Frequency Domain Magnetic Resonance Spectroscopy Research Articles

ChemInform Abstract: New Directions in Electron Paramagnetic Resonance Spectroscopy on Molecular Nanomagnets

AbstractReview: 204 refs.

High-frequency and -field EPR and FDMRS study of the [Fe(H 2O) 6] 2+ ion in ferrous fluorosilicate

The complex [Fe(H 2O) 6]SiF 6 is one of the most stable and best characterized high-spin Fe(II) salts and as such, is a paradigm for the study of this important transition metal ion. We describe high-frequency and -field electron paramagnetic resonance studies of both pure [Fe(H 2O) 6]SiF 6 and [Zn(H 2O) 6]SiF 6 doped with 8% of Fe(II). In addition, frequency domain magnetic resonance spectroscopy was applied to these samples. High signal-to-noise, high resolution spectra were recorded which allowed an accurate determination of spin Hamiltonian parameters for Fe(II) in each of these two, related, environments. For pure [Fe(H 2O) 6]SiF 6, the following parameters were obtained: D = +11.95(1) cm −1, E = 0.658(4) cm −1, g = [2.099(4), 2.151(5), 1.997(3)], along with fourth-order zero-field splitting parameters: B 4 0 = 17 ( 1 ) × 10 - 4 cm - 1 and B 4 4 = 18 ( 4 ) × 10 - 4 cm - 1 , which are rarely obtainable by any technique. For the doped complex, D = +13.42(1) cm −1, E = 0.05(1) cm −1, g = [2.25(1), 2.22(1), 2.23(1)]. These parameters are in good agreement with those obtained using other techniques. Ligand-field theory was used to analyze the electronic absorption data for [Fe(H 2O) 6]SiF 6 and suggests that the ground state is 5A 1, which allows successful use of a spin Hamiltonian model. Density functional theory and unrestricted Hartree–Fock calculations were performed which, in the case of latter, reproduced the spin Hamiltonian parameters very well for the doped complex.

Inelastic neutron scattering and frequency-domain magnetic resonance studies ofS=4andS=12Mn6single-molecule magnets

We investigate the magnetic properties of three ${\text{Mn}}_{6}$ single-molecule magnets by means of inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The experimental data reveal that small structural distortions of the molecular geometry produce a significant effect on the energy-level diagram and therefore on the magnetic properties of the molecule. We show that the giant spin model completely fails to describe the spin-level structure of the ground spin multiplets. We analyze theoretically the spin Hamiltonian for the low-spin ${\text{Mn}}_{6}$ molecule $(S=4)$ and we show that the excited $S$ multiplets play a key role in determining the effective energy barrier for the magnetization reversal, in analogy to what was previously found for the two high spin ${\text{Mn}}_{6}$ $(S=12)$ molecules [S. Carretta et al., Phys. Rev. Lett. 100, 157203 (2008)].

Universal Theoretical Approach to Extract Anisotropic Spin Hamiltonians.

Monometallic Ni(II) and Co(II) complexes with large magnetic anisotropy are studied using correlated wave function based ab initio calculations. Based on the effective Hamiltonian theory, we propose a scheme to extract both the parameters of the zero-field splitting (ZFS) tensor and the magnetic anisotropy axes. Contrarily to the usual theoretical procedure of extraction, the method presented here determines the sign and the magnitude of the ZFS parameters in any circumstances. While the energy levels provide enough information to extract the ZFS parameters in Ni(II) complexes, additional information contained in the wave functions must be used to extract the ZFS parameters of Co(II) complexes. The effective Hamiltonian procedure also enables us to confirm the validity of the standard model Hamiltonian to produce the magnetic anisotropy of monometallic complexes. The calculated ZFS parameters are in good agreement with high-field, high-frequency electron paramagnetic resonance spectroscopy and frequency domain magnetic resonance spectroscopy data. A methodological analysis of the results shows that the ligand-to-metal charge transfer configurations must be introduced in the reference space to obtain quantitative agreement with the experimental estimates of the ZFS parameters.

