Comportement thermique des propionates hydrates de calcium,strontium et baryum

Résumé Les propionates monohydratés de calcium, strontium et baryum ont été étudiés par radiocristallographie (sur monocristal et sur poudre) et par TG et ATD (à l'air et en milieu inerte). Ils cristallisent dans les systèmes monoclinique (Ca) ou orthorhombique (Sr, Ba). Ils perdent leur eau en une fois (Ca, Sr) ou deux fois (Ba) avec formation d'un hémihydrate intermédiaire. Tous les sels anhydres, d'abord amorphes aux rayons X, cristallisent vers 200°. Le propionate de baryum anhydre cristallisé subit une transformation polymorphique à 240°. Après 300° à l'air et 350° sous atmosphère inerte, la fusion et la décomposition se produisent simultanément: il y a dégagement de diéthylcétone et le résidu solide est du carbonate métallique bien cristallisé.

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The monohydrates of calcium, strontium and barium propionates have been investigated by radiocrystallography (using monocrystals and the powder method) and by TG and DTA (in air and in an inert atmosphere). These compounds crystallize in the monoclinic system (Ca) or the orthorhombic system (Sr, Ba). Dehydration occurs in a single stage for calcium and strontium propionates and in two stages for barium propionate. The compound Ba(C₂H₅CO₂) 0.5 H₂O is well-defined. All the anhydrous salts, first amorphous to X-rays, crystallize at about 200°. Barium propionate undergoes another phase transition at 240°. Melting and simultaneous decomposition take place after 300° in air and after 350° in an inert atmosphere, leading to evolution of diethylketone and to a solid residue of well-crystallized metal carbonate.

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Zusammenfassung Die Propionatmonohydrate von Calcium, Strontium und Barium wurden mittels Radiokristallographie (an Einkristallen und an Pulver), sowie mittels TG und DTA (in Luft und in inertem Medium) untersucht. Sie kristallisieren in monoklinen (Ca) oder orthorhombischen (Sr, Ba) Systemen. Sie verlieren ihr Kristallwasser auf einmal (Ca, Sr) oder zweimal (Ba) unter Bildung eines intermediären Hemihydrats. Sämtliche Anhydrosalze, welche sich zuerst bei Röntgenbestrahlung amorph verhalten, kristallisieren in der Nähe von 200°. Das kristalline wasserfreie Bariumpropionat erfährt eine polymorphe Änderung bei 240°. Nach 300° in Luft und 350° in inerter Atmosphäre treten Schmelzen und Zersetzung gleichzeitig auf: Diäthylaceton wird freigesetzt und der feste Rückstand besteht aus kristallinem Metallcarbonat.

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, , , . - () — Sr, Ba. , — . (₂₅)₂. 0,5₂. 200°. 240° . 300° 350° , .

     

Skeletal rearrangements in mass spectra. Part I. Bis-aryl compounds

Examination of a large number of spectra indicates that bis-unsaturated compounds commonly undergo a skeletal rearrangement in which part or all or the bridging moiety is eliminated, often with concomitant loss of hydrogen atoms. The spectra of labeled azobenzene, diphenyl sulfide, and their p,p′-dimethyl derivatives show that scrambling of hydrogen atoms precedes or accompanies such rearrangements, in contrast to the loss of a p-methyl group from the latter derivatives. These results are rationalized in terms of the radical site formed on one unsaturated functional group attacking the polarizable π-electrons of the other unsaturated group.     

Correlating second harmonic optical responses of single Ag nanoparticles with morphology

Femtosecond laser excited second harmonic (SH) activity from single Ag nanoparticles is reported. A correlation of SH single-particle measurements with high-resolution imaging of particle morphology by TEM was achieved by creating position markers on an optical and electron transparent substrate (Si3N4 thin film, approximately 100 nm). We compared the SH activity of single Ag nanoparticles (nanospheres versus nanorods) and cluster structures (composed of two or multiple particles, e.g., dimers and trimers). The direct correlation of single-particle structures and SH activity, spectral and power dependence, strongly suggests one-photon resonant driven nonlinear oscillator response mechanism.     

How homogeneous are the trehalose, maltose, and sucrose water solutions? An insight from molecular dynamics simulations

The structural properties resulting from the reciprocal influence between water and three well-known homologous disaccharides, namely, trehalose, maltose, and sucrose, in aqueous solutions have been investigated in the 4-66 wt % concentration range by means of molecular dynamics computer simulations. Hydration numbers clearly show that trehalose binds to a larger number of water molecules than do maltose or sucrose, thus affecting the water structure to a deeper extent. Two-dimensional radial distribution functions of trehalose solutions definitely reveal that water is preferentially localized at the hydration sites found in the trehalose dihydrate crystal, this tendency being enhanced when increasing trehalose concentration. Over a rather wide concentration range (4-49 wt %), the fluctuations of the radius of gyration and of the glycosidic dihedral angles of trehalose indicate a higher flexibility with respect to maltose and sucrose. At sugar concentrations between 33 and 66 wt %, the mean sugar cluster size and the number of sugar-sugar hydrogen bonds formed within sugar clusters reveal that trehalose is able to form larger clusters than sucrose but smaller than maltose. These features suggest that trehalose-water mixtures would be more homogeneous than the two others, thus reducing both desiccation stresses and ice formation.     

