Høgni Weihe | University of Copenhagen (original) (raw)
Papers by Høgni Weihe
Inorganic Chemistry, 2000
The superexchange interactions in four three-center model systems A-L-B, for A and B being parama... more The superexchange interactions in four three-center model systems A-L-B, for A and B being paramagnetic centers and L a diamagnetic bridging ligand, are analyzed by valence bond configuration interaction models in combination with fourth-order perturbation theory. We analyze the four distinct cases where a bridging ligand orbital simultaneously interacts with half-filled orbitals localized on A and B (case i), a half-filled orbital localized on A and an empty orbital localized on B (case ii), a full orbital localized on A and a half-filled orbital localized on B (case iii), and finally a full orbital localized on A and an empty orbital localized on B (case iv). In all four cases we compare our new results using localized orbitals with the equivalent results obtained using the Anderson ansatz of delocalized (magnetic) orbitals. The effective metal-to-metal electron transfer energy U eff in the old formalism with delocalized orbitals is expressed in terms of the metal-to-metal electron transfer energy U and the ligand-to-metal electron transfer energy ∆ using localized orbitals. We find that the old formalism containing only U eff is in general not sufficient. For cases i and ii we show that U eff can be regarded as an effective U strongly reduced with respect to the free ion as a result of hybridization effects, whereas the same reduction of U for the cases iii and iv is not possible. The relevance and applicability of our theoretical results is demonstrated on magnetochemical data from the literature.
Dalton Transactions, 2010
We show that high-temperature expansion of the partition function is a computationally convenient... more We show that high-temperature expansion of the partition function is a computationally convenient tool to interpretation of magnetic properties of spin clusters wherein the spin centers are interacting via an isotropic Heisenberg exchange operator. High-temperature expansions up to order 12 are used to reproduce temperature dependent magnetic susceptibilities of a Cr(III) tetramer, a decanuclear Fe(III) wheel, a 3x3 grid arrangement of nine Mn(II) ions, and a dodecanuclear Ni(II) wheel. Excellent agreement with experiments as well as with results from computationally more elaborate methods is achieved.
Journal of the American Chemical Society, 1998
ABSTRACT The angular dependence of the magnetic properties of all known octahedrally coordinated ... more ABSTRACT The angular dependence of the magnetic properties of all known octahedrally coordinated oxo-bridged dimers with trivalent 3d transition metals are interpreted in terms of the most simple concepts. These concepts are based on a combination of kinetic exchange and the chemically intuitive angular overlap model. The use of an angular overlap model allows the separation of the σ donor properties of the bridging oxide ligand into sσ and pσ contributions. An estimate of the sσ and the pσ angular overlap model parameters is given. An analysis of the magnetic properties of the known oxo-bridged dimers allows us to predict the magnetic properties of unknown oxo-bridged dimers with trivalent 3d transition metal ions.
Comments on Inorganic Chemistry, 2000
The superexchange interaction in cyano-bridged tmnsition metal dimers is analysed by a valence bo... more The superexchange interaction in cyano-bridged tmnsition metal dimers is analysed by a valence bond confgumtion interaction model in combination w i t h fourtb order perturbation theory. Ferroand antiferromagnetic contributions to the exchange splitting arc cxprrsscd in t u m s of metal-ligand hybridization matrix elements, metal-to-ligand, ligand-to-metaI, metal-to-mtal charge transfer energies, and intra-atomic exbange integrals.
Journal of the American Chemical Society, 1997
ABSTRACT The magnetic properties of oxo-bridged oxo-(carboxylato)-bridged and oxo-bis(carboxylato... more ABSTRACT The magnetic properties of oxo-bridged oxo-(carboxylato)-bridged and oxo-bis(carboxylato)-bridged iron(III) dimers are found to depend on the iron−(μ-O) distance as well as on the iron−(μ-O)−iron angle. With an angular and radial overlap model we account for both these dependencies. The use of an angular overlap model allows us to separate the σ donor properties of oxide as a ligand into the sσ and pσ contributions.
Inorganic Chemistry, 1997
Expressions in the early literature for kinetic exchange in magnetically coupled systems are crit... more Expressions in the early literature for kinetic exchange in magnetically coupled systems are critically analyzed by second-order perturbation theory and substituted by corrected versions. The widely held belief that kinetic exchange is always antiferromagnetic is found to be incorrect. Ferromagnetic kinetic exchange terms are found to be more important than hitherto assumed. The quantity I /U, where I stands for intraatomic exchange interactions and U is the energy difference between the ground electron configuration and a charge-transfer configuration, plays a crucial role in the competition between ferromagnetic and antiferromagnetic contributions. I /U readily exceeds the value (1)/(5) considered as an upper limit by Anderson. I is found to be proportional to the number of unpaired electrons on a given magnetic center.
