Diamagnetic Host Research Articles

31P Magic Angle Spinning NMR Study of Flux-Grown Rare-Earth Element Orthophosphate (Monazite/Xenotime) Solid Solutions: Evidence of Random Cation Distribution from Paramagnetically Shifted NMR Resonances

We present (31)P magic angle spinning nuclear magnetic resonance spectra of flux-grown solid solutions of La(1-x)Ce(x)PO4 (x between 0.027 and 0.32) having the monoclinic monazite structure, and of Y(1-x)M(x)PO4 (M = V(n+), Ce(3+), Nd(3+), x between 0.001 and 0.014) having the tetragonal zircon structure. Paramagnetically shifted NMR resonances are observed in all samples due to the presence of paramagnetic V(n+), Ce(3+), and Nd(3+) in the diamagnetic LaPO4 or YPO4. As a first-order observation, the number and relative intensities of these peaks are related to the symmetry and structure of the diamagnetic host phase. The presence of paramagnetic shifts allows for increased resolution between NMR resonances for distinct atomic species which leads to the observation of low intensity peaks related to PO4 species having more than one paramagnetic neighbor two or four atomic bonds away. Through careful analysis of peak areas and comparison with predictions for simple models, it was determined that solid solutions in the systems examined here are characterized by complete disorder (random distribution) of diamagnetic La(3+) or Y(3+) with the paramagnetic substitutional species Ce(3+) and Nd(3+). The increased resolution given by the paramagnetic interactions also leads to the observation of splitting of specific resonances in the (31)P NMR spectra that may be caused by local, small-scale distortions from the substitution of ions having dissimilar ionic radii.

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.

The efficacy of travertine as a palaeoenvironmental indicator: palaeomagnetic study of neotectonic examples from Denizli, Turkey

This study has aimed to evaluate whether a discernible environmental signature is recorded in tectonic travertine by applying palaeomagnetic study to examples from the Denizli region in western Turkey. Palaeomagnetic sampling in 7 quarry exposures through short stratigraphic intervals of bedded travertine has determined variations in magnetic susceptibility and palaeofield direction; the former is a potential proxy of climatically-controlled atmospheric dust input and the latter is a possible indicator of directional changes resulting from geomagnetic secular variation of the ancient magnetic field and hence a measure of the rate of travertine accretion. Most sites record normal polarity as predicted from emplacement during the Brunhes Chron, although one had reversed polarity evidently imparted during the Matuyama Chron and confirming longer-term preservation of remanence. A few sites with coherent directions widely different from the recent field axis appear to have slumped or tectonically rotated since emplacement. Within-section groupings of palaeomagnetic directions are tight with lack of dispersion indicating that secular variation has been averaged over protracted periods of time. Magnetic remanence is therefore a diagenetic phenomenon, as expected from prolonged infiltration through porous bedded travertine. Magnetic susceptibilities are mostly very weak and dominated by the diamagnetic host, but some positive values record paramagnetic and ferromagnetic constituents. We find that environmental signatures may be revealed in bedded travertine by magnetic susceptibility. Palaeomagnetic directions provide no reliable constraint on incremental growth although fissure emplacements, including an additional example reported in this study, can record a short-term record of secular variation and yield estimates for the duration of fissure activity. In contrast the tufa-like deposits laid down by geothermal waters spilling out at the surface are highly porous and susceptible to later fluid seepage and only atmospheric dust landing on the surface can potentially record environmental effects. Isotopic and palaeomagnetic systems are homogenised over long intervals of time and unable to record short-term near-surface changes.

EPR Spectroscopy of 4, 4′‐Bis(tert‐butyl)‐2, 2′‐bipyridine‐1, 2‐dithiolatocuprates(II) in Host Lattices with Different Coordination Geometries

