Field-dependent [M – 2H]+. formation in the field desorption mass spectrometric analysis of hydrocarbon samples

High-boiling hydrocarbons often yield abundant [M — 2H]^+. ions under the conditions of field desorption mass spectrometry (FDMS). This work evaluated [M — 2H]^+. formation from various saturated and aromatic compounds. The most intense [M — 2H]^+. signals observed resulted from the analysis of saturated compounds containing long and branched chains. The presence of an aromatic ring in a molecule, however, strongly diminished [M — 2H]^+. formation during FDMS. Experiments involving manipulation of the applied potential between the FD anode and cathode reveal the strong field dependence of this phenomenon. At a potential difference of 10 kV, strong [M — 2H]^+. formation occurred for samples including squalane, hexatriacontane (C₃₆ n-alkane) and a Polywax 655 mixture. Analyses of the same samples conducted at a potential difference of 4 kV produced only weak (if measurable) [M — 2H^+.] signals. The magnitude of [M — 2H]^+. formation also decreased as the sample quantity decreased.     

Vibrational study of (CH3)4NSbCl6 and [(CH3)4N]2SiF6

(CH3)4NSbCl6 and [(CH3)4N]2SiF6 are face-centred cubic compounds at ambient temperature with a = 11.548 and 11.172 A, respectively. The vibrational spectra of these two compounds are discussed in relation to their crystal structure and other compounds containing (CH3)4N+ ions. The assignment of the observed bands is discussed. The Raman and infrared spectra of [(CH3)4N]2SiF6 show that the cations and anions are not distorted inside the crystals and are weakly hydrogen-bonded to each other. The infrared spectrum of (CH3)4NSbCl6 confirms a cubic structure for this compound at ambient temperature, in which cations are in octahedral environments and can be interpreted as being disordered groups. No evidence could be found for the existence of hydrogen bonding between cations and anions in this compound. The phase transition of (CH3)4NSbCI6 is studied by means of Raman spectroscopy. It is believed to be governed by the reorientational motions of tetramethylammonium cations and may be classified as an 'order-disorder' type.     

Dichlorobis(pyridine‐κN)bis(3,3,3‐trifluoropropyl‐κC1)tin(IV)

In the title compound, [Sn(C₃H₄F₃)₂Cl₂(C₅H₅N)₂], the Sn atom lies on an inversion centre and is octahedrally coordinated by two Cl atoms, two tri­fluoro­propyl groups and two N atoms in an all‐trans configuration. The electronegative tri­fluoro­propyl groups increase the electrophilic properties of the Sn atom, and the Sn—Cl and Sn—N bonds are shortened in comparison with those reported for analogous compounds.     

Synthesis of simplified hybrid inhibitors of type 1 17beta-hydroxysteroid dehydrogenase via cross-metathesis and sonogashira coupling Reactions

[structure: see text] The inhibitor of type 1 17beta-hydroxysteroid dehydrogenase EM-1745 (1) exhibits affinity for both the substrate (estrone or estradiol) and the cofactor (NAD(P)H) binding domains. However, to increase its bioavailability, this compound needs to be simplified. The efficient and convergent synthesis of simplified substrate/cofactor hybrid inhibitors (compounds 2) involving a cross-metathesis and a Sonogashira coupling reaction as key steps is described. Compounds 2a-c were also tested as enzyme inhibitors and compared to EM-1745.     

Aqueous-phase, thermal Pauson-Khand reactions in the presence of surfactants

Efficient stoichiometric Pauson-Khand reactions were realized in water as the only solvent, and in the presence of surfactants as additives. Use of the cationic surfactant CTAB provided good yields of cyclopentenones from the thermal reactions of pre-formed dicobalthexacarbonyl complexes of enynes and alkynes. The water-CTAB medium was found to be less effective with Co₄(CO)₁₂ as the promoter of the reaction. Incidentally, these results have provided a strictly water-based PKR protocol under very mild and convenient conditions.     