Inorganic Chemistry, 2006
The electronic structure of a trigonally distorted vanadium(III) complex, [V(urea)6](ClO4)3, and ... more The electronic structure of a trigonally distorted vanadium(III) complex, [V(urea)6](ClO4)3, and its deuterated analogue, [V(urea-d4)6](ClO4)3 has been investigated with Raman, luminescence, and high-frequency high-field electron paramagnetic resonance spectroscopies and with magnetic measurements. A broad electronic Raman transition is observed at around 1400 cm(-1) and attributed to a transition to the (3)E (D3) component of the (3)T1g (O(h)) ground state. The same splitting is seen in the near-infrared luminescence spectrum in the form of a similarly broad peak at 8450 cm(-1), 1400 cm(-1) lower in energy than the (1)E --> (3)A2 spin-flip transition. Powder high-frequency and high-field electron paramagnetic resonance spectra, magnetic susceptibilities, and magnetization studies give a precise measurement of the zero-field splitting and of the g values in this complex (D = 6.00(2) cm(-1), E = 0.573(6) cm(-1), g(x) = 1.848(2), g(y) = 1.832(4), and g(z) = 1.946(7)). A set of angular overlap model parameters is proposed that reproduces all spectroscopic observations, and an analysis of the influence of the bonding of urea on the trigonal distortion of the complex is given.
Inorganic Chemistry, 2001
The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-U... more The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-UV of (Et4N)3Cr2F9 are reported. In the energy range 25800-27700 cm(-1) the 4A2 --> 2A1 (O notation) ligand field transition can be resolved in detail for the first time in any Cr3+ compound. This allows the determination of the antiferromagnetic ground-state exchange splitting with great accuracy: J = 25.9 cm(-1) and j = 0.27 cm(-1) using the Hamiltonian H = J(S(A).S(B)) - j(S(A).S(B))2, where j leads to deviations from the regular Landé pattern. The temperature dependence of the magnetic susceptibility is nicely reproduced by these parameters. A comparison with Cs3Cr2Cl9 and Cs3Cr2Br9 reveals an exponential dependence of the ground-state splitting upon the Cr-Cr distance in the [Cr2X9]3- dimers. This is the result of a dominant sigma-type orbital exchange pathway along the Cr-Cr axis.
Chemical Physics, 1998
Low temperature magnetization data obtained for titanium (III) doped caesium gallium alum show a ... more Low temperature magnetization data obtained for titanium (III) doped caesium gallium alum show a spectacular deviation from Brillouin behavior with the magnetization highly dependent on the strength of the applied field at a given value of B/T. This arises from unprecedented higher-order contributions to the magnetization and these measurements afford the determination of the Zeeman coefficients to third order. This anomalous magnetic behavior is a manifestation of Jahn-Teller coupling giving rise to low-lying vibronic states which mix into the ground state through the magnetic field.
Acta Crystallographica Section C-crystal Structure Communications, 2002
ABSTRACT The crystal structure of the low-spin (S = 1) MnIII complex [Mn(CN)2(C10H24N4)]ClO4, or ... more ABSTRACT The crystal structure of the low-spin (S = 1) MnIII 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 MnIII 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.
![Research paper thumbnail of Magnetism, Spectroscopy and Structure of Tri-mu-hydroxobis{[N,N',N''-trimethyl-1,1,1-tris(aminomethyl)ethane]chromium(III)} Chloride Tetrahydrate](https://attachments.academia-assets.com/50227918/thumbnails/1.jpg)
](Acta Chemica Scandinavica, 1993
Journal of Magnetic Resonance, 2010
Utilizing a standard spin Hamiltonian for an S=32 spin system, we fit complete X-band powder EPR ... more Utilizing a standard spin Hamiltonian for an S=32 spin system, we fit complete X-band powder EPR spectra of the Cr(oxalate)33- anion diluted into K3[Co(oxalate)3]·3H2O. By using models for the bandshape and bandwidths of varying degree of flexibility, we show that the successful outcome of such a fitting endeavour very much depends on the used bandshape–bandwidth model. The best results are obtained when the bandwidth model incorporates anisotropic intrinsic bandwidths in addition to being able to account for inhomogeneous line broadening caused by distributions of the spin Hamiltonian parameters.
