The phase diagram for the binary system K2CrO4CaCrO4

The phase diagram for the binary system K₂CrO₄CaCrO₄ has been determined for CaCrO₄ concentrations up to 60 mole%, using the techniques of differential thermal analysis, X-ray diffraction, and drop calorimetry. Essential features of the phase diagram are: the solid-solid phase transition for pure K₂CrO₄ at 670°C, β-K₂CrO₄ ? α-K₂CrO₄; a eutectoid reaction at 14 mole% CaCrO₄ and 548°C, β-K₂CrO₄ ? α-K₂CrO₄ + K₂CrO₄ · CaCrO₄; a peritectoid event at 50 mole% CaCrO₄ and 640°C, β-K₂CrO₄ + CaCrO₄ ? K₂CrO₄ · CaCrO₄; and a eutectic reaction at 51 mole% CaCrO₄ and 678°C, L ? β-K₂CrO₄ + CaCrO₄. X-ray diffraction studies lead to the determination of the unit cell dimensions for the K₂CrO₄ · CaCrO₄ double salt, a C-centered monoclinic form with a₀ = 7.615(6) Å, b₀ = 22.797(15) Å, c₀ = 9.777(9) Å, β = 115.45(5)°.     

The crystal structure of CuVo3(II), a high-pressure ilmenite phase

The crystal structure of a second high-pressure copper vanadate phase, CuVO₃(II), has been determined and refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.042 (R_w = 0.051). The space group is rhombohedral, $\text{R}\text{3}$, with hexagonal unit cell a = 4.966(2) and c = 14.084(5) Å [a_R = 5.501(2) Å and α = 53.66(3)°]. The structure is the fully ordered ilmenite-type and, on the basis of published magnetic data and the interatomic distances, the valence distribution Cu⁺V⁵⁺O₃ is proposed. This represents a unique example of Cu⁺ in an octahedral environment.     

Heat transfer from a rarefied plasma flow to a metallic or nonmetallic particle

Heat transfer from a plasma flow to a metallic or nonmetallic spherical particle is studied in this paper for the extreme case of free-molecule flow regime. Analytical expressions are derived for the heat flux due to, respectively, atoms, ions, and electrons and for the floating potential on the sphere exposed to a two-temperature plasma flow. It has been shown that the local or average heat flux density over the whole sphere is independent of the sphere radius and approximately in direct proportion to the gas pressure. The presence of a macroscopic relative velocity between the plasma and the sphere causes substantially nonuniform distributions of the local heat flux and enhances the total heat flux to the sphere. The heat flux is also enhanced by the gas ionization. Appreciable difference between metallic and nonmetallic spheres is found in the distributions along the oncoming flow direction of the floating potential and of the local heat flux densities due to ions and electrons. The total heat flux to the whole sphere is, however, almost the same for these different spheres. For a fixed value of the electron temperature, the heat flux decreases with increasing temperature ratio T_e/T_h.     

Electrical,magnetic, and EPR studies of the quaternary chalcogenides Cu2AIIBIVX4 prepared by iodine transport

Electrical, magnetic, and electron paramagnetic resonance (EPR) measurements have been made on crystals and powders of several quaternary chalcogenides of the type Cu₂A^IIB^IVX₄, where A^II = Zn, Mn, Fe, or Co, B^IV = Si, Ge, or Sn, and X = S or Se. The electrical properties of these compounds are extrinsic, but their magnetic properties do not appear to be affected by impurities. The magnetic moments of the Cu₂MnBX₄ compounds decrease with increasing covalency of the MnX bond, and those of Cu₂FeGeS₄ and Cu₂CoGeS₄ reflect an orbital contribution to the moment. Both the Weiss constants and magnetic ordering temperatures in these compounds show an evolution from antiferromagnetism to ferromagnetism with increasing separation between the moments. Magnetic measurements on single crystals of Cu₂MnGeS₄, Cu₂CoGeS₄, and Cu₂FeGeS₄ indicate that only the latter is anisotropic. EPR measurements on crystals and powders of Cu₂ZnGeS₄ doped nominally with 0.1% Mn reveal that Mn²⁺ experiences an axial distortion and that the bond ionicity is the same as in ZnS.     