The influence of substituent groups on the resonance stabilization of benzene. An ab initio computational study

Accurate G3(MP2) calculations of the enthalpies of formation (Delta(f)H298) of organic molecules permit replication and extension of calculations that were formerly dependent on experimental thermochemical results. A case in point is Kistiakowski's classical calculation of the total stabilization enthalpy of benzene relative to that of cyclohexene, called for many years the "resonance energy". This paper investigates extension of the classical calculation to substituted benzenes. Slight modification of the usual procedure for Delta(f)H298 determination permits exclusion of all empirical information, leaving a purely ab initio result. Stabilization enthalpies relative to the corresponding 4-substituted cyclohexenes are presented for benzene, toluene, aniline, phenol, phenylacetylene, styrene, ethylbenzene, and phenylhydrazine. In the process of calculating these stabilization enthalpies, we have also obtained 42 values of Delta(f)H298 for monosubstituted benzenes, cyclohexenes, and cyclohexanes, 24 of which are not in the standard reference literature. For the remaining 18 G3(MP2) results, the unsigned mean difference between calculated Delta(f)H298 values and experimental results is +/-0.91 kcal x mol(-1).     

Mass spectrometry analysis of synthetically myristoylated peptides

Myristoylpeptides were synthesized in order to determine if a neutral loss of 210 Da, C14H26O (the mass of the myristoyl moiety), was universal and observable by both liquid chromatography electrospray ionization quadrupole ion trap (LC-ESI-QIT) and matrix-assisted laser desorption/ionization time-of-flight time-of-flight (MALDI-ToF/ToF) mass spectrometry. Myristoylation was successfully introduced on the N-terminus, internally on the amino group of lysine and arginine. Larger peptides and the arginine compounds needed elevated temperatures for myristoylation. To our knowledge, this is the first report of a chemically-synthesized myristoylated arginine in a peptide. Collision energy studies for the LC-ESI-QIT instrument showed that modified peptides and a loss of 210 Da could be detected under commonly used conditions (energy level between 30 and 42%) with picomole amounts of sample. The loss of myristoyl moiety is observed on the MALDI-Tof/Tof mass spectrometer as well. Due to the hydrophobic properties of the myristoyl moiety, it is not surprising that the modified peptides all formed at least dimers, and in some cases trimers. We were also able to distinguish a mixture of two mono-myristoylated peptides. MS3 data from the LC-ESI-QIT instrument on a di-myristoylated peptide indicates the loss of 210 Da at either the N-terminus or lysine. We were also able to analyze a mixture of modified and unmodified peptides on the MALDI-ToF/ToF instrument. The data presented in this paper demonstrates the constant neutral loss of the 210 Da, C14H26O, from both N-terminally and internally myristoylated peptides can be identified unambiguously using LC-ESI-QIT or MALDI-ToF/ToF mass spectrometers. This will be a useful tool in determining the myristoylation status of candidate proteins after enzyme digestion, and in elucidating the modification sites of internal myristoyl proteins.     

Improved short-lived nuclide activation analysis

A novel technique has been developed to increase the sensitivity, precision and accuracy of short-lived nuclide activation analysis and especially of delayed fission neutron counting, by improving the counting statistics. This has been achieved by approaching the irradiated sample to the detector during the counting period in order to compensate for the rapid radioactive decay by variation of the counting efficiency, permitting prolongation of the counting period with constant count rate. The sample holder movement with approximately exponentially decreasing velocity is implemented by a variable DC motor with a rotary to linear movement converter, which is being replaced by a more reliable stepping motor.     

Elastic properties of crosslinked poly(vinyl alcohol) gels: Network topology

Current network theory exhibits inconsistencies which show up particularly clearly in deformation of networks prepared by crosslinking a polymer in solution. A check of theory can be obtained if one knows precisely the number of crosslinks in the network and if a range of deformations is applied to the network. In an effort to explore this problem we have examined the relation of shear modulus to crosslink density, primary molecular weight, and polymer concentration for a series of poly(vinyl alcohol) gels at low to intermediate concentrations. Aqueous poly(vinyl alcohol) solutions were crosslinked to form infinite networks using terephthalaldehyde. We find a large discrepancy with these poly(vinyl alcohol) gels between measured shear modulus and that calculated from classical elasticity theory assuming quantitative reaction of crosslinking. The ratio of measured to calculated modulus is independent of crosslink density for a given primary molecular weight and concentration. It shows linear dependence on polymer concentration prior to crosslinking and extrapolates to a critical concentration which is consistent with the effective sizes of the polymer molecules.     

Notes on Kuhn's ω″ molecular orbital method

(1) It is shown that, contrary to statements sometimes made, it is perfectly possible to obtain consistent sets of charges in Kuhn's ω″ molecular-orbital method, whatever the values of the parameters ω, ω′, ω″.

(2) Particular applications are made to the benzyl cation, and to the non-alternants fulvene, heptafulvene and azulene. It appears that including the ω-terms decreases the magnitude of the largest of the net atomic charges as calculated by the simple Hückel method, and results in an overall smoothing-out process. Further inclusion of the ω′-terms continues this smoothing-out process, but inclusion of the ω″-terms may sometimes slightly reverse this process.

(3) The charge distributions obtained for fulvene and azulene in (1) and (2) lead to molecular dipole moments which are still much too large.

In view of (3) it is not clear that the additional work involved in the inclusion of ω′ and ω″ is justified by the greater accuracy thus obtained.     

Syngas reactions : II. The homogeneous catalyzed carbonylation and cyclization of allylic substrates

Carbon monoxide insertion and/or addition to allylic precursors may lead to the formation of both linear and cyclic carbonylation products. In examining these competing reaction paths, rhodium, platinum, palladium and nickel-based homogeneous catalysts have been developed which are particularly useful for the selective synthesis of γ-butyrolactam, N-alkyl-2-pyrrolidones, vinylacetate and phenylacetate esters and diesters from a variety of allylic and benzylic substrates. The extension of this catalysis to the carbonylation of certain vinylic and propargyl congeners had also been considered.