Magnetic field induced dipolar couplings in the pretransitional region of a nematic liquid crystal

13C and 1H NMR spectroscopy have been used to study the orientational order which develops when a nematogenic compound, 4-pentyl-4'-cyanobiphenyl (5CB), approaches the transition from the isotropic to the nematic phase at T(NI). The experiments yield values of field induced dipolar couplings, (1), between all of the directly bonded carbon and hydrogen nuclei in the molecule, and 2D(HH)B, the geminal dipolar coupling between protons in the first methylene group in the alkyl chain. The temperature dependence of these couplings shows that in each case they follow a divergence behavior governed by (T - T*)(-1),where T* is a temperature determined from the experimental data and which is close to but less than T(NI). Experiments performed at spectrometer field strengths of 11.75 and 18.79 T confirm the prediction that the induced couplings should depend on the square of the applied field strength. It was found that, within experimental error, T* is the same for each field-induced coupling, and that T(NI) - T* is the same at 11.75 and 18.79 T. It is shown that the set of field-induced couplings 1D(CHi)B obtained at a temperature close to T(NI) can be used to derive a conformer distribution for 5CB in the isotropic phase.     

Two aluminotriphosphates with closely related intersecting tunnel structures involving tetrahedral “AlP” chains and layers: AAl3(P3O10)2, A=Rb, Cs

Two new aluminotriphosphates, RbAl₃(P₃O₁₀)₂ and CsAl₃(P₃O₁₀)₂, were synthesized by solid-state reaction. They crystallize in non-centrosymmetric space groups: C222₁ with , , , (Z=4) for RbAl₃(P₃O₁₀)₂ and C2ce with , , , (Z=4) for CsAl₃(P₃O₁₀)₂. Their 3D frameworks, built up of corner sharing P₃O₁₀ groups, AlO₄ tetrahedra and AlO₆ octahedra, exhibit several remarkable features. The AlO₄ tetrahedra and P₃O₁₀ groups are directly associated through the corners, forming helical columns in the Rb-phase and “helicoid” layers in the Cs-phase. The simultaneous presence of AlO₄ and AlO₆ species, rather rare in phosphates, leads to the formation of closely related [Al₃P₆O₂₄]_∞ layers in both structures, which differ by their stacking mode. The presence of intersecting tunnels running along 〈110〉 and [001] directions, with Rb⁺ and Cs⁺ sitting at the intersections, shows the opened character of these two structures.     

End‐group modified poly(methyl vinyl ether): Characterization and LCST demixing behavior in water

A range of hydrophilic poly(methyl vinyl ether) (PMVE) polymers was synthesized by living cationic polymerization of methyl vinyl ether (MVE), having different hydrophilic or hydrophobic chain‐end functionalities. The dissimilar end‐groups were either introduced by end‐capping of the growing polymer chain with LiBH₄, methanol, and water or by functional initiation with 2‐bromo‐(3,3‐diethoxy‐propyl)‐2‐methylpropanoate. The synthesized PMVEs were characterized by ¹H NMR, size exclusion chromatography, and matrix‐assisted laser desorption ionization time of flight, displaying a narrow polydispersity. Modulated temperature DSC was applied to study the influence of the nature of the end‐groups on the solubility behavior of PMVE in water. Terminal‐modification with a hydroxyl function improves the solubility, whereas a Br‐containing end‐group causes the polymer to be insoluble in water at room temperature; however, the special type III lower critical solution temperature demixing behavior being maintained. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 461–469, 2006     

Study by Raman spectromicroscopy of the effect of tensile deformation on the molecular structure of Bombyx mori silk

The effect of mechanical deformation on the polarized Raman spectra of a silk monofilament of the silkworm Bombyx mori has been studied in order to detect tensile stress-induced changes of the fibroin conformation and reorientation up to the breaking point. For this study, a motorized mechanical stretcher has been specifically designed to fit under the Raman microscope, thus allowing the simultaneous recording of the stress, strain and polarized Raman spectra. Due to its twin drive screw design, this stretcher allows to stretch the silk fiber symmetrically and thus to probe the same area of the fiber during the entire elongation procedure. The results obtained have allowed to clarify the assignment of some vibrational bands of silk. They also show that the intensity of some Raman bands is affected by tensile deformation. In particular, the amide III/amide I intensity ratio decreases as the strain increases and the variation is proportional to the stress applied on the fiber. These variations in intensity suggest that the alignment of the protein chains decreases with strain which might be due to the reorganization of the amorphous phase. The Raman spectra also show that several bands shift toward low wavenumbers as the strain applied increases indicating that the force constant and/or dihedral angles of the peptide bonds, in particular those involved in the β-sheets, are affected by drawing. Therefore, a local stress is directly applied on the β-sheets and the wavenumber shift observed is proportional to the stress applied on the fiber.     

