A Thermodynamic, 13C NMR,and 17O NMR Study of Isomeric 4,7-Dihydro-1,3-dioxepins and 4,5-Dihydro-1,3-dioxepins

The relative thermodynamic stabilities of 4,7-dihydro-1,3-dioxepin (4,6-dioxacycloheptene, 1a) and 4,5-dihydro-1,3-dioxepin (3,5-dioxacycloheptene, 1b), and of a number of their 2-substituted derivatives, have been determined by base-catalyzed chemical equilibration in DMSO solution. Without exception, the 4,5-dihydro isomer is the dominating species at thermodynamic equilibrium. The relative stability of the b form is promoted by the presence of a single alkyl group on C-2, whereas two alkyl groups on C-2 have an opposite effect. In general, the thermodynamic parameters H _m and _Sm , of isomerization vary unexpectedly with the pattern of substitution at C-2. These trends appear to be derived from significant substituent-induced conformational changes in the b isomer, as suggested by ¹³C and ¹⁷O NMR chemical shift data.     

Points of {varepsilon} Differentiability of Lipschitz Functions from Rn to Rn-1

This paper proves that for every Lipschitz function f : RⁿR^m,m < n, there exists at least one point of -differentiabilityof f which is in the union of all m-dimensional affine subspacesof the form q0 +span{q1,q2,...,q_m}, where q_j (j = 0,1,...,m)are points in Rⁿ with rational coordinates. 2000 MathematicsSubject Classification 26B05, 26B35.     

Modeling NMR parameters by DFT methods as an aid to the conformational analysis of cis-fused 7a(8a)-methyl octa(hexa)hydrocyclopenta[d][1,3]oxazines and [3,1]benzoxazines

The energies of the preferred conformations of four 7a-methyl octa(or hexa)hydrocyclopenta[d][1,3]oxazines, five 8a-methyl octa(or hexa)hydro[3,1]benzoxazines, and 8a-methyl hexahydro[1,3]benzoxazinone, all cis-fused, were investigated by DFT methods. Following geometry optimization at the B3LYP/6-31G(d,p) level, both the proton chemical shifts and the vicinal coupling constants between H-4a and the H-4 and H-5 protons were calculated at the B3LYP/cc-pVTZ level and compared to the previously experimentally measured values. The agreement between the calculated and the experimental chemical shifts was found to be good. Similarly, the agreement between the calculated and the experimental vicinal coupling constants was also found to be good, thus providing a methodology for determining the conformational equilibria of such systems that is comparable in many respects to experimental approaches such as variable-temperature NMR or to the use of model coupling constant values, when available, from analogous compounds.     

Structural characterization of isomeric 2,3,5‐substituted tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones prepared by cycloaddition of N‐methyl‐C‐arylnitrones to N‐phenyl‐ or N‐methylmaleimide

1,3‐Dipolar cycloaddition reactions of N‐methyl‐C‐arylnitrones with N‐phenyl‐ or N‐methylmaleimide were studied. The reaction of p‐dimethylamino‐, 4‐benzyloxy‐3‐methoxy‐, p‐nitro‐ and p‐chloro‐substituted phenylnitrones with N‐phenylmaleimide gave cis and trans cycloadducts but that of the corresponding phenylnitrones with N‐methylmaleimides only the cis adducts in the case of p‐dimethylamino and 4‐benzyloxy‐3‐methoxy substitution. All cis adducts attain a biased conformation whereas the trans forms are shown (by ¹H NMR at 233 K and ¹³C NMR at 208 K) to be mixtures of two invertomers, namely o‐(N‐lone pair antiperiplanar to 3H; minor) and i‐conformations (3H‐C‐C‐3aH dihedral angle close to 90°; major). PM3 and DFT calculations at the B3LYP/6–31G(d) level of theory prove qualitatively that these two conformers of the trans adduct are of comparable stability and represent energy minima.     