AbstractA series of new heteroleptic MN2S2 transition metal complexes with M = Cu2+ for EPR measurements and as diamagnetic hosts Ni2+, Zn2+, and Pd2+ were synthesized and characterized. The ligands are N2 = 4, 4′‐bis(tert‐butyl)‐2, 2′‐bipyridine (tBu2bpy) and S2 =1, 2‐dithiooxalate, (dto), 1, 2‐dithiosquarate, (dtsq), maleonitrile‐1, 2‐dithiolate, or 1, 2‐dicyanoethene‐1, 2‐dithiolate, (mnt). The CuII complexes were studied by EPR in solution and as powders, diamagnetically diluted in the isostructural planar [NiII(tBu2bpy)(S2)] or[PdII(tBu2bpy)(S2)] as well as in tetrahedrally coordinated[ZnII(tBu2bpy)(S2)] host structures to put steric stress on the coordination geometry of the central CuN2S2 unit. The spin density contributions for different geometries calculated from experimental parameters are compared with the electronic situation in the frontier orbital, namely in the semi‐occupied molecular orbital (SOMO) of the copper complex, derived from quantum chemical calculations on different levels (EHT and DFT). One of the hosts, [NiII(tBu2bpy)(mnt)], is characterized by X‐ray structure analysis to prove the coordination geometry. The complex crystallizes in a square‐planar coordination mode in the monoclinic space group P21/a with Z = 4 and the unit cell parameters a = 10.4508(10) Å, b = 18.266(2) Å, c = 12.6566(12) Å, β = 112.095(7)°. Oxidation and reductions potentials of one of the host complexes, [Ni(tBu2bpy)(mnt)], were obtained by cyclovoltammetric measurements.

Structure and luminescent properties of Cs2Sr(VO3)4:Mn2+

We have developed a procedure for thermally stimulated synthesis of a cesium strontium metavanadate, Cs2Sr(VO3)4:Mn2+ (0.01, 0.50, 1.00, 5.00 at % Mn2+), using MnO-containing starting mixtures. The EPR spectrum of the material containing 0.01 at % Mn2+ shows a hyperfine structure due to the incorporation of a small amount of manganese into the diamagnetic double metavanadate host. The luminescent and optical properties of Cs2Sr(VO3)4:Mn2+ depend on manganese content. In contrast to higher doping levels, doping with 0.01 at % Mn2+ increases the integrated emission intensity of the vanadate by 10% and improves its chromaticity characteristics (approaching them to those of white light). We assume that this is due to the reduction in the density of vacancy-type growth defects, such as oxygen vacancies.

Synthesis and Electronic Properties of Thermoelectric and Magnetic Nanoparticle Composite Materials

Application of a magnetic field greatly enhances the thermoelectric efficiency of bismuth-antimony (Bi-Sb) alloys. We synthesized a hybrid of Bi-Sb alloy and magnetic nanoparticles, expecting improvement of the thermoelectric performance due to the magnetic field generated by the nanoparticles. Powder x-ray diffraction and magnetic measurements of the synthesized hybrid Bi 0.88 Sb 0.12 (FeSb) 0.05 sample indicated that the ferromagnetic FeSb nanoparticles, with a size of about 30 nm, were distributed in the main phase of the Bi-Sb alloy. The FeSb nanoparticles act as soft ferromagnets in the diamagnetic host Bi-Sb alloy. The electrical resistivity ρ of the host Bi 0.88 Sb 0.12 sample decreased concomitantly with decreasing temperature, showing a shoulder at 80 K. In contrast, ρ for the hybrid sample was enhanced below 100 K because of carrier scattering by the nanoparticles. The temperature dependence of the Seebeck coefficient S was also altered by the nanoparticle addition. In contrast, the addition of magnetic nanoparticles only slightly influenced the thermal conductivity κ. These results indicate that the addition of magnetic nanoparticles to thermoelectric materials modulates the electronic structures but does not influence the lattice system.

Preparation and Characterization of (CNSSS)2(A)2(A = AsF6−, SbF6−, Sb2F11−) Containing the O2-like 5,5′-Bis(1,2,3,4-trithiazolium) Dication: The Second Example of a Simple Nonsterically Hindered Main-Group Diradical That Retains Its Paramagnetism in the Solid State