![Research paper thumbnail of Magnetic susceptibility and EPR spectra of (μ-hydroxo) bis [pentaamminechromium (III)] chloride monohydrate](https://attachments.academia-assets.com/50227917/thumbnails/1.jpg)
](Inorganic Chemistry, 1997
From magnetic susceptibility measurements the J-value of (µ-hydroxo)bis[pentaamminechromium(III)]... more From magnetic susceptibility measurements the J-value of (µ-hydroxo)bis[pentaamminechromium(III)] chloride hydrate was found to be 32.3 cm -1 . The EPR spectra of the complex diluted in the analogous cobalt(III) complex have been recorded down to 4 K. The spectrum from the quintet state has been identified by the temperature variation of the intensity. Below 200 K the quintet spectra consist of two spectra originating from two different sites in the crystal. Both spectra have been computer simulated, and the following two parameter sets were obtained for the two different ions in the crystal: D e ) -0.0512(1) cm -1 , E e ) -0.0263(1) cm -1 ; D e ) -0.0498 cm -1 , E e ) -0.0263 cm -1 .
Inorganica Chimica Acta, 2005
The syntheses, structures and magnetic properties of five new manganese (III) cyclam complexes, t... more The syntheses, structures and magnetic properties of five new manganese (III) cyclam complexes, trans-[Mn(cyclam)(OH 2 ) 2 ]-(CF 3 SO 3 ) 3 AE H 2 O, trans-[Mn(cyclam)I 2 ]I, trans-[Mn(cyclam)(ONO) 2 ]ClO 4 , trans-[Mn(cyclam)(OClO 3 ) 2 ]ClO 4 and trans-[Mn(cyclam) (CH 3 COO)(CH 3 COOH)](ClO 4 ) 2 , are reported. Cyclam is the tetradentate amine ligand 1,4,8,11-tetraazacyclotetradecane. The complexes all exhibit pronounced tetragonal elongation of the coordination octahedron with the four cyclam nitrogens occupying the four equatorial positions. The magnetic properties are consistent with the formulation of the complexes as high-spin d 4 systems. trans-[Mn(cyclam)(OH 2 ) 2 ](CF 3 SO 3 ) 3 AE H 2 O is shown to be a convenient starting material for the syntheses of trans cyclam complexes. [Mn(cyclam)(CH 3 COO)(CH 3 COOH)](ClO 4 ) 2 exhibits extremely short intermolecular hydrogen bonds resulting in a pseudo-chain structure. The tilt of the axial ligands with respect to the equatorial plane containing the manganese and the cyclam nitrogen atoms is discussed.
Zeitschrift Fur Anorganische Und Allgemeine Chemie, 2001
ABSTRACT The syntheses and X-ray structure determinations of single crystals and powders are repo... more ABSTRACT The syntheses and X-ray structure determinations of single crystals and powders are reported for the compounds (Et4N)3M2F9 (M = V3+, Cr3+, Fe3+). The compounds are isostructural and represent a new structure type for A3M2X9 compounds. They crystallize in the hexagonal space group P63/m with Z = 2. Their lattice parameters are (Et4N)3V2F9: a = 13.3017(3), c = 10.7714(2) Å; (Et4N)3Cr2F9: a = 13.2691(3), c = 10.7148(3) Å; and (Et4N)3Fe2F9: a = 13.3040(3), c = 10.7650(3) Å.Strukturbestimmung von (Et4N)3M2F9 (M = V, Cr, Fe) mittels Einkristallund Pulverröntgendiffraktion: Ein neuer A3M2X9 Strukturtyp Die Verbindungen (Et4N)3M2F9 (M = V3+, Cr3+, Fe3+) wurden synthetisiert und ihre Kristallstrukturen aus Röntgendiffraktionsmessungen an Einkristallen und Pulvern bestimmt. Alle drei Verbindungen sind isostrukturell und repräsentieren einen neuen A3M2X9 Strukturtyp. Sie kristallisieren in der hexagonalen Raumgruppe P63/m mit Z = 2. Ihre Gitterkonstanten betragen für (Et4N)3V2F9: a = 13.3017(3), c = 10.7714(2) Å; (Et4N)3Cr2F9: a = 13.2691(3), c = 10.7148(3) Å; und für (Et4N)3Fe2F9: a = 13.3040(3), c = 10.7650(3) Å.
High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 ... more High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 T as well as Q-band ($f$ = 34.1 GHz) EPR were performed on single crystals of the molecular wheel CsFe$_8$. In this molecule, eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic (AF) Heisenberg exchange interactions, form a nearly perfect ring. The angle-dependent EPR data allow for the accurate determination of the spin Hamiltonian parameters of the lowest spin multiplets with SleqS \leqSleq 4. Furthermore, the data can well be reproduced by a dimer model with a uniaxial anisotropy term, with only two free parameters JJJ and DDD. A fit to the dimer model yields JJJ = -15(2) cm$^{-1}$ and DDD = -0.3940(8) cm$^{-1}$. A rhombic anisotropy term is found to be negligibly small, EEE = 0.000(2) cm$^{-1}$. The results are in excellent agreement with previous inelastic neutron scattering (INS) and high-field torque measurements. They confirm that the CsFe$_8$ molecule is an excellent experimental model of an AF Heisenberg ring. These findings are also important within the scope of further investigations on this molecule such as the exploration of recently observed magnetoelastic instabilities.
