Vibrational spectra of cyclooctatetraenyl alkali metal complexes [C8H8M2 (M = Na, K)]: A theoretical study

The cyclooctatetraenyl dianion (C₈H₈²⁻) π-conjugated system forms a stable complex system with alkali and some transition metals. The results of vibrational analysis for C₈H₈M₂ (M = Na, K) complexes were reported here. The geometries of C₈H₈M₂ (M = Na, K) were optimized using ab initio (HF, MP2, CCSD(T)) and DFT (B3LYP) methods with 6-311G** and 6-311++G** basis sets and the harmonic frequencies were obtained. To reproduce and compare with the experimental values the structurally similar molecules C₅H₅M (M = Na, K) and benzene were studied. The scale factors obtained from these systems were applied to predict the experimental frequencies of C₈H₈M₂ (M = Na, K). The force field and vibrational spectra are analyzed and the most probable assignments are proposed for all the fundamentals based on the potential energy distribution.     

Syntheses of group 7 metal carbonyl complexes with a stable N-heterocyclic chlorosilylene

Two structurally characterized manganese [L(2)Mn(CO)(4)](+)[Mn(CO)(5)](-) (1) and rhenium [L(3)Re(CO)(3)](+)[ReCO)(5)](-) (2) silylene complexes were prepared in one pot syntheses by reacting 1 equivalent of Mn(2)(CO)(10) with 2 equivalents of stable N-heterocyclic chlorosilylene L {L = PhC(NtBu)(2)SiCl} and 1 equivalent of Re(2)(CO)(10) with 3 equivalents of L in toluene at room temperature. Both complexes 1 and 2 were characterized by single-crystal X-ray structural analysis, NMR and IR spectroscopy, EI-MS spectrometry, and elemental analysis.     

Giant flexoelectricity in polyvinylidene fluoride films

Recent studies have shown that giant flexoelectricity may exist in certain elastomers with bent-core molecular structures, which contradicts the previous theoretical estimation that the flexoelectric coupling is small in those materials. In this Letter, we report an analogous phenomenon, i.e., the giant direct flexoelectric effect, observed in a polyvinylidene fluoride (PVDF) film. Our experimental studies indicate that such an enhanced flexoelectric effect might be induced by the interaction between the energy couplings of the apolar and the polar molecular structures of the polymer film under elastic deformation.     

Fiber-based Bessel beams with controllable diffraction-resistant distance

We experimentally generate n=0 Bessel beams via higher-order cladding mode excitation with a long period fiber grating. Our method allows >99% conversion efficiency, wide or narrow conversion bandwidth, and accurate control of the number of rings in the beam. This latter property is equivalent to tuning the beam cone angle and allows for control of width and propagation distance of the center spot. We generate Bessel-like beams from LP(0,5) to LP(0,15) cladding modes and measure their propagation-invariant characteristics as a function of mode order, which match numerical simulations and a simple geometric model. This yields a versatile tool for tuning depth of focus out of fiber tips, with potential uses in endoscopic microscopy.     

THE 31P NMR SPECTRA OF ISOMERIC DIAMINOHEXA-CHLOROCYCLOTETRAPHOSPHAZATETRAENES; EXAMPLES OF A2B2 AND AA'BB' SPIN SYSTEMS

Abstract

The ³¹P nmr spectra of 2,4- and 2,6-diamino-derivatives of octachlorocyclotetraphosphazatetraene, N₄P₄Cl₆(NR¹R²)₂ (R¹ = H, R² = Bu^t; R¹ = H, R² = CH₂Ph; R¹ = Me, R² = Ph), have been measured. The 2,4- and 2,6-isomers were analysed as AA'BB' and A₂B₂ spin systems respectively. In the 2,4-isomers the spin-spin couplings ²J(PNP) and ⁴J(PNPNP) were of opposite sign.     

Identification and Characterization of the Yellow Pigment Synthesized by Cupriavidus sp. USMAHM13

Microbial pigments are gaining intensive attention due to increasing awareness of the toxicity of synthetic colours. In this study, a novel polymer-producing bacterium designated as Cupriavidus sp. USMAHM13 was also found to produce yellow pigment when cultivated in nutrient broth. Various parameters such as temperature, pH and ratio of culture volume to flask volume were found to influence the yellow pigment production. UV-Visible, Fourier transform infrared and ¹³C-nuclear magnetic resonance analyses revealed that the crude yellow pigment might probably represent new bioactive compound in the carotenoid family. The crude yellow pigment also exhibited a wide spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria with their inhibition zones and minimal inhibitory concentrations ranged from 25 to 38 mm and from 0.63 to 2.5 mg/ml, respectively. To the best of our knowledge, this is the first report on the identification and characterization of yellow pigment produced by bacterium belonging to the genus Cupriavidus.     

Using Surface Segregation To Design Stable Ru‐Ir Oxides for the Oxygen Evolution Reaction in Acidic Environments

The methods used to improve catalytic activity are well‐established, however elucidating the factors that simultaneously control activity and stability is still lacking, especially for oxygen evolution reaction (OER) catalysts. Here, by studying fundamental links between the activity and stability of well‐characterized monometallic and bimetallic oxides, we found that there is generally an inverse relationship between activity and stability. To overcome this limitation, we developed a new synthesis strategy that is based on tuning the near‐surface composition of Ru and Ir elements by surface segregation, thereby resulting in the formation of a nanosegregated domain that balances the stability and activity of surface atoms. We demonstrate that a Ru_0.5Ir_0.5 alloy synthesized by using this method exhibits four‐times higher stability than the best Ru‐Ir oxygen evolution reaction materials, while still preserving the same activity.     

Radio frequency-driven recoupling at high magic-angle spinning frequencies: homonuclear recoupling sans heteronuclear decoupling

We describe solid-state NMR homonuclear recoupling experiments at high magic-angle spinning (MAS) frequencies using the radio frequency-driven recoupling (RFDR) scheme. The effect of heteronuclear decoupling interference during RFDR recoupling at high spinning frequencies is investigated experimentally and via numerical simulations, resulting in the identification of optimal decoupling conditions. The effects of MAS frequency, RF field amplitude, bandwidth, and chemical shift offsets are examined. Most significantly, it is shown that broadband homonuclear correlation spectra can be efficiently obtained using RFDR without decoupling during the mixing period in fully protonated samples, thus considerably reducing the rf power requirements for acquisition of (13)C-(13)C correlation spectra. The utility of RFDR sans decoupling is demonstrated with broadband correlation spectra of a peptide and a model protein at high MAS frequencies and high magnetic field.     

1H and 13C NMR spectral assignments of some novel 2,4,6,8-tetraaryl-3,7-diazabicyclo[3.3.1]nonan-9-one derivatives

The (1)H and (13)C NMR spectra of 2,4,6,8-tetraaryl-3,7-diazabicyclo[3.3.1]nonan-9-ones (1-2), oximes (3-8) and O-benzyl oximes (9-12) were recorded. The chemical shifts were unambiguously assigned using 1D and 2D NMR spectral data. The results clearly indicate that the compounds exist in chair-boat conformation with equatorial and axial orientation of the aryl groups in the chair and boat forms, respectively. Since the molecules are flexible and dynamic in solution, the chair and boat forms are mutually interconvertible. In 3-12, because of the effect of oximation/oximination, all the protons in the heterobicyclic systems gave distinct signals except the benzylic protons of the boat form. In all synthesized compounds, the aryl group protons at C-6,8 are shielded by the aryl groups at C-2,4 and therefore appear in the lower frequency region than the aryl groups at C-2,4.