Selective substitution of vanadium for molybdenum in Sr2(Fe1−xVx)MoO6 double perovskites

The crystal and magnetic structures of Sr₂(Fe_1−xV_x)MoO₆ (0.03?x?0.1) compounds are refined by alternately using X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) data collected at room temperature. The refinement results reveal that the V atoms selectively occupy the Mo sites instead of the Fe sites for x?0.1. The 3d/4d cation ordering decreases with the increase of the V content. Slight distortions in the lattice and metal octahedra are shown at 300 K, and the distortions increase at 4 K. The magnetic structure at 4 K can be modeled equally well with the moments aligning along [001], [110] or [111] directions. The total moments derived from the NPD data for the [110] and [111] direction models agree well with the magnetic measurements, whereas the [001] model leads to a smaller total moment. Bond valence analysis indicates that Sr ions are properly located in the structure and Mo ions are compatible with both the Fe sites and the Mo sites. The electronic effects are suggested to be responsible for the selective occupation of the V on the Mo sites due to the different distortions of the FeO₆ and MoO₆ octahedra.     

Models for polyurethane hydrolysis under moderately acidic conditions: A comparative study of hydrolysis rates of urethanes,ureas, and amides

The hydrolysis of model structures found in the hard blocks of polyurethanes has been studied under moderately acidic and neutral conditions. These include a series of N-alkyl and N-phenyl urethanes and ureas; for comparison purposes the corresponding amides and a carbonate ester based on bisphenol A were also studied. Of all functional groups the urethane was the most resistant to hydrolysis and the amide the most labile. Also, the N-phenyl compounds were hydrolyzed faster than the corresponding alkyl derivatives. The carbonate ester underwent relatively rapid hydrolysis. Polyether containing polyurethanes, known to be relatively stable to hydrolysis, are discussed as to which polymer segments provide the weakest links under hydrolytic conditions.     

An electrochemical study of enzymatic oligonucleotide digestion

This paper describes the synthesis and application of a novel ferrocene (Fc) label that can be efficiently attached to oligonucleotides. We demonstrate how pulse electrochemical methods can be used to measure very low concentrations of ferrocene label and, importantly, show good electroanalytical discrimination between a labelled oligonucleotide and an enzyme digested labelled oligonucleotide, in which the ferrocene label nucleotide conjugate has been released. Real time in situ analysis gives a much greater understanding of the process. Potential applications include the detection of specific nucleic acid sequences and measurement of nuclease activity.     

Polyurethane elastomers with hydrolytic and thermooxidative stability. I. Polyurethanes with N-alkylated polyamide soft blocks

Segmented polyurethanes with N-alkylated amides as soft blocks as prepared. Comparisons are made with both a poly(ester urethane) and a poly(ether urethane) with the same hard block; the poly(amide urethane) is more hydrolytically stable than the polyester containing material and demonstrates greater thermooxidative stability than that with the polyether moiety. The aliphatic poly(amide urethane)s remain transparent upon exposure to uv light.     

Fluorocyclohexanes-II Cis- and trans- 1H:3H- and 1H:4H-decafluorocyclohexanes

An extension of the methods employed in the isolation of (trans) 1H/2H-decafluorocyclohexane,¹ (I) from the polyfluorocyclohexane mixture obtained by the vapour phase fluorination of benzene with cobaltic fluoride at about 150°, has afforded the four remaining members of the series of decafluorocyclohexanes [the cis- and trans-1H:3H- and 1H:4H-isomers (1H:3H/-(IV), 1H/3H-(III), 1H:4H/-(VII), and 1H/4H-(VIII), respectively)] and also the cis-1H:2H-decafluorocyclohexane (II), obtained previously^1,2 by the lithium aluminium hydride reduction of 1:2-dichlorodecafluorocyclohexane. The structures of the 1H:3H- and 1H:4H-decafluorides have been established by dehydrofluorination studies. The six decafluorocyclohexanes have been related to two new nonafluorocyclohexanes³ (IX and X) by further fluorination of the latter. 2H-Heptafluoroadipic acid has been obtained from 3H-nonafluorocyclohex-1-ene (V), one of the dehydrofluorination products of the 1H:3H-decafluorides.     