Frequency-domain magnetic-resonance spectroscopic investigations of the magnetization dynamics inMn12Acsingle crystals

A comprehensive spectroscopic study of the magnetization relaxation of ${\text{Mn}}_{12}\text{Ac}$ is presented. The single crystalline samples are investigated as a function of magnetic field and temperature using frequency-domain magnetic-resonance spectroscopy. The magnetization relaxation is followed in real-time by recording spectra as a function of delay time. The experiments are performed both in the absence of a magnetic field and in external fields either parallel (Faraday geometry) or perpendicular (Voigt geometry) to the direction of radiation. The relaxation rates obtained from spectral fits correspond well to those obtained from magnetometric measurements. The results exhibit clear signatures of quantum tunneling of the magnetization. The influence of $D$ strain and internal dipolar fields is observed.

Magnetism and magnetic resonance studies of single-molecule magnets in polymer matrices

We have studied samples of the Mn 12piv (piv = O 2C t Bu) single-molecule magnet diluted in a polystyrene matrix using magnetometry and magnetic resonance spectroscopy. The ac susceptibility measurements showed that the energy barrier remains the same going from the solid to the polymer doped sample. Furthermore, there is no concentration dependence of the energy barrier (Δ E = 61 K) for the different polymer doped samples. The frequency domain magnetic resonance spectroscopy (FDMRS) and high-frequency electron paramagnetic resonance (HFEPR) measurements showed that also the spin Hamiltonian parameters are unchanged, D = −0.46 cm −1 and B 4 0 = - 2 × 10 - 5 cm - 1 .

Large Magnetic Anisotropy in Pentacoordinate NiII Complexes

Pentacoordinate complexes in which Ni(II) is chelated by the tridentate macrocyclic ligand 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPrtacn) of formula [Ni(iPrtacn)X(2)] (X=Cl, Br, NCS) have relatively large magnetic anisotropies, revealed by the large zero-field splitting (zfs) axial parameters |D| of around 15 cm(-1) measured by frequency-domain magnetic resonance spectroscopy (FDMRS) and high-field high-frequency electron paramagnetic resonance (HF-HFEPR). The spin Hamiltonian parameters for the three complexes were determined by analyzing the FDMRS spectra at different temperatures in zero applied magnetic field in an energy window between 0 and 40 cm(-1). The same parameters were determined from analysis of HF-HFEPR data measured at different frequencies (285, 380, and 475 GHz) and at 7 and 17 K. The spin Hamiltonian parameters D (axial) and E (rhombic) were calculated for the three complexes in the framework of the angular overlap model (AOM). The nature and magnitude of the magnetic anisotropy of the three complexes and the origin of the influence of the X atoms were analyzed by performing systematic calculations on model complexes.

Frequency domain magnetic resonance spectroscopy on [Mn12]− and [Mn9]: Zero-field splittings and lineshape analysis

The zero-field splitting parameters of two non-integer spin single molecule magnets, [Mn 12 ] − and [Mn 9 ] were determined using frequency domain magnetic resonance spectroscopy. Interestingly, the resonance line is homogeneously broadened in the case of [Mn 12 ] − . The zero-field splitting parameters of two non-integer spin state single molecule magnets were determined using frequency domain magnetic resonance spectroscopy: D = −0.454 ± 0.004 cm −1 , B 4 0 = ( + 1 . 01 ± 0 . 05 ) × 10 - 5 cm - 1 for [Mn 12 ] − , and D = −0.247 ± 0.005 cm −1 , B 4 0 = ( + 4 . 6 ± 0 . 1 ) × 10 - 6 cm - 1 for [Mn 9 ]. The linewidth of the magnetic resonance lines is determined by distributions in the zero-field splitting parameters in the case of [Mn 9 ], leading to a Gaussian lineshape. Interestingly, the resonance line is homogeneously broadened in the case of [Mn 12 ] − , which is one of the few examples in single molecule magnetism. This means that the linewidth is determined by the excited spin state lifetime, which can be calculated to be 50–58 ps depending on the temperature.

