Practical aspects of carbon content determination in carburized steels by EPMA.

The carbon contents in carburized steels were investigated by electron probe microanalysis (EPMA) for a range of carbon levels in the solid solution less than 1 wt%. This article describes the difficulties encountered with the classic analytical procedure using the k ratio of X-ray intensities and the phi(rhoz) model. Here, a suitable calibration curve method is presented with emphasis on the metallographic study of standard specimens and on the carbon decontamination of samples.     

Synthesis of gamma,delta-unsaturated and delta,epsilon-unsaturated alpha-amino acids from fragmentation of gamma- and delta-lactones

A noncoded amino acid of cyclomarin A (1) was synthesized in a racemic fashion. The method employs a six-membered ring template to control the relative stereochemistry and introduction of the functional groups. Ultimately, Pd-catalyzed fragmentation of the lactone provided gamma,delta-unsaturated and delta,epsilon-unsaturated alpha-amino acids. A Pd-catalyzed ring opening of a gamma-lactone is also reported.     

Synthèse en monocristaux et structure du monouranate de sodium β-Na2UO4

β-Na₂UO₄ single crystals were synthesized from a melt with U, Nb, and Na ions. They are orthorhombic Fmmm with a = 5.802(2), B = 5.969(2), and c = 11.699(3) Å, and Z = 4. The structure was refined to R = 0.075 and R_w = 0.096 for 603 independent reflexions with I ≥ 3σ(I). The study shows that Na and O atoms are delocalized and that the structure results from the juxtaposition of four P2₁/b domains.     

Synthesis and properties of fluorine-containing heterocyclic compounds. V. Trifluoromethyl-1,8- and 1,10-phenanthrolines

The condensation of ethyl trifluoroacetoacetate with various heterocyclic amines yielded 2-(trifluoromethyl)phenanthrolin-4-ones. The structures of these compounds were established from a study of their infrared and ultraviolet spectra. Substituted phenanthrolines were prepared by nucleophilic attack on a chlorophenanthroline. The oxidation of 5-methoxy-2-trifluoromethyl-1,10-phenanthroline yielded an o-quinone which was treated with typical quinone reagents. A molecular orbital calculation was carried out in an attempt to explain the ease of reduction of the 1,10-phenanthroline system and the colors of the reduced compounds.     

Affinement de la structure de TiNb2O7 et répartition des cations

TiNb₂O₇ is monoclinic $\text{C2}\text{m}$ with a = 20.351(3), b = 3.801(2), c = 11.882(2) Å, β = 120.19°, Z = 6. The structure, determined by Wadsley in 1961 from visual estimation of intensities, has been refined to R = 0.065 for 915 reflexions with I ≥ 3σI. This study shows that Ti and Nb are not randomly distributed: like several niobo or tantalotitanates with edge-sharing octahedra, this one shows preferential occupation of one type of site by the titanium atoms.     

The first structural and spectroscopic characterization of a neptunyl polyoxometalate complex

The reaction between PW9O349- and NpO2+ has yielded the first structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analogue, and there is also spectroscopic evidence for its existence in solution. The complex is readily extracted into toluene, and this may have significance in the sequestering and/or separation of the neptunyl ion in terms of nuclear waste management.     

A capping methodology for the synthesis of lower mu-oxo-phthalocyaninato silicon oligomers

A mixed condensation of a substituted and unsubstituted silicon phthalocyanine monomer, in which the former serves as a "cap" to a growing oligomeric chain, provides unprecedented access to a mixed dimer, trimers, and a tetramer. An important element of product control can be achieved by changing the ratio of the precursor monomers.     

Two (E)‐2‐arylidene‐1,4‐di‐p‐tosyl‐1,2,3,4‐tetrahydroquinoxaline compounds: supramolecular frameworks built with C—H...O and C—H...π(arene) hydrogen bonds and π–π stacking interactions

The pyrazine ring in two N‐substituted quinoxaline derivatives, namely (E)‐2‐(2‐methoxybenzylidene)‐1,4‐di‐p‐tosyl‐1,2,3,4‐tetrahydroquinoxaline, C₃₀H₂₈N₂S₂O₅, (II), and (E)‐methyl 2‐[(1,4‐di‐p‐tosyl‐1,2,3,4‐tetrahydroquinoxalin‐2‐ylidene)methyl]benzoate, C₃₁H₂₈N₂S₂O₆, (III), assumes a half‐chair conformation and is shielded by the terminal tosyl groups. In the molecular packing of the compounds, intermolecular C—H...O hydrogen bonds between centrosymmetrically related molecules generate dimeric rings, viz. R₂²(22) in (II) and R₂²(26) in (III), which are further connected through C—H...π(arene) hydrogen bonds and π–π stacking interactions into novel supramolecular frameworks.     

Pushing the limits of the hydrogen bond enhanced halogen bond—the case of the C–H hydrogen bond

C–H hydrogen bonds have remarkable impacts on various chemical systems. Here we consider the influence of C–H hydrogen bonds to iodine atoms. Positioning a methyl group between two iodine halogen bond donors of the receptor engendered intramolecular C–H hydrogen bonding (HBing) to the electron-rich belt of both halogen bond donors. When coupled with control molecules, the role of the C–H hydrogen bond was evaluated. Gas-phase density functional theory studies indicated that methyl C–H hydrogen bonds help bias a bidentate binding conformation. Interaction energy analysis suggested that the charged C–H donors augment the halogen bond interaction—producing a >10 kcal mol⁻¹ enhancement over a control lacking the C–H⋯I–C interaction. X-ray crystallographic analysis demonstrated C–H hydrogen bonds and bidentate conformations with triflate and iodide anions, yet the steric bulk of the central functional group seems to impact the expected trends in halogen bond distance. In solution, anion titration data indicated elevated performance from the receptors that utilize C–H Hydrogen Bond enhanced Halogen Bonds (HBeXBs). Collectively, the results suggest that even modest hydrogen bonds between C–H donors and iodine acceptors can influence molecular structure and improve receptor performance.

C–H hydrogen bonds to iodine halogen bond donors are shown to improve halogen bonding and molecular preorganization.     

Coordination-induced bond weakening of water at the surface of an oxygen-deficient polyoxovanadate cluster

Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V₆O₅(OH₂)(OMe)₈ (calix = 4-tert-butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O–H)_avg of the aquo ligand as 62.4 ± 0.2 kcal mol⁻¹, indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton–electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.

Coordination-induced bond weakening facilitates H-atom transfer from polyoxovanadate surface.