The reaction of NC-CN with a 1:1 mixture of S(4)(MF(6))(2) and S(8)(MF(6))(2) (M = As, Sb) (stoichiometrically equivalent to four "S(3)MF(6)" units) results in the quantitative formation of S(3)NCCNS(3)(MF(6))(2) [7(MF(6))(2)], which is the thermodynamic sink in this reaction. The Sb(2)F(11)(-) salt 7(Sb(2)F(11))(2) is prepared by the addition of an excess of SbF(5) to 7(AsF(6))(2). Crystal structure determinations for all three salts show that 7(2+) can be viewed as two R-CNS(3)(+) radical cations joined together by a C-C single bond. The two rings are coplanar and in a trans orientation due to electrostatic N(delta-)...S(delta+) interactions. The classically bonded alternative (quinoidal structure), in which the octet rule is obeyed, is not observed and is much higher in energy based on calculated estimates and a simple comparison of pi bond energies. Calculated molecular orbitals (MOs) support this, showing that the MO corresponding to the quinoidal structure lies higher in energy than the nearly degenerate singly occupied MOs of 7(2+). The vibrational spectra of 7(2+) in all salts were assigned based on a normal-coordinate analysis and theoretical vibrational frequencies calculated at the PBE0/6-31G* level. In the solid state, 7(2+) is a planar disjoint diradical with essentially degenerate open-shell singlet and triplet states. The disjoint nature of the diradical cation 7(2+) is established by magnetic susceptibility studies of the Sb(2)F(11)(-) salt doped in an isomorphous diamagnetic host material (CNSNS)(2)(Sb(2)F(11))(2) [10(Sb(2)F(11))(2)]. Intramolecular spin coupling is extremely weak corresponding to a singlet-triplet gap (DeltaE(ST) = 2J) of <+/-2 cm(-1). CASPT2[12,12]/6-311G* calculations support a triplet ground state with a small singlet-triplet gap. The single-crystal electron paramagnetic resonance (EPR) of 7(Sb(2)F(11))(2) doped in 10(Sb(2)F(11))(2) is in agreement with the triplet state arising from the weak coupling between the unpaired electrons residing in p(pi) orbitals in each of the rings. Variable-temperature susceptibility data for bulk samples of 7(A)(2) (A = SbF(6)(-), AsF(6)(-), Sb(2)F(11)(-)) are analyzed by employing both 1D chain and 2D sheet magnetic models. These studies reveal significant intermolecular exchange approximating that of a 1D chain for the SbF(6)(-) salt with |J| = 32 cm(-1). The exchange coupling is on the same order of magnitude as that for the AsF(6)(-) salt, although in this case it is likely that there are complex exchange pathways where no particular one is dominant. Intermolecular exchange in the Sb(2)F(11)(-) salt is an order of magnitude weaker. In solution, the EPR spectrum of 7(2+) shows a broad triplet resonance as well as a sharp resonance that is tentatively attributed to a rotomer of the 7(2+)/anion pair, which is likely the origin of the green species given on dissolution of the red 7(2+) salts in SO(2)/AsF(3)/MF(5). We account for the many similarities between O(2) and 7(2+), which are the only simple nonsterically hindered nonmetal diradicals to retain their paramagnetism in the solid state. 7(2+) is also the first isolable, essentially sulfur-based diradical as evidenced by calculated spin densities.

Study of defect-induced ferromagnetism in hydrogenated anatase TiO2:Co

Electronic and magnetic properties of Co-doped TiO2 polycrystalline pellets (Ti0.95Co0.05O2) have been investigated using x-ray diffraction, x-ray photoemission, magnetization, and resistance measurements. The as-synthesized and hydrogenated specimens crystallize in the anatase type tetragonal structure containing very small (∼4.4%) rutile phase. The dopant ions of Co are found to be divalent and well incorporated into TiO2 lattice, substituting the Ti site within the anatase phase, with no evidence of metallic Co or any other oxides of Co. The Co doping induces a weak ferromagnetic ordering in the diamagnetic TiO2 host matrix. Interestingly, when the Co-doped TiO2 is annealed in hydrogen atmosphere, it shows a giant enhancement in magnetization. However, an extended reheating in air causes this H-induced magnetization to vanish and the sample regains the as-prepared status. Our findings indicate that ferromagnetism originates from the doped matrix rather than any magnetic clusters and strongly correlated with oxygen vacancies in the doped TiO2. The induced ferromagnetic ordering is found to be a reversible process with regard to oxygen vacancy defects that could be induced or removed, respectively, upon introduction or removal of these defects. At the request of the authors and editor, this article is being retracted effective 15 August 2014.

Supramolecular host–guest systems constructed of pyrrole derivatives and 3D-coordination polymers

Pyrrole derivatives have been shown to be completely or partially oxidized within the expandable channels of the 3D-coordination polymers [(R3Sn)3Fe(CN)6] n and [(R3Sn)(R 3 ′ Sn)2Fe(CN)6] n , R and R′ = Me, n-Bu, or Ph, to give novel class of supramolecular host–guest systems. The structures and physical properties of these host–guest systems depend on the reaction time, nature of the host and guest, the space empty within the network of the 3D-coordination polymers. Pyrrole undergoes oxidative polymerization in the channels of the 3D-coordination polymers to form semiconducting diamagnetic supramolecular host–guest systems. Whereas N-methylpyrrole and 2,5-dimethylpyrrole are not polymerized under these experimental conditions, but give paramagnetic charge transfer (CT) supramolecular host–guest systems.