Synthesis and Spectroscopic Characterization of a New Family of Ni4 Spin Clusters

A new family of tetranuclear Ni complexes [Ni(4)(ROH)(4)L(4)] (H(2)L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni(4)O(4)] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni(II) centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximately -0.93 cm(-)(1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.

Studies of an Enneanuclear Manganese Single-Molecule Magnet

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)] with the tripodal ligand H(3)thme (1,1,1-tris(hydroxymethyl)ethane) affords the enneanuclear complex [Mn(9)O(7)(O(2)CCH(3))(11)(thme)(py)(3)(H(2)O)(2)] 1.1MeCN.1Et(2)O. The metallic skeleton of complex 1 comprises a series of 10 edge-sharing triangles that describes part of an idealized icosahedron. Variable temperature direct current (dc) magnetic susceptibility data collected in the 1.8-300 K temperature range and in fields up to 5.5 T were fitted to give a spin ground state of S = (17)/(2) with an axial zero-field splitting parameter D = -0.29 cm(-)(1). Ac susceptibility studies indicate frequency-dependent out-of-phase signals below 4 K and an effective barrier for the relaxation of the magnetization of U(eff) = 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. Inelastic neutron scattering (INS) studies on complex 1 confirm the S = (17)/(2) ground state and analysis of the INS transitions within the zero-field split ground state leads to determination of the axial anisotropy, D = -0.249 cm(-)(1), and the crystal field parameter, B(4)(0) = 7(4) x 10(-)(6) cm(-)(1). Frequency domain magnetic resonance spectroscopy (FDMRS) determined the same parameters as D = -0.247 cm(-)(1) and B(4)(0) = 4.6 x 10(-)(6) cm(-)(1). DFT calculations are fully consistent with the experimental findings of two Mn(II) and four Mn(III) ions "spin up" and three Mn(IV) ions "spin down" resulting in the S = (17)/(2) spin ground state of the molecule, with D = -0.23 cm(-)(1) and U = 26.2 K.

High-frequency/high-field EPR spectroscopy of the high-spin ferrous ion in hexaaqua complexes

Electron paramagnetic resonance (EPR) at conventional magnetic fields and microwave frequencies, respectively, B0 < or = 1.5 T, nu < or = 35 GHz, has been widely applied to odd electron-number (S = 1/2) transition metal complexes. This technique is less successfully applied to high-spin systems that have even electron configurations, e.g. Fe2+ (S = 2). The recently developed technique of high-frequency and high-field EPR (HFEPR), employing swept fields up to 25 T combined with multiple, sub-THz frequencies readily allows observation of EPR transitions in such high-spin systems. A parallel spectroscopic technique is frequency-domain magnetic resonance spectroscopy (FDMRS), in which the frequency is swept while at zero, or at discrete applied magnetic fields. We describe here the application of HFEPR and FDMRS to two simple high-spin (HS) ferrous (Fe2+) salts: ferrous perchlorate hydrate, [Fe(H2O)6](ClO4)2 and (NH4)2[Fe(H2O)6](SO4)2, historically known as ferrous ammonium sulfate. Both compounds contain hexaaquairon(II). The resulting spectra were analyzed using a spin Hamiltonian for S = 2 to yield highly accurate spin-Hamiltonian parameters. The complexes were also studied by powder DC magnetic susceptibility and zero-field Mössbauer effect spectroscopy for corroboration of magnetic resonance results. In the case of [Fe(H2O)6](ClO4)2, all the magnetic techniques were in excellent agreement and gave as consensus values: D = 11.2(2) cm(-1), E = 0.70(1) cm(-1). For (NH4)2[Fe(H2O)6](SO4)2, FDMRS and HFEPR gave D = 14.94(2) cm(-1), E = 3.778(2) cm(-1). We conclude that the spin-Hamiltonian parameters for the perchlorate best represent those for the isolated hexaaquairon(II) complex. To have established electronic parameters for the fundamentally important [Fe(H2O)6]2+ ion will be of use for future studies on biologically relevant systems containing high-spin Fe2+.