical, optical Zeeman, and zero-field optically detected magnetic resonance (ODMR) spectroscopy at T ? 4 K. The observed electronic transitions in absorption are assigned as ¹A_u ← ¹A_g (ν₀ = 34131 cm^?1) from its normal polarizatio These correspond to the expected lowest lying ^1,3nπ* excitations in trans-α-dicarbonyls. The ³A_u → ¹A_g phosphorescence is also observed. Monitoring the phosphorescence intensity, the fine structure splittings and principal axes' orientation and the kinetic parameters of the ³A_u s The fine structure constants are X = 2510.0, Y = ?1800.3, and Z = ?709.7 MHZ where the x axis is in-plane and parallel to the carbo The absolute signs of the constants have been established by optical Zeeman measurements. The τ_x zero-field spin state has the largest total phosphorescence rate, radiative rate, and populating rate. The τ_x activity in the 0 - 0 band is polarized mainly along the x axis. However, considerable normal polarization associated with an in-p     

Low spin ferric hemoglobin complexes

A caloulation has been made of the energy eigenfunotions and eigenvalues of low spin ferric ion in complexes with a strong cubic crystal field including the effects of tetragonal and rhombic distortions and of spin-orbit coupling among the ground state components and with excited states. Using the resultant, spin-orbit coupled eigenfunotions as a basis set, the magnetic susceptibility, the components of magnetic field energy, and the lattice and valence contributions to an electric field gradient at the iron nucleus were all calculated as a function of rhombic, tetragonal, and spin-orbit coupling strength used as parameters: R, u and . All of the calculated results agree reasonable well with experiment for the values of parameters R=1000 cm^–1, u=2000 cm^–1 and the free ion value (=420 cm^–1. These values of parameters were selected for the excellent fit they gave of the calculated values of g _x, g_y and g _z compared with the experimental ones obtained from single crystal electron spin resonance of ferrihemoglobin azide. With them, a value of 2.29 Bohr magnetons was calculated for the effective magnetic moment compared to the experimental value of 2.35. The total field gradient calculated under the same conditions, predicts a nuclear quadrupole moment Q in the range of. 107 –127. Barns, which is smaller than the range predicted from the high spin ferric ion results. Reasons for this discrepancy are discussed.

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Zusammenfassung Ausgehend von einem starken kubischen Ligandenfeld und unter Berücksichtigung tetragonaler (R) und rhombischer (u) Verzerrung sowie der Spin-Bahn-Kopplung () werden Eigenfunktionen und Energien fur Low-Spin-Ferrihämoglobinkomplexe berechnet. Mit den Parametern R=1000 cm^–1, u=2000 cm^–1, =420 cm^–1 erhält man für Suszeptibilität, elektrischen Feldgradienten am Fe und g-Werte gute Übereinstimmung mit experimentellen Daten. Aus dem berechneten Feldgradienten folgt ein Quadrupolmoment des Fe⁵⁷ von 0.107–0.127 Barn, im Gegensatz zu den viel höheren Resultaten bei High-Spin-Fe(III)-Verbindungen; diese Diskrepanz wird diskutiert.

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Résumé Les fonctions propres et les énergies du complexe Ferrihémoglobine «low spin» sont calculées pour un fort champ de ligandes à symétrie cubique, en tenant compte des distortions tétragonale (R) et rhomboédrique (u), ainsi que du couplage spin-orbite (). Avec les parametres R=1000 cm^–1, u=2000 cm^–1, =420 cm^–1, on trouve pour la susceptibilité, le gradient du champ électrique à l'emplacement de Fe et le facteur g des valeurs en bon accord avec les données expérimentales. On déduit du gradient de champ calculé un moment quadrupolaire de Fe⁵⁷ de 0,107 à 0,127 Barn, en désaccord avec les résultats beaucoup plus élevés obtenus à partir des associations Fe (III) «high spin». Ce désaccord fait l'objet d'une discussion.

