Mössbauer effect study of Gd- and Eu-doped CaSO4

Mössbauer spectra have been taken of rare earth doped CaSO₄ using the¹⁵¹Eu and¹⁵⁵Gd transitions. Site assignments have been made to the five principal Gd³⁺ configurations principally on the basis of point charge calculations. We were unable to distingiush different Eu³⁺ sites and no conversion of Eu³⁺ to Eu²⁺ could be observed after prolonged x-irradiation.     

Mössbauer study of the crystallization products of a Fe75Zr25 amorphous alloy

The crystallization of the Fe₇₅Zr₂₅ as-milled amorphous alloy has been studied by Mössbauer spectroscopy. The process begins at around 880 K with the complete and fast crystallization of the amorphous phase into the crystalline α-Fe and Fe₂Zr ones but, from around 915 K, there is a sudden phase transformation into the Fe₂₃Zr₆ phase, which remains stable after cooling. A fitting for the Mössbauer contribution of the Fe₂₃Zr₆ phase, consistent with its crystalline structural features, has been obtained.     

Mössbauer spectra of organotin amino-acid and glutathione derivatives

Mössbauer parameters are reported for compounds in which organotin entities are bonded to thiolate, carboxylate and sulfonate groups of simple ligands (L-cysteine and derivatives, DL-penicillamine, cysteic acid and Glutathione reduced) which have biological relevance. The relationship between the stereochemistry about the tin atom and the corresponding quadropole splitting value is examined and the extension to the use of Mössbauer spectroscopy to determine the binding of organotins in more complex biochemical systems is discussed.     

Structure and magnetism of a new pyrazolate bridged iron(II) spin crossover complex displaying a single HS-HS to LS-LS transition

The dinuclear iron(II) complex [(pypzH)(NCSe)Fe([micro sign]-pypz)(2)Fe(NCSe)(pypzH)].2H(2)O displays a single, sharp spin crossover transition between the [HS-HS] and [LS-LS] states and is structurally characterised above and below the T(1/2)= 225 K value     

Anion dependent redox changes in iron bis-terdentate nitroxide {NNO} chelates

The reaction of [Fe(II)(BF(4))(2)]·6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(?)), produces the mononuclear transition metal complex [Fe(II)(L(?))(2)](BF(4))(2) (1) which has been investigated using temperature dependent susceptibility, Mo?ssbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mo?ssbauer measurements reveal an octahedral low spin Fe(2+) environment where the pyridyl donors from L(?) coordinate equatorially while the oxygen containing the radical from L(?) coordinates axially forming a linear O(?)··Fe(II)··O(?) arrangement. Magnetic susceptibility measurements show a strong radical-radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe(2+) center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe(III)(L(-))(2)](BPh(4))·0.5H(2)O (2) has been fully characterized with Fe-ligand and N-O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L(-) form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.     

Exploring Problem‐based Learning in the Context of High School Science: Design and Implementation Issues

This paper reports on the experiences of a small collaborative inquiry group consisting of a high school science teacher, Deidre, and two university researchers, the authors of this paper, as they explored an active, inquiry‐based approach to teaching and learning referred to as Problem‐Based Learning or PBL ( Barrows, 1994 ; Barrows & Tamblyn, 1980 ). Although PBL is not new and has an established tradition in medical education and other professional schools, the use and scholarship of PBL at the secondary level is only starting to emerge. This small‐scale collaboration allowed the co‐inquirers to delve into the complexities of PBL and to examine its feasibility as a curriculum and instructional approach in the context of high school science teaching and learning. The three collaborators adopted an action‐based inquiry method referred to as Collaborative Inquiry (CI), a “process consisting of repeated episodes of reflection and action through which a group of peers strives to answer a question of importance to them” ( Bray, Lee, Smith, & Yorks, 2000 , p. 6). Data collection methods and sources included student‐generated work, classroom observation, student interviews, and audio‐taped planning meetings. The outcomes of the study focus on the issues that arose during PBL design and implementation, such as selecting a PBL topic, determining the level of structure to be incorporated into the PBL experience, selecting appropriate assessment approaches, facilitating groups, and providing optimal student feed‐back. In addition, outcomes related to student perceptions of PBL indicated that the majority liked learning through PBL because it promoted active learning, made science relevant, provided variety in learning, and supported group work. The authors discuss implications for the adoption of PBL in K‐12 settings.     