Inorganica chimica …, 1992
The synthesis and characterization of several new bis(r-oxo)dimanganese(III,IV) and -(IV,IV) comp... more The synthesis and characterization of several new bis(r-oxo)dimanganese(III,IV) and -(IV,IV) complexes employing tetradentate ligands related to N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen) (1) are reported. The crystal structure of one MnIV2 complex, bis(p-oxo)bis[N,N'-bis(2-pyridylmethyl)-1,3-propanediamine]dimanganese(IV,IV) perchlorate trihydrate, [(bispictn)-Mn-02-Mn(bispictn)] (C104)4.3H20 (14), has been established by three-dimensional X-ray diffraction techniques. The complex, of formula Mn2C30H44N8020C1 , crystallizes in the monoclinic space group P21/c with four molecules in a cell of dimensions a = 12.772 (7) A, b = 18.581 (7) 1, c = 18.816 (6) A, and @ = 108.81 (3)'. The structure was solved by direct methods and refined by least-squares techniques to a final agreement factor of 0.0564 based on 2247 observed independent intensities. The structure is similar to that of the bispicen analogue, but the Mn-Mn distance of 2.719 (3) A is longer than that of 2.672 (1) A in the bispicen complex, and the bridging Mn-0-Mn angles of 98.4 (4) and 97.3 (4)' are larger than those of 95.0 (2) and 95.2 (2)' in the bispicen complex. The magnetic properties of the (II1,IV) complexes are consistent with a doublet ground state, the observed J values being similar to those observed in other (II1,IV) complexes. The (IVJV) complexes have singlet ground states, with J values again consistent with those in other systems. The EPR spectra of the (II1,IV) complexes exhibit the commonly occurring 16-line spectrum, with varying degrees of anisotropy depending on the particular ligand system. The complexes all exhibit two quasi-reversible waves in the cyclic voltammograms, with redox potentials in the range 0.083-0.170 and 0.943-1.008 V (vs Ag/AgCl) for the (111,111)
Inorganic Chemistry, 1999
From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tm... more From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tmtame = N,N',N' '-trimethyl-1,1,1-tris(aminomethyl)ethane) in the visible and near UV, the exchange splittings of the (4)A(2)(4)A(2) ground as well as the (2)E(4)A(2) and (2)T(1)(4)A(2) singly and (2)E(2)E, (2)E(2)T(1), (2)T(1)(2)T(1), (2)E(2)T(2), and (2)T(1)(2)T(2) doubly excited states of the ground electron configuration are determined, the latter corresponding to simultaneous pair excitations by a single photon. The bulk of intensity in the region of these doubly excited states is found to be vibronically induced by an electric-dipole exchange mechanism. From single-crystal and glass absorption spectra a ferromagnetic energy ordering of the lowest energy ligand-to-metal charge transfer (LMCT) states is derived, whereas the ground state is antiferromagnetically split. The observed splittings are rationalized using a model based on a valence bond approach (VBCI), where the exchange interactions are derived from configuration interaction of LMCT and metal-to-metal charge transfer (MMCT) electron configurations with the ground configuration. The splittings are well reproduced by this model over a range of about 40 000 cm(-)(1). Trigonal orbital exchange parameters J(a) and J(e) are derived, revealing that the direct pathway along the Cr-Cr axis is the dominant one. This gives rise to a double exchange situation in the LMCT configuration, leading to the observed ferromagnetic energy ordering of LMCT levels. Magnetostructural correlations are established from a comparison of the title compound with the similar complex [Cr(2)(OH)(3)(tmtacn)(2)](ClO(4))(3) (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane).
Spectrochimica Acta Part A-molecular and Biomolecular Spectroscopy, 2006
We present the angular variation of the resonance magnetic fields as well as the angular variatio... more We present the angular variation of the resonance magnetic fields as well as the angular variation of the linewidths obtained from single-crystal electron paramagnetic resonance spectra of Δ(+) [Cr((−−)chxn)3λλλ]3+ (chxn = trans-1,2-cyclohexanediamine) doped into the nitrate salt of the isostructural Rh(III) ion Δ(+) [Rh((−−)chxn)3λλλ](NO3)3⋅3 H2 O. An analysis of the angular variation of the resonance magnetic fields indicates axial symmetry for the cation. An analysis of the angular variation of the linewidths, on the other hand, reveals that not all the paramagnetic sites have perfect axial symmetry.
