Experimental Zero-field Splitting Parameters Research Articles

Zero-field splitting in tetracoordinate Co(II) complexes containing heterocyclic aromatic ligands

A series of tetracoordinate mononuclear Co(II) complexes [Co(L1)2(X)2] (1-3) and [Co(L2)2(X)2] (4-6) {where L1 = 2-amino-1-methyl-benzimidazole; L2 = 4-methylquinoline; X = Cl, Br, I} have been synthesized and studied in detail. Compounds were characterized by spectroscopic methods along with their molecule structure determined by the single-crystal X-ray diffraction. All complexes have been investigated in the static magnetic field Bdc = 0.1 T using SQUID magnetometry. DC magnetic data were fitted by a spin Hamiltonian model appropriate to a zero-field splitting system. The axial zero-field splitting parameters D ranged as -14.44 (1), -13.05 (2), -7.50 (3), -6.76 (4), +7.19 (5), -5.58 (6) cm−1. The experimental zero-field splitting parameters are compared with those obtained by ab initio calculations of the CASSCF + NEVPT2 + SOC quality.

Magnetostructural relationships for Ni(II) ions at octahedral sites in [NixZn1−x(C2O4)(dmiz)2]: Computational study of zero-field splitting and using superposition model

Superposition model (SPM) analysis of the zero-field splitting parameters (ZFSPs), for short SPM/ZFSP, is utilized to investigate magnetostructural relationships in Ni(II)-based magnetic systems: the pure Ni(C2O4)(dmiz)2 and mixed NixZn1−x(C2O4)(dmiz)2 derivatives, where dmiz is 1,2-dimethylimidazole. The pure system is considered as an example of the S=1 Heisenberg antiferromagnetic one-dimensional chain. The Haldane gap exhibited by such systems is the major motivation for their studies. SPM/ZFSP is based on knowledge of the model parameters, i.e. the intrinsic parameters – b¯k(R0), power law exponents – tk, and the reference distance – R0. As a first approximation we use the model parameter sets determined earlier for Ni(II) ions in the sixfold coordinated NiO6 with axial symmetry. Matching of the SPM predicted ZFSPs with the experimental ones is carried out for several model parameter sets and various models of distortions around Ni(II) ions. The axial (D) and rhombic (E) ZFSPs obtained by high magnetic field/high-frequency EMR (HMF–EMR) for the derivatives NixZn1−x(C2O4)(dmiz)2 for x=0.09 and 0.07 are employed. Three approaches accounting for the local structural distortions with respect to the undistorted host are considered, namely, the angular distortions, the distortions of the ligand–metal bond lengths, and simultaneous distortions of the two types. This enables a thorough study of the magnetostructural relationships for the doped Ni(II) ions in NixZn1−x(C2O4)(dmiz)2 and related crystals. Major finding is that SPM/ZFSP method is capable of yielding satisfactory matching of both the size and the signs of the predicted and experimental ZFSPs. Controversies concerning the very large D-values obtained by ab initio methods as compared with those usually observed in Ni(II) systems are discussed. Several inconsistencies concerning interpretation of experimental and theoretical data for Haldane gap systems are also clarified.

W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting.

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has DT = 1.369 cm(-1), ET = 0.093 cm(-1), and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows DQ = - 0.306 cm(-1), EQ = 0.0137 cm(-1), and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has DS = - 0.203 cm(-1), ES = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the "heavy atom effect." This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

EMR studies of the internal motion of Mn4+ ions in the Sr overdoped (La1−xSrx)(Ga1−yMny)O3 (x/y up to 8) supplemented by magnetic and optical spectroscopy measurements

The effect of the Sr doping on electronic structure in single crystals of (La1−xSrx)(Ga1−yMny)O3 solid solutions (LSGM) is investigated by means of electron magnetic resonance (EMR). The EMR results are supplemented by magnetic susceptibility and optical spectroscopy measurements. The compositions with small concentration of Mn doping (y<1%) and overdoped content of Sr (the ratio x(Sr)/y(Mn) up to 8) are used to maximally enhance the role of divalent doping. The experimental results provide evidence of the holes delocalization in the overdoped compound (x(Sr)/y(Mn)>1). This delocalization is accompanied by appearance of the new charge transfer transitions in the optical spectrum and dynamical valence change of manganese atoms. Additionally we observe the thermally activated narrowing of resonance EMR lines due to the internal motion, which is characterized by the energy barrier depending strongly on the ratio x(Sr)/y(Mn). The energy barrier is found to be associated with the charge carrier (hole) self-trapped energy. Fitting the EMR spectra in three orthogonal planes to an orthorhombic spin Hamiltonian enables extracting the zero-field splitting (ZFS) parameters and the Zeeman g-factors for Mn4+ (S=3/2) ions in LSGM. The experimental ZFS parameters are modeled using superposition model analysis based on an orthorhombic symmetry approximation.