C4ν Symmetry Crystal Field and Ground State Wavefunction of the VO2+ Ion

The spin-Hamiltonian parameters obtained from EPR studies of the VO2+ ion in different diamagnetic host lattices are used to estimate the ground state wave functions. These are of dxy type with admixture of the excited states dx2−y2, dyz and dxz. The hyperfine interaction parameter and Fermi contact term for the VO2+ doped crystals are determined using the coefficients of the ground state. The various parameters are correlated to the ligand properties of the complexes.

Characterization of magnetic and electronic properties of trimetallic nitride endohedral fullerenes by SQUID magnetometry and electron paramagnetic resonance

Magnetic and electronic properties of high-purity trimetallic nitride endohedral fullerenes Er3N@C80, Lu3N@C80, Sc3N@C80 diluted in a diamagnetic host were characterized by SQUID DC magnetometry and electron paramagnetic resonance (EPR) at multiple frequencies. The Er3N@C80 sample followed the Curie–Weiss law with negligible Weiss temperature of 0.16K. Based on temperature dependence and isothermal saturation magnetization the effective magnetic moment of Er3N@C80 was estimated as 10.2μB. The magnetic behavior of these fullerenes is ascribed to quenching of the orbital moment of the metal ions due to interactions between the metal centers with the nitrogen and the fullerene cage.

EPR and luminescence studies of Eu(II) magnetically diluted in LiCl–KCl salt

High-temperature LiCl–KCl molten salt medium provides an efficient way to produce the paramagnetic Eu(II) ion to be magnetically diluted into a diamagnetic host medium. Eu(II) was formed by a dissolution and an auto-reduction processes in a high-temperature LiCl–KCl eutectic melt at 723 K by using Eu 2O 3 as a starting material. By using EPR and luminescence spectroscopic method, we studied the nature of the magnetically isolated paramagnetic Eu(II) ion diluted in a LiCl–KCl medium. With the aid of these spectroscopic tools, it was found that stable Eu(II) species was formed spontaneously at 723 K under anaerobic conditions. EPR and luminescence spectroscopy provided detailed information regarding the nature of the europium ion in a molten salt.

Synthesis, characterization, and crystal structures of two new polyoxomolybdate wheel clusters

Two new polyoxomolybdates Na 5(NH 4) 16[Mo 57Mn 6(NO) 6O 174 (OH) 3(H 2O) 24] · 44H 2O (1) and (NH 4) 21[Mo 57Cu 6(NO) 6O 168(OH) 3(H 2O) 24] · 53H 2O (2) were synthesized in aqueous solution and characterized by elemental analysis, IR, TG analysis and single-crystal X-ray diffraction. They represent interesting examples of integrating and embedding magnetic 3d metal ions into the diamagnetic polyoxomolybdate host structure.

Identification of static JT in copper(II) doped hexaimidazole M(II) lattices: M = Co and Ni: An EPR study

Single crystal EPR studies on Cu(II) doped paramagnetic host lattices, hexaimidazole M(II) dichloride tetrahydrate (M = Co and Ni), isomorphous with M = Zn, have been carried out from room temperature to 77 K to understand the nature of Jahn–Teller (JT) distortion in these paramagnetic host systems. The paramagnetic impurity, doped in the present two paramagnetic host lattices, shows anisotropic EPR spectra with superhyperfine from ligands, even at room temperature. An interesting observation noticed in the EPR spectra at room temperature is that there are more resonances corresponding to the second site in the paramagnetic hosts than in the diamagnetic host at 4.2 K. This difference in behavior between the diamagnetic and paramagnetic host lattices indicates a change in the depth of the JT valleys. The spin Hamiltonian parameters are evaluated for Cu(II) ion in both the host lattices and the relaxation times have been calculated for the ion in cobalt host lattice only.