The solubility of H2S in liquid sulfur

The need for fundamental data describing the dissolution of hydrogen sulfide, H₂S, in Claus recovered liquid sulfur prompted an examination of equilibrium H₂S solubility at typical industrial condensation temperatures and partial pressures. An FT/IR absorption technique has been described and new H₂S solubility measurements have been reported for partial H₂S pressures from 0.4 to 56 kPa and temperatures from 120 to 155 °C. The measurements were combined with previously reported values for atmospheric pressures of H₂S and used to calibrate a semi-empirical equation for the Henry's law solubility in liquid sulfur as a function of temperature. Estimations were compared to Claus plant field data and appear to underestimate the H₂S solubility for the first and second catalyst condenser stages. This underestimation was attributed to rapid condensation resulting in super-saturation of the recovered sulfur liquid, or an inaccurate measurement of the condensation temperatures.     

Internal energy distribution in electrospray ionization

Internal energies and energy distributions were studied using the 'survival yield' method developed previously. In addition to conventional benzylpyridinium salts, protonated esters (fragmenting by rearrangement) and protonated leucine enkephalin were also used, extending the validity of the technique. Fragmentation processes were studied in the cone voltage region and modeled by the RRKM-based MassKinetics program. The results show that the shapes of the energy distributions are similar to thermal distributions. The mean internal energies are very similar for all compound classes studied, and show a linear increase with collision energy in the 10-50 eV region.     

Chemical shift correlations in disordered solids

We show how two-dimensional chemical shift conditional probability distributions can be extracted from experimental NMR correlation spectra of disordered solids. We show that transverse dephasing times are of central importance in determining the resolution (and sensitivity) of these probability distributions. These conditional probability distributions provide a new source of structural information characteristic of disordered solids, which is much more sensitive to structure than the individual-atom chemical shift distributions. The structural information contained in these distributions is clearly a potentially extremely rich source for understanding disorder. This is illustrated with examples of a phosphorus-containing organic compound and with a sample of disordered cellulose where the different structural allomorphs present are identified from the distributions.     

Structural isomer differentiation of 1,6-diaza-3,8-dioxabicyclo[4.4.1]undecane and N,N'-methylenebis(oxazolidine) by mass spectrometry

The title compound types are structural isomers. The absence of literature studies dealing with the mass spectrometric gas-phase behavior of such compounds prompted a search for the best conditions for differentiation between the two prototype examples (R,R)-5,10-diphenyl-1,6-diaza-3,8-dioxabi-cyclo[4.4.1]undecane and N,N'-methylenebis[(R)-4-phenyloxazolidine]. Attempts to differentiate between the isomers by NMR were inconclusive and X-ray crystallography had to be used. The best mass spectrometric results were obtained using gas-phase ionization techniques, particularly chemical ionization, whereby protonated molecules were observed. The fragmentation pathways were elucidated from MS/MS studies and from experiments performed on hexadeuterated samples of each compound.     

Silica-Grafted Tris(neopentyl)aluminum: A Monomeric Aluminum Solid Co-catalyst for Efficient Nickel-Catalyzed Ethene Dimerization

A silica-supported monomeric alkylaluminum co-catalyst was prepared via surface organometallic chemistry by contacting tris(neopentyl)aluminum and partially dehydroxylated silica. This system, fully characterized by solid-state ²⁷Al NMR spectroscopy augmented by computational studies, efficiently activates (ⁿBu₃P)₂NiCl₂ towards dimerization of ethene, demonstrating comparable activity to previously reported dimeric diethylaluminum chloride supported on silica. Three types of aluminum surface species have been identified: monografted tetracoordinated Al species as well as two types of bisgrafted Al species—tetra- and pentacoordinated. Of them, only the monografted Al species is proposed to be able to activate the (ⁿBu₃P)₂NiCl₂ complex and generate the active cationic species.