     

trans‐Bis(cyano‐κC)(1,4,8,11‐tetraazacyclotetradecane‐κ4N)manganese(III) perchlorate,a low‐spin manganese(III) complex

The crystal structure of the low‐spin (S = 1) Mn^III complex [Mn(CN)₂(C₁₀H₂₄N₄)]ClO₄, or trans‐[Mn(CN)₂(cyclam)](ClO₄) (cyclam is the tetradentate amine ligand 1,4,8,11‐tetra­aza­cyclo­tetra­decane), is reported. The structural parameters in the Mn(cyclam) moiety are found to be insensitive to both the spin and the oxidation state of the Mn ion. The difference between high‐ and low‐spin Mn^III complexes is that a pronounced tetragonal elongation of the coordination octahedron occurs in high‐spin complexes and a slight tetragonal compression is seen in low‐spin complexes, as in the title complex.     

A Heteroleptic Push–Pull Substituted Iron(II) Bis(tridentate) Complex with Low‐Energy Charge‐Transfer States

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)]²⁺ with a strongly electron‐withdrawing ligand (dcpp, 2,6‐bis(2‐carboxypyridyl)pyridine) and a strongly electron‐donating tridentate tripyridine ligand (ddpd, N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reported. Both ligands form six‐membered chelate rings with the iron center, inducing a strong ligand field. This results in a high‐energy, high‐spin state (⁵T₂, (t_2g)⁴(e_g*)²) and a low‐spin ground state (¹A₁, (t_2g)⁶(e_g*)⁰). The intermediate triplet spin state (³T₁, (t_2g)⁵(e_g*)¹) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low‐energy π^* orbitals of dcpp allow low‐energy MLCT absorption plus additional low‐energy LL′CT absorptions from ddpd to dcpp. The directional charge‐transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)]³⁺ and [Fe(dcpp)(ddpd)]⁺, augmented by density functional calculations. The combined effect of push–pull substitution and the strong ligand field paves the way for long‐lived charge‐transfer states in iron(II) complexes.     

Simplified formula for the1 A 1⇄5 T 2 spin transition temperature in Fe(II) complexes

A formula relating the¹A₁?⁵T₂ spin transition temperature (T_c) in Fe(II) complexes to characteristics of the compounds is derived. With certain assumptions, T_c is determined by the splitting parameter Δ_LS of e_g- and t_2g-orbitals for the low-spin complexes and by the frequency ratio of normal vibrations of the low- and high-spin phases. For the group of compounds possessing spin transitions, the values of Δ_LS are found and analyzed. Correlations between T_c and Δ_LS are established; the values of the change in the probability of the Mössbauer effect are correlated with those of entropy of spin transition. The correlations are substantiated. It is concluded that for mononuclear Fe(II) complexes possessing sharp spin transitions, T_c may not be significantly higher than for Fe(Phy)₂(BF₄)₂ (T_c=282 K).     

Preparation and mössbauer studies of tetraaquabis[maleato(1-)]-iron(II)

Summary Tetraaquabis[maleato(1-)]iron(II), Fe(C₄H₃O₄)₂ · 4H₂O has been synthesised. The compound is monomeric and is isomorphous with the manganese(II) analogue. The temperature dependence of quadrupole splitting and optical spectral data giveca. 10000 cm^–1,ca. 650 cm^–1,ca. 190 cm^–1 andca. 1400 cm^–1 for the 10 Dq, tetragonal field, rhombic field and the splitting of the E_g level, respectively. The quadrupole splitting data indicate that distortion from cubic symmetry increases in the order oxalate < malonate < maleate. DTA, TGA and Mössbauer studies indicate elimination of one molecule of maleic acid at 235 °C. The resulting product shows a less distorted structure and this is attributed to the formation of maleato(2-)iron(II).     

Electron Paramagnetic Resonance Spectra of PrIons Doped in Sr2SnO4and Ba2SnO4

The isotropic electron paramagnetic resonance (EPR) spectra of powders with Pr⁴⁺doped inM₂SnO₄(M=Sr, Ba) with K₂NiF₄-type structure were measured at 4.2 K. A very large hyperfine interaction with the¹⁴¹Pr nucleus was observed. The results were analyzed based on the weak field approximation, andgvalues and hyperfine coupling constantsAwere obtained. The measuredgvalues are much smaller than ∣−10/7∣, showing that the crystal field effect on the behavior of a 4felectron is large. The value of ∣g∣ decreases from 0.646 (Pr⁴⁺/Ba₂SnO₄) to 0.615 (Pr⁴⁺/Sr₂SnO₄), which is caused by the increase of the crystal field due to the shrinking of the lattice. In contrast, the hyperfine coupling constants are almost constant:A=0.0605 cm⁻¹.     