Crystal Environments Probed by EPR Spectroscopy. Variations in the EPR Spectra of CoII(octaethylporphyrin) Doped in Crystalline Diamagnetic Hosts and a Reassessment of the Electronic Structure of Four-Coordinate Cobalt(II)

The powder and single-crystal EPR spectra of Co(II)(OEP) (OEP is the dianion of octaethylporphyrin) doped into a range of diamagnetic crystals including simple four-coordinate hosts, H(2)(OEP), the triclinic B form of Ni(II)(OEP), the tetragonal form of Ni(II)(OEP) and Zn(II)(OEP); five-coordinate hosts, micro-dioxane)[Zn(II)(OEP)](2) and (py)Zn(II)(OEP); six-coordinate hosts, (py)(2)Zn(II)(OEP) and (py)(2)Mg(II)(OEP); and hosts containing fullerenes, C(60).2Zn(II)(OEP).CHCl(3), C(70).Ni(II)(OEP).C(6)H(6).CHCl(3), and C(60).Ni(II)(OEP).2C(6)H(6) have been obtained and analyzed. Spectra were simulated using a program that employed the exact diagonalization of the 16 x 16 complex spin Hamiltonian matrix. The EPR spectra of these doped samples are very sensitive to the environment within each crystal with the crystallographic site symmetry determining whether axial or rhombic resonance patterns are observed. For Co(II)(OEP) doped into tetragonal Ni(II)(OEP) (which displays a very large g( perpendicular ) of 3.405 and a very small g( parallel ) of 1.544) and several other crystals containing four-coordinate metal sites, the g components could not be fit using existing theory with the assumption of the usual z(2) ground state. However, reasonable agreement of the observed EPR parameters could be obtained by assuming that the unpaired electron resides in an xy orbital in the four-coordinate complexes.

Magnetic properties of a homologous series of vanadium jarosite compounds.

Redox-based, hydrothermal synthetic methodologies have enabled the preparation of a new series of stoichiometrically pure jarosites of the formula, AV(3)(OH)(6)(SO(4))(2) with A = Na(+), K(+), Rb(+), Tl(+), and NH(4)(+). These jarosites represent the first instance of strong ferromagnetism within a Kagomé layered framework. The exchange interaction, which is invariant to the nature of the A(+) ion (theta(CW) approximately equal to +53(1) K), propagates along the d(2) magnetic sites of the triangular Kagomé lattice through bridging hydroxyl groups. An analysis of the frontier orbitals suggests this superexchange pathway to possess significant pi-orbital character. Measurements on a diamagnetic host jarosite doped with magnetically dilute spin carriers, KGa(2.96)V(0.04)(OH)(6)(SO(4))(2), reveal significant single-ion anisotropy for V(3+) ion residing in the tetragonal crystal field. This anisotropy confines the exchange-coupled moments to lie within the Kagomé layer. Coupling strengths are sufficiently strong to prevent saturation of the magnetization when an external field is applied orthogonal to the Kagomé layer. Antiferromagnetic ordering of neighboring ferromagnetic Kagomé layers becomes dominant at low temperatures, characteristic of metamagnetic behavior for the AV(3)(OH)(6)(SO(4))(2) jarosites. This interlayer exchange coupling decreases monotonically with increasing layer spacing along the series, A = Na(+), K(+), Rb(+), NH(4)(+), and Tl(+), and it may be overcome by the application of external field strengths in excess of approximately 6 kOe.

Frequency dependence of electron spin relaxation for threeS = 1/2 species doped into diamagnetic solid hosts

Electron spin lattice relaxation rates (1/T1) were measured as a function of temperature at two or three microwave frequencies for threeS = 1/2 species in temperature ranges with different dominant relaxation processes. Between 10 and 50 K the contribution from the direct process to the relaxation rate was substantially greater at 94 than at 9.5 GHz for a vanadyl porphyrin doped into zinc tetratolylporphyrin. For bis(diethyldithiocarbamato)copper(II) doped into the diamagnetic Ni(II) analog the relaxation rate between 25 and 100 K is dominated by the Raman process and exhibits little frequency dependence between 9.2 and 94 GHz. For 4-hydroxy-2,2,6,6-tetramethylpiperidinoloxy (tempol) doped into a diamagnetic host the relaxation rate between about 40 and 100 K is dominated by the Raman process. In this temperature range, relaxation rates at 3.2, 9.2, and 94 GHz exhibit little frequency dependence. Above about 130 K, the relaxation rate for tempol decreases in the order S-band s> X-band > W-band. The relaxation rates in this temperature range fit a model in which 1/T1 is dominated by a thermally activated process that is assigned as rotation of the methyl groups on the nitroxyl ring.

