Transport in amorphous solid water films: implications for self-diffusivity

Thermal desorption spectroscopy is employed to examine transport mechanisms in structured, nanoscale films consisting of labeled amorphous solid water (ASW, H(2)(18)O, H(2)(16)O) and organic spacer layers (CCl(4), CHCl(3)) prior to ASW crystallization (T approximately 150-160 K). Self-transport is studied as a function of both the ASW layer and the organic spacer layer film thickness, and the effectiveness of these spacer layers as a bulk diffusion "barrier" is also investigated. Isothermal desorption measurements of structured films are combined with gas uptake measurements (CClF(2)H) to investigate water self-transport and changes in ASW film morphology during crystallization and annealing. CCl(4) desorption is employed as a means to investigate the effects of ASW film thickness and heating schedule on vapor-phase transport. Combined, these results demonstrate that the interlayer mixing observed near T approximately 150-160 K is inconsistent with a mechanism involving diffusion through a dense phase; rather, we propose that intermixing occurs via vapor-phase transport through an interconnected network of cracks/fractures created within the ASW film during crystallization. Consequently, the self-diffusivity of ASW prior to crystallization (T approximately 150-160 K) is significantly smaller than that expected for a "fragile" liquid, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.     

Structure of a surfactant-templated silicate framework in the absence of 3d crystallinity

The structure of a novel molecularly ordered two-dimensional (2D) silicate framework in a surfactant-templated mesophase has been established by using a combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction, and quantum chemical and empirical force-field modeling. These materials are unusual in their combination of headgroup-directed 2D crystalline framework ordering, zeolite-like ring structures within the layers, and long-range mesoscopic organization without three-dimensional (3D) atomic periodicity. The absence of registry between the silicate sheets, resulting from the liquidlike disorder of the alkyl surfactant chains, has presented significant challenges to the determination of framework structures in these and similar materials lacking 3D crystalline order. Double-quantum (29)Si NMR correlation experiments establish the interactions and connectivities between distinct intra-sheet silicon sites from which the structure of the molecularly ordered inorganic framework is determined.     

Stability of Gorenstein categories

We show that an iteration of the procedure used to define theGorenstein projective modules over a commutative ring R yieldsexactly the Gorenstein projective modules. Specifically, givenan exact sequence of Gorenstein projective R-modules

such that the complexes Hom_R(G, H) and Hom_R(H,G) are exact for each Gorenstein projective R-module H, themodule Coker() is Gorensteinprojective. The proof of this result hinges upon our analysisof Gorenstein subcategories of abelian categories.     

Second Dissociation Constant of Bis-[(2-hydroxyethyl)amino]acetic Acid (BICINE) and pH of Its Buffer Solutions from 5 to 55 °C

The values of the second dissociation constant, K ₂, and related thermodynamic quantities of the ampholyte bis[(2-hydroxyethyl)amino]acetic acid (BICINE) have been determined at temperatures from 5 to 55 ^∘C. The pH values of six equimolal buffer solutions, and four buffer solutions having ionic strengths (I = 0.16 mol⋅kg⁻¹) similar to those in blood plasma, have been evaluated at 12 temperatures from 5 to 55 ^∘C using the Bates–Guggenheim convention. The liquid junction potentials (E _j ) between the buffer solutions of BICINE and saturated KCl solution of the calomel electrode at 25 and 37 ^∘C have been estimated by measurement with a flowing junction cell. These values of E _j have been used to ascertain the operational pH values at 25 and 37 ^∘C. The pK ₂ values at 25 and 37 ^∘C are 8.333 and 8.156, respectively. The thermodynamic quantities associated with the second acid dissociation have been calculated from the values of pK ₂ as a function of temperature. The zwitterionic buffer BICINE was shown to be useful as a pH standard in the region close to that of blood serum.     