Vinylidene complexes of transition metals : III. Derivatives of cyclopentadienyltricarbonyl rhenium with phenylvinylidene ligands. Crystal and molecular structure of Cp(CO)2Re[CC(Ph)C(Ph)CH2]Re(CO)2Cp

The novel complexes CpRe(CCHPh)(CO)₂ and Cp₂Re₂(μ-CCHPh)(CO)₄ containing a terminal and a bridging phenylvinylidene ligand respectively and the binuclear complex Cp(CO)₂Re[CC(Ph)C(Ph)CH₂]Re(CO)₂Cp were obtained in the reaction of CpRe(CO)₃ with PhCCH.According to an X-ray study of the latter complex the unusual bridging ligand is η¹-bonded to one Re atom and η²-bonded to the other.     

Electrode processes in the oxidation of tetrahydroisoquinoline derivatives

The electrochemical oxidation of the alkaloid laudanosine (Ia) to O-methylflavinantine (II) has been studied in acetonitrile solvent. Using cyclic voltammetry, rotating disc voltammetry and preparative electrolyses on several alkaloids, simple aliphatic amines and aromatic compounds, some aspects of the mechanism of this coupling reaction are elucidated. The first anodic wave for laudanosine at platinum has E_p=0.55 V vs. Ag/Ag⁺. The electrode rapidly becomes partially passivated at potentials above 0.5 V. This is due to a film which “dissolves” below 0.5 V, at a rate independent of the potential. It is shown that the reaction (Ia)→(II) proceeds at 0.5 V by initial oxidation of the amine moiety. If acids such as sodium bicarbonate are added to the anolyte the amine is protonated causing the first wave to disappear. Oxidation at 1.1 V under these acidic conditions produces the same product, but more rapidly and in significantly higher yield because electrode filming and side reactions resulting from the amine oxidation are abrogated.     

Some structural systematics in metal carbonyl cluster chemistry

The recently developed electron-counting rules for condensed polyhedral metal carbonyl clusters have been utilised to develop a systematic approach for enumerating the structural alternatives for metal cluster compounds which are isoelectronic. The more usual cluster degradation and aggregation processes are represented in a matrix form which relates the change in electron count to the number of new bonds formed (or broken). The matrices for cluster degradation and aggregation are represented in a complementary fashion so that the search for isoelectronic clusters greatly simplified. The utility of the approach is illustrated by reference to some recent examples from metal carbonyl cluster chemistry.     

Double asymmetric induction in benchrotrene derivatives

Attachment of a Cr(CO)₃ moiety to one of the aromatic rings in rigid diaryl ketones of very low prochirality followed by addition of a chiral and optically pure reagent leads to satisfactory asymmetric induction ~40%. Asymmetric induction is smaller in the case of acyclic systems.     

Cleavage by halogens of carbon—cobalt bonds in organometallic complexes of cobalt(III) : III. Kinetics and mechanism of the reactions of organobis(dimethylglyoximato)cobalt(III) complexes with iodine in chloroform

The kinetics of the reaction between organocobaloximes, RCo(DH)₂H₂O, and iodine have been investigated. They reveal the participation of an RCo(DH)₂H₂O · I₂ intermediate which undergoes intramolecular transalkylation and acts as an electrophile towards a second organocobaloxime molecule. The trend in reactivity as the R group is varied is discussed.     

On the molecular structure of cycloheptadiene

Molecular mechanics calculations on cycloheptadiene indicate that the molecule has a structure which undergoes a wide pseudorotational motion between two C₁ forms, and a C₁ form, and this structure is in equilibrium with the C₂ form. It is shown that this equilibrium mixture is consistent with all of the available experimental data.