Interpreting the optical spectra of trace Fe2+ in layer silicates

Estimates have been made of Fe²⁺ term splittings in axial crystalline fields. It is found that all three long-wave bands in the optical spectrum are due to Fe²⁺ Fe³⁺ charge transfer, while the splitting of ⁵T_2g and ⁵E_g occurs in low-symmetry fields. Experimental evidence is presented for these calculations.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 464–468, July–August 1987.     

Dithiophosphinates of trivalent iron,ruthenium and osmium

Dithiophosphinates of Fe(III), Ru(III) and Os(III) have been prepared and their ESR studies indicate distortion from octahedral geometry. The tetragonal and rhombic splittings and the orbital reduction factors were calculated from the experimental g values for low-spin Ru(III) and Os(III) complexes. The metal-ligand covalency increases in the order Ru < Os.     

EPR studies on the high spin FeIII tetraphenylporphine with rhombic character,determination of zero-field splitting parameters from the middle kramers transition

Electron paramagnetic resonance transitions (EPR) in the middle Kramers doublet is observed for the first time in the ⁶A₁ ground state of Fe^(III) complex of tetraphenylporphine (TPP) diluted in the polycrystalline free base tetraphenylporphine (THHP₂). The system shows the EPR spectra and resulting from several distinct species of high spin (Fe^(III) TPP with various degress of rhombicity. The middle Kramers as absorptions are in general of much weaker intensity than the corresponding lowest Kramers absorptions, and their g values show marked systematic deviations from the calculated first order prediction. The deviation can be explained by carrying the Zeeman perturbation to the third order. The analysis also rendered it possible to determine the zero-field parameters D and E separately. The results indicate the similarity in the crystal field of the present system with that of cyctochrome P450.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.     

Crystal Chemistry and Physical Properties of the Nonmagnetic Kondo Compound HfAs1.7Se0.2

Single crystals of HfAs_1.7Se_0.2 are grown by chemical transport reaction and their chemical composition characterized in detail by various analytical methods. Chemical analyses and crystal structure investigations by single‐crystal X‐ray diffraction as well as powder diffraction with synchrotron radiation reveal a tetragonal PbFCl structure type with strong disorder caused by a significant arsenic deficiency (As_0.9) on the 2a site and mixed occupancy of the 2c site (As_0.8Se_0.2). HfAs_1.7Se_0.2 is a diamagnetic metal which transforms into a superconducting state at T_c=0.52 K. Similar to other PbFCl‐type arsenide selenides, the title compound displays a magnetic‐field‐independent ?AT^1/2 term in the low‐temperature electrical resistivity. This unusual term presumably originates from the electron scattering of structural two‐level systems. According to the experimental results, HfAs1.7Se0.2 appears to be a rare example of a nonmagnetic Kondo material.     

Radical-Enhanced Intersystem Crossing in a Bay-Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

A 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was attached to the bay position of perylene-3,4 : 9,10-bis(dicarboximide) (perylenebisimide, PBI) to study the radical-enhanced intersystem crossing (REISC) and electron spin dynamics of the photo-induced high-spin states. The dyads give strong visible light absorption (ϵ=27000 M⁻¹ cm⁻¹at 607 nm). Attaching a TEMPO radical to the PBI unit transforms the otherwise non-radiative decay of S₁ state (fluorescence quantum yield: Φ_F=2.9 %) of PBI unit to ISC (singlet oxygen quantum yield: Φ_Δ=31.8 %, Φ_F=1.6 %). Moreover, the REISC is more efficient as compared to the heavy atom effect-induced ISC (Φ_Δ=17.8 % for 1,8-dibromoPBI). For the dyad, ISC takes 245 ps and triplet state lifetime is 1.5 μs, much shorter than the native PBI (τ_T=126.6 μs). X- and Q-band time-resolved electron paramagnetic resonance spectroscopy shows that the exchange interaction in the photoexcited radical-chromophore dyad is larger than the triplet zero-field splitting (ZFS) and the difference of Zeeman energies of the radical and chromophore. The inversion of electron spin polarization from emissive to absorptive was observed and attributed to the initial completion of the quartet state population and the subsequent depopulation processes induced by the zero-field splitting.