Thermal Stability of Periodic Mesoporous SiCO Glasses

Periodic Mesoporous Organosilicas (PMOs) with 2D-hexagonal and cubic Pm3n structures have been prepared from bis(trialkoxysilyl)ethane and cetyltrimethylammonium chloride. The two samples have been pyrolyzed under argon up to 1000 °C. Study by X-ray diffraction (synchrotron radiation) allows the thermal stability of both structures to be followed as a function of the pyrolysis temperature. While the 2D-hexagonal structure collapses after pyrolysis at 800 °C, the cubic Pm3n structure is retained up to 1000 °C. Further characterizations were performed by ²⁹Si MAS-NMR, N₂ adsorption-desorption experiments and elemental analysis. At 1000 °C, the first sample can be described as a mixture of silica and a free C phase, while the cubic one is a true SiCO glass with mixed SiC_xO_{4 – x} units (x = 0,1,2) and a very large surface area of 730 m²/g.     

Solid state (47,49)Ti, (87)Sr and (137)Ba NMR characterisation of mixed barium/strontium titanate perovskites

Solid state ^47,49Ti, ¹³⁷Ba, ⁸⁷Sr NMR spectra have been recorded on Ba_xSr_1−xTiO₃ (0 x 1) perovskite samples prepared by the powder sintering method. Multinuclear solid state NMR shows great potential for characterising such systems since the quadrupolar parameters are very sensitive to any geometric deformation around the studied nucleus. ^47,49Ti NMR powder lineshapes appear strongly influenced by the presence of even a small amount of barium (or strontium) in the coordination second sphere of the probed titanium site: substitution of strontium by barium induces the broadening of the peaks, due to quadrupolar effects, while the isotropic chemical shift increases. ¹³⁷Ba NMR spectra exhibit a distribution of the quadrupolar interaction, that could be tentatively quantified, C_Q increasing with the amount of strontium. Preliminary results were also obtained on ⁸⁷Sr NMR showing behaviour comparable to ¹³⁷Ba NMR, i.e. a broadening of the peaks due to an increasing quadrupolar interaction with the amount of barium distorting the environment of the strontium sites.     

Total Synthesis of the Antimicrotubule Agent (+)-Discodermolide Using Boron-Mediated Aldol Reactions of Chiral Ketones

With a similar mechanism of action to taxol, the title compound 1 is a particularly promising candidate for development in cancer chemotherapy. This efficient synthesis, based on stereocontrolled aldol reactions, should help to overcome the scarce natural supply of 1 from the rare sponge source.     

Graph dismantling problems

Problems involving the dismantling of a digraph (graph) by removal of arcs (edges) are investigated. Some of these problems have good characterizations related to the familiar results about Euler trails, others are NP-complete.     

Stereochemical studies of oligomers. IV: structure of bis-3,3,5-trimethylcyclohexyl phthalate

C₂₆H₃₈O₄,Mr=414.6, is monoclinic,P2₁/c, witha=15.059(2),b=7.348(2),c=23.164(3) Å,β=90.35(16)°,V _c =2563.13(85) ų,D _x =1.07(4) g cm^?3,μ(CuKα)=5.28 cm^?1. The structure was determined by direct methods and refined by full-matrix least squares toR=0.074 for 2479 observed reflections. The two cyclohexane rings both adopt a chair conformation.     

Obtaining thermochemical data by the extended kinetic method

A microcanonical analysis of the extended kinetic method is performed using statistical rate calculations based on orbiting transition state theory. The model systems simulate polydentate bases M which exhibit losses of entropy upon protonation of up to 35 kJ mol(-1) K(-1). It is shown that the correlations using the natural logarithm of the ratio of rate constants vs the proton affinity of the reference bases, at several effective temperatures, lead to correct proton affinity and protonation entropy of the base M of interest. A systematic underestimate of the latter quantity (by 5-15%), mainly due to the use of a linear rather than a polynomial curve fitting procedure, is noted, however. When considering experimental data, more severe underestimates are observed for the protonation entropies of polydentate bases (by 50-90%). The origins of these considerable discrepancies are beyond the limits of the present modeling and remain to be determined.     

New scaffolds for supramolecular chemistry: upper-rim fully tethered 5-methyleneureido-5'-methyl-2,2'-bipyridyl cyclodextrins

Seven upper-rim fully tethered cyclodextrins (URFT-CDs) have been synthesised in a good average coupling yield using the one-step "phosphine imide" approach and their metal complexation behaviour with lanthanides and transition metals was explored. We observed that the A-TE-E light conversion process (antennae effect) occurs in the URFT-CD lanthanide complexes. A molecular redox switch based on the corresponding iron complexes is also reported. A reversible intramolecular translocation of the FeII and FeIII ions, between two distinct binding cavities has been monitored spectroscopically and achieved by chemical triggering. Finally, a negative allosteric control of ion recognition through the formation of a CD pseudocryptand is discussed.