X-ray diffraction study of TiO2 thin films on mica

Thin layers of polycrystalline TiO₂ were deposited on flaky muscovite and phlogopite particles. Deposition was performed in water slurry using aqueous TiCl₄ as titanium source. The effect of heat treatments on the structure of TiO₂ thin films was investigated at different layer thicknesses. On muscovite mica the films crystallized as anatase TiO₂. On phlogopite mica the increasing layer thickness favored partial rutile formation at higher calcination temperatures, although the films could not be converted to pure rutile. The preferred orientation of the TiO₂ films was evident. The most intense reflections were measured from (004) and (105) planes. The crystallite size of TiO₂ was strongly dependent on calcination temperature and TiO₂ layer thickness. It varied between 15 and 47 nm for films deposited on phlogopite, and 15 and 57 nm for films deposited on muscovite.     

Die photometrische Bestimmungsmethode des Eisens mit Tri-n-butylammoniumsalz und 7-Jod-8-oxychinolin-5-sulfos?ure

Zusammenfassung Es wird ein Verfahren zur Bestimmung von Fe³⁺-Ionen als Tri-n-butylammoniumsalz des Eisen-(III)-Ferronkomplexes beschrieben.Das Eisen wird mit Amylalkohol als grüngefärbte Verbindung ausgeschüttelt, die im p_h-Bereich von 2–7,5 auf längere Zeit farbkonstant ist.Der Deutschen Forschungsgemeinschaft, dem Fonds der Chemie und der Firma Ernst Leitz, Optische Werke, Wetzlar, danken wir für Unterstützung.     

Colloid stability of lipid/polyelectrolyte decorated latex

The colloid stability of supramolecular assemblies composed of the synthetic cationic lipid dioctadecyldimethylammonium bromide (DODAB) on carboxymethyl cellulose (CMC) supported on polystyrene amidine (PSA) microspheres was evaluated via turbidimetry kinetics, dynamic light scattering for particle sizing, zeta-potential analysis, and determination of DODAB adsorption on CMC-covered particles. At 0.1 g L(-1) CMC and 2 x 10(11) PSA particles/mL, CMC did not induce significant particle flocculation, and a vast majority of CMC-covered single particles were present in the dispersion so that this was the condition chosen for determining DODAB concentration (C) effects on particle size and zeta potentials. At 0.35 mM DODAB, charge neutralization, maximal size, and visible precipitation indicated extensive flocculation and minimal colloid stability for the DODAB/CMC/PSA assembly. At 0.1 g L(-1) CMC, isotherms of high affinity for DODAB adsorption on CMC-covered particles presented a plateau at a limiting adsorption of 700 x 10(17) DODAB molecules adsorbed per square meter PSA which was well above bilayer deposition on a smooth particle surface. The polyelectrolyte layer on hydrophobic particles was swelled and fluffy (ca. 11-nm hydrodynamic thickness), and maximal adsorption of DODAB lipid onto this layer produced a compressed composite cationic film with 20 mV of zeta potential and about 10-nm mean thickness. The assembly of cationic lipid/CMC layer/polymeric particle was stable only well above charge neutralization of the polyelectrolyte by the cationic lipid, at relatively large lipid concentrations (at and above 1 mM DODAB) with charge neutralization leading to extensive particle aggregation.     

Evolution of wood surface free energy after heat treatment

Surface free energies of pine and beech wood were investigated before and after heat treatment using the Lifshitz-van der Waals/acid-base approach from contact angles measured by the Wilhelmy method. The results obtained showed that the decrease of the electron-donating component of the acid-base component was the major parameter affecting the wetting of the modified wood's surface. The Lifshitz-van der Waals component was slightly modified after heat treatment indicating that the atomic and molecular interactions due to permanent or induced dipoles between wood macromolecules were weakly modified. Modification of the surface chemical composition was studied by X-ray photoelectron spectroscopy (XPS) and titration of acidity. XPS indicated an important decrease of the O/C ratio after heat treatment explaining the decrease of the electron-donating component (γ⁻) of the surface free energy. The decarboxylation and degradation of glucuronic acids present in hemicelluloses, demonstrated by titration of carboxylic acid functions of wood, had only limited effect on the electron-accepting component (γ⁺).