Linear chain complexes linked by hydrogen bonded O2H3– and O3H5– units: structural and magnetic comparison of trans-dihydroxo-, -diaqua-, and -aquahydroxo-chromium(iii) complexes

The syntheses and characterization of a series of chromium(III) complexes of the general types trans-[(A)4Cr(OH)(H2O)]2+, trans-[(A)4Cr(OH)2]+, and trans-[(A)4Cr(H2O)2]3+ are described. The ligands (A)4 used include the tetradentate ligand 1,4,8,11-tetraazacyclotetradecane (cyclam), its hexamethylated derivative C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (cyca), and the bidentate ligand 2-aminomethylpyridine (pico). The cobalt(III) aquahydroxo complexes with cyclam and cyca are also reported. The trans-diaqua and trans-dihydroxo complexes are monomeric, but the trans-aquahydroxo complexes form linear chains. The complexes trans-[Cr(cyclam)(OH)(H2O)](CF3SO3)2 1 and trans- [Co(cyclam)(OH)(H2O)](CF3SO3)2 3 and their corresponding perchlorates (2 and 4, respectively) have been isolated and the structures of the chromium triflate salt 1 and the cobalt perchlorate salt 4 have been determined. Both of these complexes 1 and 4 form infinite chains in the solid state, adjacent metal centers being bridged by hydrogen bonds between hydroxo and aqua ligands. The Cr⋯Cr separation in 1 is 6.085 Å whereas the Co⋯Co separation in 4 is 6.078 Å. The complex trans-[(pico)2Cr(OH2)2](NO3)3·H2O 8 (where pico is picolylamine, 2-aminomethylpyridine) crystallizes with only one monomeric formula in a unit cell. In contrast to the structures of 1 and 4 (and, presumably, of 2 and 3 also), the two complexes trans-[Cr(cyclam)(OH)2]ClO4·H2O 5 and 8 are monomeric in the solid state. The chromium complex trans-[Cr(cyca)(OH)(H2O)](ClO4)2·H2O 6 crystallizes with two monomeric formula units in a unit cell and the cobalt analogue 7 is isomorphous with the chromium complex. The structures of these complexes are also linear chains, but in this case there is an intervening water molecule between the aqua and hydroxo ligands on adjacent chromium centers. EPR spectroscopy of the chromium complex 1 doped in the diamagnetic cobalt host 3 reveals the presence of weak magnetic interactions.

1–370 GHz EPR Linewidths for K3CrO8: A Comprehensive Test for the Anderson–Weiss Model

Electron paramagnetic resonance (EPR) measurements have been carried out over the frequency range of 1–370 GHz on single crystals of potassium peroxychromate (K3CrO8) with the view of examining the current models of exchange narrowing of EPR signals in solids. K3CrO8has a simple (tetragonal) lattice structure, can be grown as single crystals pure or diluted with an isostructural diamagnetic host K3NbO8, and its paramagnetism can be described by a very simple (S=12,I= 0) spin Hamiltonian. The measurements were made at various orientations of single crystals in the Zeeman field, with emphasis on the principal directions of theg-tensor. For essentially all orientations, the linewidth decreases monotonically for measurements at resonance frequencies, ω0, from 1 to about 100 GHz, and then starts to increase at higher ω0. In order to delineate the spin exchange effects from other sources of line broadening, the measurements were repeated with a diluted spin system, K3NbO8containing ≅0.5 mole % of K3CrO8, representing the broadening effect of all the magnetic field dependent terms, such as the broadening due to theg-strain and sample holder/waveguide magnetization at the high field utilized, up to 14 T. Using these data, the K3CrO8linewidths were analyzed in terms of the current models of spin exchange narrowing in three-dimensional systems. A reasonably good agreement was found with the Anderson–Weiss model, when modified for various line broadening effects. The accuracy of the analysis procedure was confirmed by the comparison of the presently determined values of the exchange constant,J,and the dipolar field,Hp,with their values obtained by dc magnetic susceptibility measurements and theoretical analysis, respectively; the agreement was within 5% forJ(=1.35 K) and about 25% forHp(160 G). However, some deviations and unusual splittings were noted in measurements at 370 GHz, whose origin remains unclear.

Polarization of quadrupolar nuclei

A method of obtaining high polarization and pure spin states of impurity nuclei with a moderately strong quadrupole interaction in solid diamagnetic hosts whose nuclei have spin 1/2, a large g factor (like 1H and 19F), and a high degree of polarization is proposed. The method employs cross-relaxation transitions of the impurity nuclei with the host spins (with adiabatic variation of the external magnetic field) and simple radio-frequency pulses that invert the host nuclei or give rise to two-spin resonance of the host and impurity nuclei.