The encapsulation of ferrocyanide by copper(II) complexes of tripodal tetradentate ligands. Novel H-bonding networks incorporating heptanuclear and pentanuclear heterometallic assemblies

Substitution of the weakly binding aqua ligand in [Cu(tren)OH2](2+) and [Cu(tpa)OH2](2+) (tren = tris(2-aminoethyl)amine; tpa = tris(2-pyridylmethyl)amine) by a cyano ligand on ferricyanide results in the assembly of heteropolynuclear cations around the cyanometalate core. In water, the reduction of the Fe(III) core to Fe(II) generates complexes that feature heteropolycations in which ferrocyanide is encapsulated by the Cu(II) moieties: [(Cu(tpa)CN)6Fe][ClO4]8-3H2O 1, [(Cu(tren)CN)6Fe][ClO4]8-10H2O 2, [(Cu(tren)CN)6Fe][Fe(CN)6]2[ClO4]2-15.8H2O 3, and [(Cu(tren)CN)6Fe][(Cu(tren)CN)4Fe(CN)2][Fe(CN)6)]4-6DMSO-21H2O 4. The formation of discrete molecules, in preference to extended networks or polymeric structures, has been encouraged through the use of branched tetradentate ligands in conjunction with copper(II), a metal center with the propensity to form five-coordinate complexes. Complex 3 crystallizes in the monoclinic space group P2(1)/c (#14) with a = 14.8674(10), b = 25.9587(10), c = 27.5617(10) A, beta = 100.8300(10) degrees, and Z = 4, and it is comprised of almost spherical heptanuclear cations, [(Cu(tren)CN)6Fe](8+), whose charge is balanced by two ferricyanide and two perchlorate counteranions. Complex 4 crystallizes in the triclinic space group P1 (# 1) with a = 14.8094(8), b = 17.3901(7), c = 21.1565(11) A, alpha = 110.750(3), beta = 90.206(2), gamma = 112.754(3) degrees, and Z = 1, and it is comprised of the heptanuclear [(Cu(tren)CN)6Fe](8+) cation and pentanuclear [(Cu(tren)CN)4Fe(CN)2](4+) cation, whose terminal cyano ligands are oriented trans to each other. The charge is balanced exclusively by ferricyanide counteranions. In both complexes, H-bonding interactions between hydrogens on primary amines of the tren ligand, terminal cyano groups of the ferricyanide counterions, and the solvent of crystallization generate intricate 3D H-bonding networks.     

FeIII Quinolylsalicylaldimine Complexes: A Rare Mixed‐Spin‐State Complex and Abrupt Spin Crossover

A new synthesis of (8‐quinolyl)‐5‐methoxysalicylaldimine (Hqsal‐5‐OMe) is reported and its crystal structure is presented. Two Fe^III complexes, [Fe(qsal‐5‐OMe)₂]Cl ? solvent (solvent=2 MeOH ? 0.5 H₂O ( 1 ) and MeCN ? H₂O ( 2 )) have been prepared and their structural, electronic and magnetic properties studied. [Fe(qsal‐5‐OMe)₂] Cl ? 2 MeOH ? 0.5 H₂O ( 1 ) exhibits rare crystallographically independent high‐spin and low‐spin Fe^III centres at 150 K, whereas [Fe(qsal‐5‐OMe)₂]Cl ? MeCN ? H₂O ( 2 ) is low spin at 100 K. In both structures there are extensive π–π and C? H???π interactions. SQUID magnetometry of 2 reveals an unusual abrupt stepped‐spin crossover with T_1/2=245 K and 275 K for steps 1 and 2, respectively, with a slight hysteresis of 5 K in the first step and a plateau of 15 K between the steps. In contrast, 1 is found to undergo an abrupt half‐spin crossover also with a hysteresis of 10 K. The two compounds are the first Fe^III complexes of a substituted qsal ligand to exhibit abrupt spin crossover. These conclusions are supported by ⁵⁷Fe Mössbauer spectroscopy. Both complexes exhibit reversible reduction to Fe^II at ?0.18 V and irreversible oxidation of the coordinated qsal‐5‐OMe ligand at +1.10 V.     

Minerals,sediments and coals

A survey is presented of some of the recent uses of Mössbauer spectroscopy in studying chemical changes in mineral systems. The changes considered include those occurring naturally, such as weathering, leaching, soil formation, lake and river sedimentation and coal oxidation, and some induced by man, such as chemical leaching, precipitate ageing, coal hydroliquefaction and ash resulting from coal combustion.