Inorganic …, 2008
The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O ... more The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O (M(III) = Co(III), Cr(III), Fe(III)) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn-M(III)-Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm(-1) established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Cr(CN)(6)].2H(2)O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Fe(CN)(6)].2H(2)O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100-800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)-iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)(6)](3-) anion, giving rise to an M(s) approximately +/-9/2 ground state, isolated by approximately 11.5 cm(-1) from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.
Inorganic Chemistry, 2000
The superexchange interactions in four three-center model systems A-L-B, for A and B being parama... more The superexchange interactions in four three-center model systems A-L-B, for A and B being paramagnetic centers and L a diamagnetic bridging ligand, are analyzed by valence bond configuration interaction models in combination with fourth-order perturbation theory. We analyze the four distinct cases where a bridging ligand orbital simultaneously interacts with half-filled orbitals localized on A and B (case i), a half-filled orbital localized on A and an empty orbital localized on B (case ii), a full orbital localized on A and a half-filled orbital localized on B (case iii), and finally a full orbital localized on A and an empty orbital localized on B (case iv). In all four cases we compare our new results using localized orbitals with the equivalent results obtained using the Anderson ansatz of delocalized (magnetic) orbitals. The effective metal-to-metal electron transfer energy U eff in the old formalism with delocalized orbitals is expressed in terms of the metal-to-metal electron transfer energy U and the ligand-to-metal electron transfer energy ∆ using localized orbitals. We find that the old formalism containing only U eff is in general not sufficient. For cases i and ii we show that U eff can be regarded as an effective U strongly reduced with respect to the free ion as a result of hybridization effects, whereas the same reduction of U for the cases iii and iv is not possible. The relevance and applicability of our theoretical results is demonstrated on magnetochemical data from the literature.
Dalton Transactions, 2010
We show that high-temperature expansion of the partition function is a computationally convenient... more We show that high-temperature expansion of the partition function is a computationally convenient tool to interpretation of magnetic properties of spin clusters wherein the spin centers are interacting via an isotropic Heisenberg exchange operator. High-temperature expansions up to order 12 are used to reproduce temperature dependent magnetic susceptibilities of a Cr(III) tetramer, a decanuclear Fe(III) wheel, a 3x3 grid arrangement of nine Mn(II) ions, and a dodecanuclear Ni(II) wheel. Excellent agreement with experiments as well as with results from computationally more elaborate methods is achieved.
Journal of the American Chemical Society, 1998
ABSTRACT The angular dependence of the magnetic properties of all known octahedrally coordinated ... more ABSTRACT The angular dependence of the magnetic properties of all known octahedrally coordinated oxo-bridged dimers with trivalent 3d transition metals are interpreted in terms of the most simple concepts. These concepts are based on a combination of kinetic exchange and the chemically intuitive angular overlap model. The use of an angular overlap model allows the separation of the σ donor properties of the bridging oxide ligand into sσ and pσ contributions. An estimate of the sσ and the pσ angular overlap model parameters is given. An analysis of the magnetic properties of the known oxo-bridged dimers allows us to predict the magnetic properties of unknown oxo-bridged dimers with trivalent 3d transition metal ions.
Comments on Inorganic Chemistry, 2000
The superexchange interaction in cyano-bridged tmnsition metal dimers is analysed by a valence bo... more The superexchange interaction in cyano-bridged tmnsition metal dimers is analysed by a valence bond confgumtion interaction model in combination w i t h fourtb order perturbation theory. Ferroand antiferromagnetic contributions to the exchange splitting arc cxprrsscd in t u m s of metal-ligand hybridization matrix elements, metal-to-ligand, ligand-to-metaI, metal-to-mtal charge transfer energies, and intra-atomic exbange integrals.
Journal of the American Chemical Society, 1997
ABSTRACT The magnetic properties of oxo-bridged oxo-(carboxylato)-bridged and oxo-bis(carboxylato... more ABSTRACT The magnetic properties of oxo-bridged oxo-(carboxylato)-bridged and oxo-bis(carboxylato)-bridged iron(III) dimers are found to depend on the iron−(μ-O) distance as well as on the iron−(μ-O)−iron angle. With an angular and radial overlap model we account for both these dependencies. The use of an angular overlap model allows us to separate the σ donor properties of oxide as a ligand into the sσ and pσ contributions.
Inorganic Chemistry, 1997
Expressions in the early literature for kinetic exchange in magnetically coupled systems are crit... more Expressions in the early literature for kinetic exchange in magnetically coupled systems are critically analyzed by second-order perturbation theory and substituted by corrected versions. The widely held belief that kinetic exchange is always antiferromagnetic is found to be incorrect. Ferromagnetic kinetic exchange terms are found to be more important than hitherto assumed. The quantity I /U, where I stands for intraatomic exchange interactions and U is the energy difference between the ground electron configuration and a charge-transfer configuration, plays a crucial role in the competition between ferromagnetic and antiferromagnetic contributions. I /U readily exceeds the value (1)/(5) considered as an upper limit by Anderson. I is found to be proportional to the number of unpaired electrons on a given magnetic center.