Pulsed electron spin nutation spectroscopy for weakly exchange-coupled multi-spin molecular systems with nuclear hyperfine couplings: a general approach to bi- and triradicals and determination of their spin dipolar and exchange interactions

Weakly exchange-coupled biradicals have attracted much attention in terms of their dynamic nuclear polarisation application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits in quantum information processing/quantum-computing technology. Analogues multi-partite molecular systems are important in entering a new phase of the relevant fields. Many stable organic biradicals known so far have nitrogen nuclei at their electron spin sites, where singly occupied molecular orbitals are dominating and large hyperfine couplings occur. A salient feature of such weakly exchange-coupled molecular systems in terms of electronic spin structures is underlain by small zero-field splitting (ZFS) parameters comparable with nuclear hyperfine and/or exchange interactions. Pulse-based electron spin nutation (ESN) spectroscopy of weakly exchange-coupled biradicals, applicable to oriented or non-oriented media, has proven to be a useful and facile approach to the determination of ZFS parameters, which reflect relatively short distances between unpaired electron spins. In the present study, we first treat two-dimensional single-crystal ESN spectroscopy (Q-band) of a 15N-labelled weakly exchange-coupled biradical, showing the nuclear hyperfine effects on the ESN phenomena from both the experimental and theoretical side. ESN spectroscopy is transition moment spectroscopy, in which the nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be treated. The results provide a testing ground for the simplified but general approach to the ESN analysis. In this study, we have invoked single-crystal electron-electron double resonance measurements on a typical biradical well incorporated in a diamagnetic host lattice and checked the accuracy of our ESN analysis for the spin dipolar tensor and exchange interaction. Next, we extend the general approach to analogues multi-partite molecular systems such as stable organic triradical, in which the exchange interaction can be governed by a significant amount of the delocalisation of three unpaired spins over the molecular frame of a triangular structure. The triangular structure maintains π-conjugation in which each spin-bearing nitroxide at the vertex participates and the exchange interaction is greatly controlled by the dihedral angle between the π-conjugation plane and nitroxide moiety at the vertex. In this context, the ZFS parameters do not correspond to spin distances (1.0 nm) in a straightforward manner, but reflect a salient electronic structure associated with both the π-electron network and the symmetry property of the triradical under study. Thus, both the D-value and exchange interaction J have been controlled in this study. In order to interpret the experimental ZFS parameters and exchange interaction, which is three-order of magnitude reduced in the present poly(methyl methacrylate) polymer matrix compared with that in the crystal, sophisticated quantum chemical calculations of the ZFS tensor and exchange interaction were carried out and reproduced the experimental values, concluding that the present triradical of the triangular structure undergoes significant twisting at the nitroxide sites in the polymer matrix. In this study, we observed electron spin resonance forbidden transitions between the Ms-manifolds belonging to the spin-doublet ground state and spin-quartet excited state. The observation enables us to derive the magnitude of the exchange coupling.

Modeling Spectroscopic Properties of Ni2+ Ions in the Haldane Gap System Y2BaNiO5

Modeling of spin Hamiltonian parameters enables correlation of crystallographic, spectroscopic, and magnetic data for transition ions in crystals. In this paper, based on the crystallographic data and utilizing the point-charge model and superposition model, the crystal field parameters (CFPs) are estimated for Ni2+(3d 8) ions in the Haldane gap system Y2BaNiO5. The CFPs serve as input for the perturbation theory expressions and the crystal field analysis package for microscopic spin Hamiltonian modeling of the zero-field splitting parameters (ZFSPs) D and E. Results of an extensive literature search of the pertinent crystallographic data, experimental ZFSPs, and model parameters are briefly outlined. The modeling aims at verification of the experimental ‘single ion anisotropy’ parameters and explanation of the controversy concerning the maximal rhombic distortion |E/D| ≈1/3 reported for Ni2+ ions in Y2BaNiO5. The preliminary results call for reanalysis of some magnetic studies of the Haldane gap systems.

High-spin organic molecules with dominant spin-orbit contribution and unprecedentedly large magnetic anisotropy

High-spin organic molecules with dominant spin-orbit contribution to magnetic anisotropy are reported. Quintet 4-azido-3,5-dibromopyridyl-2,6-dinitrene (Q-1), quintet 2-azido-3,5-dibromopyridyl-4,6-dinitrene (Q-2), and septet 3,5-dibromopyridyl-2,4,6-trinitrene (S-1) were generated in solid argon matrices by ultraviolet irradiation of 2,4,6-triazido-3,5-dibromopyridine. The zero-field splitting (ZFS) parameters, derived from electron spin resonance spectra, show unprecedentedly large magnitudes of the parameters D: ∣D(Q1)∣ = 0.289, ∣D(Q2)∣ = 0.373, and ∣D(S1)∣ = 0.297 cm(-1). The experimental ZFS parameters were successfully reproduced by density functional theory calculations, confirming that magnetic anisotropy of high-spin organic molecules can considerably be enhanced by the "heavy atom effect." In bromine-containing high-spin nitrenes, the spin-orbit term is dominant and governs both the magnitude and the sign of magnetic anisotropy. The largest negative value of D among septet trinitrenes is predicted for 1,3,5-trinitrenobenzene bearing three heavy atoms (Br) in positions 2, 4, and 6 of the benzene ring.