Quantitative ultraviolet measurements on wetted thin-layer chromatography plates using a charge-coupled device camera

This paper presents the first study of the UV imaging of spots on thin-layer chromatographic plates whilst still wet with solvent. Imaging of spots of benzophenone during and after development was carried out using a charge-coupled device camera. Limits of detection were found to be 5ng on a wetted plate and 3ng for a dry plate and the relationship between peak area and sample loading was found to be linear in the low nanogram range over an order of magnitude for both wet and dry modes with r(2) values>0.99. It was found that UV measurements on wet glass-backed plates suffer from low sensitivity; however, the use of aluminium-backed plates gave increased sensitivity. The apparent absorption coefficient epsilon(app) of 10AUm(2)g(-1) at 254nm is consistent with reflection of the light from the aluminium surface with a double pass through the sorbent layer, and suggests that use of aluminium-backed plates should enable monitoring of separations by UV absorbance during TLC development.     

Salen Supported Molecular Borosilicates

Various Salen ligands (Salen( ^t Bu)H₂=N, N-ethylenebis(3,5-di-tertbutyl(2-hydroxy)benzylidenimine) were used to prepare borosilyl and -O bridged borosilyl compounds having the formula, L{B(OSiMe₃)₂}₂ (L=Salen( ^t Bu) (1), Salpen( ^t Bu) (2), Salben( ^t Bu) (3), Salhen( ^t Bu) (4) and L(BOSiMe₃)₂(-O) (L=Salen( ^t Bu) (5) and Salben( ^t Bu) (6)). In the case of 5 and 6 the formation of the B–O–B linkage takes precedence over the formation of a B–O–Si linkage. All of the compounds were characterized by Mp, elemental analysis, ¹H and ¹¹B NMR, IR, MS and in the case of 1, 2, and6 by X-ray crystallography.     

Bonds Intersecting Cycles In A Graph

Let G be a k-connected graph G having circumference c ≥ 2k. It is shown that for k ≥ 2, then there is a bond B which intersects every cycle of length c-k + 2 or greater.     

Analysis of the bond‐valence method for calculating 29Si and 31P magnetic shielding in covalent network solids

²⁹Si and ³¹P magnetic‐shielding tensors in covalent network solids have been evaluated using periodic and cluster‐based calculations. The cluster‐based computational methodology employs pseudoatoms to reduce the net charge (resulting from missing co‐ordination on the terminal atoms) through valence modification of terminal atoms using bond‐valence theory (VMTA/BV). The magnetic‐shielding tensors computed with the VMTA/BV method are compared to magnetic‐shielding tensors determined with the periodic GIPAW approach. The cluster‐based all‐electron calculations agree with experiment better than the GIPAW calculations, particularly for predicting absolute magnetic shielding and for predicting chemical shifts. The performance of the DFT functionals CA‐PZ, PW91, PBE, rPBE, PBEsol, WC, and PBE0 are assessed for the prediction of ²⁹Si and ³¹P magnetic‐shielding constants. Calculations using the hybrid functional PBE0, in combination with the VMTA/BV approach, result in excellent agreement with experiment. © 2016 Wiley Periodicals, Inc.     

Ammonium hexafluorosilicate salts

The preparation and characterization of the ammonium hexafluorosilicate salts, 2[R]⁺ [SiF₆]²⁻ (where R=piperidinium (2), methylammonium (3), quinolinium (4), acridinium (5), 2,2,6,6-tetramethylpiperidinium (6), and propylammonium (7)) is described.The salts were prepared from the reaction of the corresponding alkylammonium fluoride with silica gel. The compounds were characterized by NMR, IR, mass spectrometry and in the case of 1 (piperidinium fluoride), 2-4 by X-ray crystallography. Compounds 1-3 crystallize in the orthorhombic crystal system (space groups Iba2, Fdd2, and Pnnm, respectively), with Z=8, 14, and 4, respectively. Compound 4 crystallizes in the triclinic space group P-1, with Z=2. Compounds 1-4 exhibit hydrogen bonding.