Inorganic Chemistry, 2006
The electronic structure of a trigonally distorted vanadium(III) complex, [V(urea)6](ClO4)3, and ... more The electronic structure of a trigonally distorted vanadium(III) complex, [V(urea)6](ClO4)3, and its deuterated analogue, [V(urea-d4)6](ClO4)3 has been investigated with Raman, luminescence, and high-frequency high-field electron paramagnetic resonance spectroscopies and with magnetic measurements. A broad electronic Raman transition is observed at around 1400 cm(-1) and attributed to a transition to the (3)E (D3) component of the (3)T1g (O(h)) ground state. The same splitting is seen in the near-infrared luminescence spectrum in the form of a similarly broad peak at 8450 cm(-1), 1400 cm(-1) lower in energy than the (1)E --> (3)A2 spin-flip transition. Powder high-frequency and high-field electron paramagnetic resonance spectra, magnetic susceptibilities, and magnetization studies give a precise measurement of the zero-field splitting and of the g values in this complex (D = 6.00(2) cm(-1), E = 0.573(6) cm(-1), g(x) = 1.848(2), g(y) = 1.832(4), and g(z) = 1.946(7)). A set of angular overlap model parameters is proposed that reproduces all spectroscopic observations, and an analysis of the influence of the bonding of urea on the trigonal distortion of the complex is given.
Inorganic Chemistry, 2001
The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-U... more The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-UV of (Et4N)3Cr2F9 are reported. In the energy range 25800-27700 cm(-1) the 4A2 --> 2A1 (O notation) ligand field transition can be resolved in detail for the first time in any Cr3+ compound. This allows the determination of the antiferromagnetic ground-state exchange splitting with great accuracy: J = 25.9 cm(-1) and j = 0.27 cm(-1) using the Hamiltonian H = J(S(A).S(B)) - j(S(A).S(B))2, where j leads to deviations from the regular Landé pattern. The temperature dependence of the magnetic susceptibility is nicely reproduced by these parameters. A comparison with Cs3Cr2Cl9 and Cs3Cr2Br9 reveals an exponential dependence of the ground-state splitting upon the Cr-Cr distance in the [Cr2X9]3- dimers. This is the result of a dominant sigma-type orbital exchange pathway along the Cr-Cr axis.
Chemical Physics, 1998
Low temperature magnetization data obtained for titanium (III) doped caesium gallium alum show a ... more Low temperature magnetization data obtained for titanium (III) doped caesium gallium alum show a spectacular deviation from Brillouin behavior with the magnetization highly dependent on the strength of the applied field at a given value of B/T. This arises from unprecedented higher-order contributions to the magnetization and these measurements afford the determination of the Zeeman coefficients to third order. This anomalous magnetic behavior is a manifestation of Jahn-Teller coupling giving rise to low-lying vibronic states which mix into the ground state through the magnetic field.
Acta Crystallographica Section C-crystal Structure Communications, 2002
ABSTRACT The crystal structure of the low-spin (S = 1) MnIII complex [Mn(CN)2(C10H24N4)]ClO4, or ... more ABSTRACT The crystal structure of the low-spin (S = 1) MnIII 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 MnIII 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.
Acta Chemica Scandinavica, 1993
Journal of Magnetic Resonance, 2010
Utilizing a standard spin Hamiltonian for an S=32 spin system, we fit complete X-band powder EPR ... more Utilizing a standard spin Hamiltonian for an S=32 spin system, we fit complete X-band powder EPR spectra of the Cr(oxalate)33- anion diluted into K3[Co(oxalate)3]·3H2O. By using models for the bandshape and bandwidths of varying degree of flexibility, we show that the successful outcome of such a fitting endeavour very much depends on the used bandshape–bandwidth model. The best results are obtained when the bandwidth model incorporates anisotropic intrinsic bandwidths in addition to being able to account for inhomogeneous line broadening caused by distributions of the spin Hamiltonian parameters.
Inorganic Chemistry, 1997
From magnetic susceptibility measurements the J-value of (µ-hydroxo)bis[pentaamminechromium(III)]... more From magnetic susceptibility measurements the J-value of (µ-hydroxo)bis[pentaamminechromium(III)] chloride hydrate was found to be 32.3 cm -1 . The EPR spectra of the complex diluted in the analogous cobalt(III) complex have been recorded down to 4 K. The spectrum from the quintet state has been identified by the temperature variation of the intensity. Below 200 K the quintet spectra consist of two spectra originating from two different sites in the crystal. Both spectra have been computer simulated, and the following two parameter sets were obtained for the two different ions in the crystal: D e ) -0.0512(1) cm -1 , E e ) -0.0263(1) cm -1 ; D e ) -0.0498 cm -1 , E e ) -0.0263 cm -1 .
