High temperature stability of maghemite in partially oxidized basalt lava

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J_S-T) curves with Curie temperatures of about 580°C, has revealed considerable amounts of maghemite (γ-Fe₂O₃) in many samples. In view of the expected instability, of maghemite at temperatures above 350°C, this reversibility is rather surprising. Here we report Mössbauer studies on heated lava samples, showing high content of maghemite. The samples were kept at 600°C in oxidizing, reducing, and inactive atmospheres, respectively, for different lengths of time, and then analyzed with Mössbauer spectroscopy at room temperature. The Mössbauer spectra showed that maghemite is stable in the oxidizing atmosphere for at least several hours. In the inactive atmosphere a considerable amount of maghemite still exists after two hours heating. In the reducing atmosphere maghemite had transformed to magnetite after only 30 minutes.     

X‐Ray Diffraction and Mössbauer Spectroscopy Studies of Pressure‐Induced Phase Transitions in a Mixed‐Valence Trinuclear Iron Complex

The mixed‐valence complex Fe₃O(cyanoacetate)₆(H₂O)₃ ( 1 ) has been studied by single‐crystal X‐ray diffraction analysis at pressures up to 5.3(1) GPa and by (synchrotron) Mössbauer spectroscopy at pressures up to 8(1) GPa. Crystal structure refinements were possible up to 4.0(1) GPa. In this pressure range, 1 undergoes two pressure‐induced phase transitions. The first phase transition at around 3 GPa is isosymmetric and involves a 60° rotation of 50 % of the cyanoacetate ligands. The second phase transition at around 4 GPa reduces the symmetry from rhombohedral to triclinic. Mössbauer spectra show that the complex becomes partially valence‐trapped after the second phase transition. This sluggish pressure‐induced valence‐trapping is in contrast to the very abrupt valence‐trapping observed when compound 1 is cooled from 130 to 120 K at ambient pressure.     

Structures and Properties of Molecular Torsion Balances to Decipher the Nature of Substituent Effects on the Aromatic Edge‐to‐Face Interaction

Various recent computational studies initiated this systematic re‐investigation of substituent effects on aromatic edge‐to‐face interactions. Five series of Tröger base derived molecular torsion balances (MTBs), initially introduced by Wilcox and co‐workers, showing an aromatic edge‐to‐face interaction in the folded, but not in the unfolded form, were synthesized. A fluorine atom or a trifluoromethyl group was introduced onto the edge ring in ortho‐, meta‐, and para‐positions to the C?H group interacting with the face component. The substituents on the face component were varied from electron‐donating to electron‐withdrawing. Extensive X‐ray crystallographic data allowed for a discussion on the conformational behavior of the torsional balances in the solid state. While most systems adopt the folded conformation, some were found to form supramolecular intercalative dimers, lacking the intramolecular edge‐to‐face interaction, which is compensated by the gain of aromatic π‐stacking interactions between four aryl rings of the two molecular components. This dimerization does not take place in solution. The folding free enthalpy ΔG_fold of all torsion balances was determined by ¹H NMR measurements by using 10 mM solutions of samples in CDCl₃ and C₆D₆. Only the ΔG_fold values of balances bearing an edge‐ring substituent in ortho‐position to the interacting C?H show a steep linear correlation with the Hammett parameter (σ_meta) of the face‐component substituent. Thermodynamic analysis using van′t Hoff plots revealed that the interaction is enthalpy‐driven. The ΔG_fold values of the balances, in addition to partial charge calculations, suggest that increasing the polarization of the interacting C?H group makes a favorable contribution to the edge‐to‐face interaction. The largest contribution, however, seems to originate from local direct interactions between the substituent in ortho‐position to the edge‐ring C?H and the substituted face ring.     

Synthesis of Cyclic Alkenylsiloxanes by Semihydrogenation: A Stereospecific Route to (Z)‐Alkenyl Polyenes

Cyclic alkenylsiloxanes were synthesized by semihydrogenation of alkynylsilanes—a reaction previously plagued by poor stereoselectivity. The silanes, which can be synthesized on multigram scale, undergo Hiyama–Denmark coupling to give (Z)‐alkenyl polyene motifs found in bioactive natural products. The ring size of the silane is crucial: five‐membered cyclic siloxanes also couple under fluoride‐free conditions, whilst their six‐membered homologues do not, enabling orthogonality within this structural motif.