The calculation of zero-field splitting parameters for Fe3+ ions doped in rutile TiO2 crystal by superposition model analysis

Study of Mn2+/Fe3+:TiO2 (anatase) is extended to Fe3+:TiO2 (rutile). Fe3+ environment is investigated using superposition model (SPM). The zero-field splitting (ZFS) parameters are calculated for Fe3+ ions at the substitutional sites and interstitial positions in rutile TiO2. Orthorhombic (D2h) symmetry aspects bearing on EMR data interpretation are elucidated. Inconsistencies concerning site symmetry are clarified. Three approaches are considered with adjustable model parameters yielding best agreement between the SPM-calculated and EMR-measured ZFS parameters. The predicted lattice distortions involve simultaneous variation of the ligand distances and bond angles and compare favorably with those for Rh4+/Ir4+:TiO2 (rutile). The SPM analysis indicates that satisfactory agreement between the theoretical and experimental ZFS parameters is achievable.

Superposition model analysis of the zero-field splitting parameters of Fe 3+ doped in TlInS 2 crystal – Low symmetry aspects

In this study, using the superposition model (SPM) and crystallographic data, the second- and fourth-rank zero-field splitting (ZFS) parameters (ZFSPs) have been calculated for Fe 3+ ions at two In sites in TlInS 2 single crystal. Since crystallographic data indicate C 1 site symmetry, a full triclinic ZFS Hamiltonian has been employed. Model parameters were adjusted so to obtain best agreement between the SPM calculated ZFSPs and the second-rank ZFSPs measured by electron magnetic resonance (EMR). The effects of site symmetry on the theoretically predicted ZFSPs and interpretation of the available experimental ZFSPs have been considered. The experimental second-rank ZFSPs were obtained using approximated orthorhombic spin Hamiltonian. Hence, to facilitate comparison with the SPM calculated ZFSPs, the ascent in symmetry method has been applied to the crystallographic data to quantify the structural approximation from triclinic to orthorhombic and to tetragonal site symmetry. Our considerations provide additional structural information, especially concerning the low site symmetry aspects, pertinent for Fe 3+ impurity centers in TlInS 2 and related crystals. Our SPM analysis of ZFSPs indicates that satisfactory so tentative agreement can be achieved between the theoretical predictions and the experimentally measured values of the second-rank ZFSPs. The procedure proposed here may serve as a general framework for modelling of ZFSPs for other ion-host systems. More accurate modelling for Fe 3+ ions in TlInS 2 may be carried out when better quality EMR results taking into account the low symmetry effects in EMR spectra and the values of the fourth-rank ZFSPs become available. The results of SPM calculations support earlier suggestions that Fe 3+ ions substitute for In 3+ ions in TlInS 2.

Multifrequency EPR Study of Metallofullerenes: Eu@C82 and Eu@C74

In this work, the cage structure of a family of europium metallofullerenes, Eu@C74 and three isomers of Eu@C82, were investigated. The analogy of the electronic states of three isomers of Eu@C82 with those of three isomers of Ca@C82 was confirmed by UV−vis−near-IR absorption spectra. The symmetries Cs, C2, and C2v were assigned for three isomers of Eu@C82 by comparing each corresponding isomer of Ca@C82. The D3h cage structure of Eu@C74, which is expected only for the C74 from the isolated pentagon rule, was also confirmed by comparing its photoabsorption spectrum with that of Ca@C74. Multifrequency electron paramagnetic resonance (EPR) spectroscopy was employed to determine the zero-field splitting (ZFS) parameters of the metallofullerenes, which are closely related to the surrounding cage structures. The experimental X- and W-band EPR spectra were completely reproduced by computer simulations based on a spin Hamiltonian considering the ZFS terms up to fourth order. The introduction of the fourth-order ZFS terms into the Hamiltonian resulted in a precise determination of the second-order ZFS terms. A nonvanishing rhombicity parameter E of Eu@C74 demonstrated the reduction of the symmetry from D3h to C2v due to the positioning of Eu2+ ion at the off-center of C74 cage. Moreover, in this work, the semiphenomenological superposition-exchange model (SPEM) was invoked to examine a relationship between the molecular structures and ZFS parameters. All of the experimental ZFS parameters were satisfactory reproduced in terms of the SPEM, leading to confirmation of the molecular symmetries. The semiphenomenological analysis also enabled us to select the most appropriate cage for Eu@C82 with C2 symmetry from among three possible isomers.