Inorganica Chimica Acta, 2005
The syntheses, structures and magnetic properties of five new manganese (III) cyclam complexes, t... more The syntheses, structures and magnetic properties of five new manganese (III) cyclam complexes, trans-[Mn(cyclam)(OH 2 ) 2 ]-(CF 3 SO 3 ) 3 AE H 2 O, trans-[Mn(cyclam)I 2 ]I, trans-[Mn(cyclam)(ONO) 2 ]ClO 4 , trans-[Mn(cyclam)(OClO 3 ) 2 ]ClO 4 and trans-[Mn(cyclam) (CH 3 COO)(CH 3 COOH)](ClO 4 ) 2 , are reported. Cyclam is the tetradentate amine ligand 1,4,8,11-tetraazacyclotetradecane. The complexes all exhibit pronounced tetragonal elongation of the coordination octahedron with the four cyclam nitrogens occupying the four equatorial positions. The magnetic properties are consistent with the formulation of the complexes as high-spin d 4 systems. trans-[Mn(cyclam)(OH 2 ) 2 ](CF 3 SO 3 ) 3 AE H 2 O is shown to be a convenient starting material for the syntheses of trans cyclam complexes. [Mn(cyclam)(CH 3 COO)(CH 3 COOH)](ClO 4 ) 2 exhibits extremely short intermolecular hydrogen bonds resulting in a pseudo-chain structure. The tilt of the axial ligands with respect to the equatorial plane containing the manganese and the cyclam nitrogen atoms is discussed.
Zeitschrift Fur Anorganische Und Allgemeine Chemie, 2001
ABSTRACT The syntheses and X-ray structure determinations of single crystals and powders are repo... more ABSTRACT The syntheses and X-ray structure determinations of single crystals and powders are reported for the compounds (Et4N)3M2F9 (M = V3+, Cr3+, Fe3+). The compounds are isostructural and represent a new structure type for A3M2X9 compounds. They crystallize in the hexagonal space group P63/m with Z = 2. Their lattice parameters are (Et4N)3V2F9: a = 13.3017(3), c = 10.7714(2) Å; (Et4N)3Cr2F9: a = 13.2691(3), c = 10.7148(3) Å; and (Et4N)3Fe2F9: a = 13.3040(3), c = 10.7650(3) Å.Strukturbestimmung von (Et4N)3M2F9 (M = V, Cr, Fe) mittels Einkristallund Pulverröntgendiffraktion: Ein neuer A3M2X9 Strukturtyp Die Verbindungen (Et4N)3M2F9 (M = V3+, Cr3+, Fe3+) wurden synthetisiert und ihre Kristallstrukturen aus Röntgendiffraktionsmessungen an Einkristallen und Pulvern bestimmt. Alle drei Verbindungen sind isostrukturell und repräsentieren einen neuen A3M2X9 Strukturtyp. Sie kristallisieren in der hexagonalen Raumgruppe P63/m mit Z = 2. Ihre Gitterkonstanten betragen für (Et4N)3V2F9: a = 13.3017(3), c = 10.7714(2) Å; (Et4N)3Cr2F9: a = 13.2691(3), c = 10.7148(3) Å; und für (Et4N)3Fe2F9: a = 13.3040(3), c = 10.7650(3) Å.
High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 ... more High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 T as well as Q-band ($f$ = 34.1 GHz) EPR were performed on single crystals of the molecular wheel CsFe$_8$. In this molecule, eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic (AF) Heisenberg exchange interactions, form a nearly perfect ring. The angle-dependent EPR data allow for the accurate determination of the spin Hamiltonian parameters of the lowest spin multiplets with SleqS \leqSleq 4. Furthermore, the data can well be reproduced by a dimer model with a uniaxial anisotropy term, with only two free parameters JJJ and DDD. A fit to the dimer model yields JJJ = -15(2) cm$^{-1}$ and DDD = -0.3940(8) cm$^{-1}$. A rhombic anisotropy term is found to be negligibly small, EEE = 0.000(2) cm$^{-1}$. The results are in excellent agreement with previous inelastic neutron scattering (INS) and high-field torque measurements. They confirm that the CsFe$_8$ molecule is an excellent experimental model of an AF Heisenberg ring. These findings are also important within the scope of further investigations on this molecule such as the exploration of recently observed magnetoelastic instabilities.
