Silicon and phosphorus alkoxide mixture: Sol-gel study by spectroscopic technics

Sol-gel synthesis of mixtures of tetraethoxysilane and a phosphorus alkoxide [P(OEt)₃ or (OEt)₂P-O-P(OEt)₂ or PO(OEt)₃] have been studied by ¹H, ¹³C²⁹Si and ³¹P liquid and solid state NMR, infrared and raman spectroscopies. This study shows different behaviors towards hydrolysis for these three different phosphates and phosphites. P(OEt)₃ almost instantly reacts with water to form an intermediate species HPO(OEt)₂, which slowly evolves first to HPO(OH)(OEt), then to HPO(OH)₂ a few days later. For (OEt)₂P-O-P(OEt)₂, the P-O-P bond is broken when water is added, then the same intermediates are formed faster. PO(OEt)₃ is hydrolyzed much slower than the other alkyl phosphates. After ten months, triethoxyphosphate is quantitatively present in the sol with little PO(OH)(OEt)₂ species. All these hydrolyzed species are well characterized. Only the system which contains the tetraethoxysilane and the triethoxyphosphite P(OEt)₃ forms a few P-O-P and P-O-Si bonds in the gel. Hydrolysis of tetraethoxysilane is much faster than that of phosphorus alkoxides and the conventional Q₂, Q₃ and Q₄ condensed silicon species form the gel three dimensional network.     

An inclusion complex between acetonitrile andp-tert-butylcalix[6]arene

p-tert-Butylcalix[6]arene forms a complex with 1.5 acetonitrile molecules, that are included in the macrocycle cavity. This complex crystallizes in the monoclinic system: space groupP2₁/m,a=14.043(4),b=16.916(11),c=14.199(11) Å,=98.27(6)°,V=3338(4) ų,Z=2 (formula units). Refinement led to a final conventionalR value of 0.113 for 1654 reflections. Thepinched cone conformation of the calixarene is the same as that of the neutral and freeR-calix[6]arene already described. The two acetonitrile molecules interact with some phenyl ring -clouds, a methylene bridge and sometert-butyl groups of the calixarene. Supplementary Data relating to this article (atomic coordinates for hydrogen atoms, anisotropic displacement parameters, selected geometrical parameters and observed and calculated structure factors) are deposited with the British Library as Supplementary Publication No. SUP 82188 (16 pages).     

Metabolomic analysis of saponins in crude extracts of Quillaja saponaria by liquid chromatography/mass spectrometry for product authentication

Analysis of 50% aqueous methanolic extracts of bark of Quillaja saponaria Molina (quillaja) by liquid chromatography/mass spectrometry (LC/MS), using negative ion electrospray, revealed over 100 saponins. The majority could be assigned to known structures or generalised variations of these from the product ion spectra obtained by serial mass spectrometry in a quadrupole ion trap mass spectrometer. Ten saponins contained a fatty acid domain terminated with both a pentose and deoxyhexose unit, a feature thus far only reported in QS-III. Twenty saponins were based on a hydroxylated derivative of quillaic acid, whereas only six 22beta-hydroxyquillaic acid saponins have been described. The occurrence of pairs of saponins differing only by the presence of a rhamnose or xylose unit in the C-3-substituted saccharide was readily observed in two-dimensional mass maps, and these showed the presence of the unreported 'rhamnose partner' of QS-III. However, one sample labelled as Q. saponaria appeared to lack all saponins containing rhamnose in the C-3 saccharide. Methods to authenticate saponin extracts of quillaja by LC/MS are suggested based on the general metabolomic profile, the occurrence of specific major saponins covering known structural variations, or the presence of saponins containing the unusual fatty acid domain, revealed by neutral loss analysis.     

A two-dimensional model for collisional energy transfer in bimolecular ion-molecule dynamics: M++(H2; D2; or HD)→(MH++H; MD++D; MH++D; or MD++H)

Summary Guided ion beam kinetic energy thresholds in the ion-molecule reactions M⁺+H₂MH⁺+H, where M⁺ is a closed-shell atomic ion B⁺, Al⁺, or Ga⁺, were found to exceed by 0.4 to ca. 5 eV the thermodynamic energy requirements (or the theoretically computed barrier heights) for these reactions. In addition, the formation of MD⁺ occurs at a significantly lower threshold than MH⁺ when M⁺ reacts with HD. Moreover, the measured reaction cross-sections for the production of MH⁺ or MD⁺ product ions are very small (10^–17 to 10^–20 cm²), being largest for B⁺ and smallest for Ga⁺. A previous paper from this group proposed that collisional-to-internal energy transfer is the rate-limiting step for this class of reactions. It also suggested, based on a dynamical resonance picture, that collisions occurring at or near C_2v symmetry are more effective than other collisions even though C_2v geometries provide no lower potential energy barriers than others. By examining the collision paths characteristic of flux early in the bimolecular collision and searching for geometries along such paths where collisional-to-internal energy transfer is optimal, our earlier efforts predicted reaction thresholds in reasonable agreement with the (previously perplexing) experimental data. In the present work, we introduce a model Hamiltonian whose classical and quantum dynamics we apply to the M⁺+H₂, D₂, HD reactive collisions. We calculate the classical collisional-to-internal energy transfer cross-sections and find energy transfer thresholds that resemble the experimental reaction thresholds but whose isotopic mass trends are not entirely consistent with experiment. We then use a Green function method and a local quadratic approximation to the potential surface to obtain analytical expressions for the isotopic mass dependences of the collisional-to-vibrational energy transfer and for the subsequent fragmentation of the three-atom system. Finally, we analyze the origin of the threshold energy asymmetry in the M⁺+HD reactions.     