Experimental design optimization of the analysis of gasoline by capillary gas chromatography

An optimization procedure for multi-step temperature programmed capillary GC was investigated for the detailed hydrocarbon analysis of gasoline. A set of twelve responses, represented by the value of the resolution of the most difficult separations, was selected from several isothermal analyses. The variation of these responses versus the temperature programming conditions was then modeled using a Doehlert matrix for experimental design. Optimal experimental conditions for the twelve responses were then obtained from a response surface optimization. The predicted and experimental resolutions were in good agreement.     

Transmission surface-plasmon resonance (T-SPR) measurements for monitoring adsorption on ultrathin gold island films

Evaporation of ultrathin (1.3-10 nm nominal thickness) gold films onto quartz or mica leads to the formation of a layer of rather uniform gold islands on the transparent support. The morphology of ultrathin gold island films of various thicknesses was studied by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. The surface plasmon (SP) absorption characteristic of such films is highly sensitive to the surrounding medium, with the plasmon band changing in intensity and wavelength upon binding of various molecules to the surface. The binding process can be monitored quantitatively by measuring the changes in the gold SP absorption, by using transmission UV/Vis spectroscopy. The method, termed transmission surface plasmon resonance (T-SPR) spectroscopy, is shown to be applicable to both chemically and physically adsorbed molecules, in liquid or gas phase, with measurements carried out either ex situ or in situ (real-time measurements) using a variety of molecular probes. Binding to a preformed molecular layer on the Au surface produces a similar response, suggesting the possible use of T-SPR for selective sensing. The sensitivity of T-SPR spectroscopy in detecting molecular binding to the gold depends strongly on the film preparation conditions, and may be comparable to that obtained in surface plasmon resonance (SPR) sensing.     

Control of n-alkanes crystallization by ethylene-vinyl acetate copolymers

The crystallization of paraffins from their solution at low temperature was investigated in the presence of ethylene-vinyl acetate (EVA) copolymers that allow the control of the size of the crystals. Depending on the type of solvent and distribution of the paraffin lengths, the mechanisms of crystal formation and growth are different. Precipitation of the EVA prior to the paraffins leads to the nucleation of a large number of crystals, whereas the adsorption of EVA on the surface of the growing crystals slows down the crystal growth. EVA can act either as a nucleating agent or as a growth inhibitor. These two mechanisms were identified from the analysis of the temperature of crystallization (cloud point), the chemical composition of the crystals, and the observations of the crystal habit. The EVA was able to co-crystallize with the paraffins in crystals of an orthorhombic structure and the melting enthalpies of the crystalline paraffin did not depend significantly on their neighborhood. The energies of interaction between the different paraffinlike components are close to each other, so that minor changes of the experimental conditions may lead to dramatic effects. This is the basic rationale for the large behavioral diversity observed in these systems.     

Milk membrane lipid vesicle structures studied with Cryo-TEM

Milk fat globule membrane (MFGM) lipids have been studied in the presence and absence of proteins β-lactoglobulin and β-casein. The aim of this study was to relate the self-assembly structure, e.g. vesicles, formed in aqueous dispersions of MFGM lipids to the lipid composition, electrolyte composition as well as the effect of added milk proteins, i.e. β-lactoglobulin and β-casein. For this purpose, vesicles of phospholipid mixtures, containing dioleoylphosphatidylcholine (DOPC), sphingomyelin (SM), dioleoylphosphatidylethanolamine (DOPE), phosphatidylinositol (PI) and dioleoylphosphatidylserine (DOPS) at composition corresponding to that of the MFGM, were prepared by extrusion. The morphology of the formed structures of different sample compositions was studied with cryogenic transmission electron microscopy (Cryo-TEM). Mixtures of membrane lipid with a composition (e.g. 80% DOPE, 12% DOPC and 8% SM) that at high lipid content give liquid crystalline phases at the boundary of lamellar to reversed hexagonal phase rather formed microtubular structures than vesicles at high water content. A large proportion of multilamellar vesicles is formed in buffer and divalent salts than in pure water. A small increase in the interlayer spacing of the multilamellar vesicle was observed in the presence of β-casein.