Inorganica chimica …, 1992
The synthesis and characterization of several new bis(r-oxo)dimanganese(III,IV) and -(IV,IV) comp... more The synthesis and characterization of several new bis(r-oxo)dimanganese(III,IV) and -(IV,IV) complexes employing tetradentate ligands related to N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen) (1) are reported. The crystal structure of one MnIV2 complex, bis(p-oxo)bis[N,N'-bis(2-pyridylmethyl)-1,3-propanediamine]dimanganese(IV,IV) perchlorate trihydrate, [(bispictn)-Mn-02-Mn(bispictn)] (C104)4.3H20 (14), has been established by three-dimensional X-ray diffraction techniques. The complex, of formula Mn2C30H44N8020C1 , crystallizes in the monoclinic space group P21/c with four molecules in a cell of dimensions a = 12.772 (7) A, b = 18.581 (7) 1, c = 18.816 (6) A, and @ = 108.81 (3)'. The structure was solved by direct methods and refined by least-squares techniques to a final agreement factor of 0.0564 based on 2247 observed independent intensities. The structure is similar to that of the bispicen analogue, but the Mn-Mn distance of 2.719 (3) A is longer than that of 2.672 (1) A in the bispicen complex, and the bridging Mn-0-Mn angles of 98.4 (4) and 97.3 (4)' are larger than those of 95.0 (2) and 95.2 (2)' in the bispicen complex. The magnetic properties of the (II1,IV) complexes are consistent with a doublet ground state, the observed J values being similar to those observed in other (II1,IV) complexes. The (IVJV) complexes have singlet ground states, with J values again consistent with those in other systems. The EPR spectra of the (II1,IV) complexes exhibit the commonly occurring 16-line spectrum, with varying degrees of anisotropy depending on the particular ligand system. The complexes all exhibit two quasi-reversible waves in the cyclic voltammograms, with redox potentials in the range 0.083-0.170 and 0.943-1.008 V (vs Ag/AgCl) for the (111,111)
Inorganic Chemistry, 1999
From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tm... more From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tmtame = N,N',N' '-trimethyl-1,1,1-tris(aminomethyl)ethane) in the visible and near UV, the exchange splittings of the (4)A(2)(4)A(2) ground as well as the (2)E(4)A(2) and (2)T(1)(4)A(2) singly and (2)E(2)E, (2)E(2)T(1), (2)T(1)(2)T(1), (2)E(2)T(2), and (2)T(1)(2)T(2) doubly excited states of the ground electron configuration are determined, the latter corresponding to simultaneous pair excitations by a single photon. The bulk of intensity in the region of these doubly excited states is found to be vibronically induced by an electric-dipole exchange mechanism. From single-crystal and glass absorption spectra a ferromagnetic energy ordering of the lowest energy ligand-to-metal charge transfer (LMCT) states is derived, whereas the ground state is antiferromagnetically split. The observed splittings are rationalized using a model based on a valence bond approach (VBCI), where the exchange interactions are derived from configuration interaction of LMCT and metal-to-metal charge transfer (MMCT) electron configurations with the ground configuration. The splittings are well reproduced by this model over a range of about 40 000 cm(-)(1). Trigonal orbital exchange parameters J(a) and J(e) are derived, revealing that the direct pathway along the Cr-Cr axis is the dominant one. This gives rise to a double exchange situation in the LMCT configuration, leading to the observed ferromagnetic energy ordering of LMCT levels. Magnetostructural correlations are established from a comparison of the title compound with the similar complex [Cr(2)(OH)(3)(tmtacn)(2)](ClO(4))(3) (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane).
Spectrochimica Acta Part A-molecular and Biomolecular Spectroscopy, 2006
We present the angular variation of the resonance magnetic fields as well as the angular variatio... more We present the angular variation of the resonance magnetic fields as well as the angular variation of the linewidths obtained from single-crystal electron paramagnetic resonance spectra of Δ(+) [Cr((−−)chxn)3λλλ]3+ (chxn = trans-1,2-cyclohexanediamine) doped into the nitrate salt of the isostructural Rh(III) ion Δ(+) [Rh((−−)chxn)3λλλ](NO3)3⋅3 H2 O. An analysis of the angular variation of the resonance magnetic fields indicates axial symmetry for the cation. An analysis of the angular variation of the linewidths, on the other hand, reveals that not all the paramagnetic sites have perfect axial symmetry.
Inorganic …, 2008
The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O ... more The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O (M(III) = Co(III), Cr(III), Fe(III)) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn-M(III)-Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm(-1) established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Cr(CN)(6)].2H(2)O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Fe(CN)(6)].2H(2)O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100-800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)-iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)(6)](3-) anion, giving rise to an M(s) approximately +/-9/2 ground state, isolated by approximately 11.5 cm(-1) from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.