Inhibition of [FeFe]-hydrogenase by formaldehyde: proposed mechanism and reactivity of FeFe alkyl complexes

The mechanism for inhibition of [FeFe]-hydrogenases by formaldehyde is examined with model complexes. Key findings: (i) CH₂ donated by formaldehyde covalently link Fe and the amine cofactor, blocking the active site and (ii) the resulting Fe-alkyl is a versatile electrophilic alkylating agent. Solutions of Fe₂[(μ-SCH₂)₂NH](CO)₄(PMe₃)₂ (1) react with a mixture of HBF₄ and CH₂O to give three isomers of [Fe₂[(μ-SCH₂)₂NCH₂](CO)₄(PMe₃)₂]⁺ ([2]⁺). X-ray crystallography verified the NCH₂Fe linkage to an octahedral Fe(ii) site. Although [2]⁺ is stereochemically rigid on the NMR timescale, spin-saturation transfer experiments implicate reversible dissociation of the Fe–CH₂ bond, allowing interchange of all three diastereoisomers. Using ¹³CH₂O, the methylenation begins with formation of [Fe₂[(μ-SCH₂)₂N¹³CH₂OH](CO)₄(PMe₃)₂]⁺. Protonation converts this hydroxymethyl derivative to [2]⁺, concomitant with ¹³C-labelling of all three methylene groups. The Fe–CH₂N bond in [2]⁺ is electrophilic: PPh₃, hydroxide, and hydride give, respectively, the phosphonium [Fe₂[(μ-SCH₂)₂NCH₂PPh₃](CO)₄(PMe₃)₂]⁺, 1, and the methylamine Fe₂[(μ-SCH₂)₂NCH₃](CO)₄(PMe₃)₂. The reaction of [Fe₂[(μ-SCH₂)₂NH](CN)₂(CO)₄]²⁻ with CH₂O/HBF₄ gave [Fe₂[(μ-SCH₂)₂NCH₂CN](CN)(CO)₅]⁻ ([4]⁻), the result of reductive elimination from [Fe₂[(μ-SCH₂)₂NCH₂](CN)₂(CO)₄]⁻. The phosphine derivative [Fe₂[(μ-SCH₂)₂NCH₂CN](CN)(CO)₄(PPh₃)]⁻ ([5]⁻) was characterized crystallographically.

The mechanism for inhibition of [FeFe]-hydrogenases by formaldehyde is examined with model complexes.     

Low-Temperature Sputtered Ultralow-Loss Silicon Nitride for Hybrid Photonic Integration

Silicon-nitride-on-insulator (Si₃N₄) photonic circuits have seen tremendous advances in many applications, such as on-chip frequency combs, Lidar, telecommunications, and spectroscopy. So far, the best film quality has been achieved with low pressure chemical vapor deposition (LPCVD) and high-temperature annealing (1200°C). However, high processing temperatures pose challenges to the cointegration of Si₃N₄ with pre-processed silicon electronic and photonic devices, lithium niobate on insulator (LNOI), and Ge-on-Si photodiodes. This limits LPCVD as a front-end-of-line process. Here, ultralow-loss Si₃N₄ photonics based on room-temperature reactive sputtering is demonstrated. Propagation losses as low as 5.4 dB m⁻¹ after 400°C annealing and 3.5 dB m⁻¹ after 800°C annealing are achieved, enabling ring resonators with highest optical quality factors of > 10 million and an average quality factor of 7.5 million. To the best of the knowledge, these are the lowest propagation losses achieved with low temperature Si₃N₄. This ultralow loss enables the generation of microresonator soliton frequency combs with threshold powers of 1.1 mW. The introduced sputtering process offers full complementary metal oxide semiconductor (CMOS) compatibility with front-end silicon electronics and photonics. This could enable hybrid 3D integration of low loss waveguides with integrated lasers and lithium niobate on insulator.     

螺二邻二亚甲基噻吩和环二邻二亚甲基噻吩的合成

Errede等曾利用氢氧化邻甲基苄基三甲基铵的热解制得螺二邻二亚甲基苯、环二邻二亚甲基苯和聚邻二亚甲基苯。Winberg等人对噻吩的季铵碱的热解有所研究,但未能触及活泼的中间体2,3-二亚甲基-2,3-二氢化噻吩Ⅷ及其二聚体螺环化合物和环状化合物。     

High-Temperature Pulsed-Field-Gradient Multidimensional NMR of Polymers

The application of pulsed-field-gradient (PFG) techniques has been particularly important in providing the ability to detect 2D and 3D NMR cross peaks from minor structural components in synthetic organic polymers. The lack of mobility in a large percentage of polymers leads to rapid T2 relaxation which prevents the use of pulse sequences, such as the HMBC experiment, that operate based on coherence transfer via small, long-range J couplings. High-temperature NMR increases molecular motion with corresponding line narrowing (e.g., polyethylenes are typically analyzed at 120 degrees C). However, until now, the requirement for high temperature has precluded the use of PFG methods. Here we present data from a new probe which is capable of performing high-temperature PFG coherence selection experiments at temperatures typical of those used in many polymer analyses. We illustrate the performance of this probe with PFG-HMBC spectra of a copolymer from ethylene/1-hexene/1-butene at 120 degrees C.     

Companion forms over totally real fields

We show that if F is a totally real field in which p splits completely and f is a mod p Hilbert modular form with parallel weight 2 < k < p, which is ordinary at all primes dividing p and has tamely ramified Galois representation at all primes dividing p, then there is a “companion form” of parallel weight k′ := p + 1 − k. This work generalises results of Gross and Coleman–Voloch for modular forms over Q.