Investigation of orientational behaviour of nematic and smectic a compounds using the Raman microprobe

The Raman microprobe technique, has been used to probe orientational order in oriented samples of nematic CB₅ and nematic and smectic CB₈ versus spatial localization of the analysed microvolume. In the nematic state, the influence of long-wavelength director fluctuations is pointed out and an elastic constant derived.     

Enzymatic, spectrophotometric determination of glucose, fructose, sucrose, and inulin/oligofructose in foods

A fast, simple, and accurate method, using only standard laboratory equipment, was developed for the quantification of glucose, fructose, sucrose, and inulin/oligofructose in different food matrixes. Samples were extracted using boiling water and hydrolyzed with sucrase and fructanase. Sugars were determined in the initial extract and in both hydrolysates using an enzymatic, spectrophotometric kit for glucose and fructose determination with hexokinase, glucose-6-phosphate dehydrogenase, and phosphoglucose isomerase. Calculations of sucrose and inulin/oligofructose were based only on fructose measurement. Glucose results of the hydrolysates were not used for inulin/oligofructose calculations because of possible interference. Released glucose by the hydrolysis of maltose or by possible partial hydrolysis of other compounds like maltodextrines, starch, lactose, or maltitol could interfere in the measurement of the sucrase and the fructanase hydrolysates. To validate the method, a wide range of different food matrixes and different amounts of inulin/oligofructose (1-54%) were analyzed. Mean recovery +/- relative standard deviation (RSD) for inulin or oligofructose was 96.0 +/- 5.3%. The RSDr for inulin/oligofructose measured on 35 food samples, analyzed in duplicate, was 5.9%. Accuracy and precision of the method were less for samples with large concentrations of sucrose, maltose, maltodextrines, or starch (ratio to inulin/oligofructose >4 to 1). Precision and accuracy were comparable with those of the ion exchange chromatographic method AOAC 997.08 and the enzymatic, spectrophotometric method AOAC 999.03. In contrast to 999.03, this method allows the accurate quantification of both GFn and Fn forms.     

H6P2W18O62·14H2O as an efficient catalyst for the green synthesis of α-aminophosphonates from α-amino acids

Abstract

A simple, efficient and eco-friendly protocol has been described for the synthesis of α-aminophosphonates via Kabachnik-Fields reaction, catalyzed by H₆P₂W₁₈O₆₂·14H₂O as a reusable catalyst. The reaction was realized by condensation of amino acids, various aromatic aldehydes and triethylphosphite under solvent-free conditions, the corresponding α-aminophosphonates were formed in good yields. All the new products were characterized by IR, ¹H, ¹³C and ³¹P-NMR analyses. This method offers advantages such as simplicity workup with the green aspects, good yields and short reaction times.     

Palladium-catalyzed homocoupling of arenediazonium salts: an operationally simple synthesis of symmetrical biaryls

A simple procedure for the intermolecular homocoupling of arenediazonium salts in air using a catalytic amount of palladium acetate is described. The optimum conditions were found to be 15 mol % palladium acetate in refluxing methanol, with no additional terminal reducing agent required. These optimized conditions were used to prepare biaryls from several arenediazonium tetrafluoroborate salts, and most examples proceeded in moderate to high yields.     

Chemical and photochemical reactivity in micellar media and microemulsions VII- Effect of the interface on the reactivity of excited states

Study of the photodimerization of coumarin in micellar media and microemulsions shows that the orientation and the concentration of molecu- les in the interfacial film can favor reactions involving just one of the excited states (the singlet state in this case).     

Cross-linked polystyrene resins containing triorganotin-4-vinylbenzoates: Assessment of their catalytic activity in transesterification reactions

Three new resins are synthesized by radical co-polymerization of triorganotin-4-vinylbenzoates (substituent at tin = Me, Bu or Ph) with styrene and 1,4-divinylbenzene. The products prepared have been characterized by FT-IR and NMR spectroscopy both in the solid state and as swollen samples, showing a predominantly tetracoordinated tin atom. The catalytic activity of the above resins in a transesterification reaction has been tested using ethyl acetate and primary, secondary or tertiary alcohol, showing good results with the former, but not with the latter ones. Focussing on the different triorganotin substituents, a comparison between the prepared resins and low molecular weight analogues evidences a lower activity of the resins, due to their inhomogeneous operating conditions. However, the triphenyltin functionalized resin shows a transesterification activity comparable to the corresponding model compound. The reaction mechanism and the effects of Lewis acidity of the different groups linked to tin have been investigated by ¹H and ¹¹⁹Sn hr-MAS NMR.