Carbaporphyrinoid chemistry has a silver lining! silver(III) oxybenzi-, oxynaphthi-, tropi-, and benzocarbaporphyrins

[structure: see text] Carbaporphyrinoid systems with semiquinone, cycloheptatriene, or indene subunits react under mild conditions with silver(I) acetate to give stable silver(III) organometallic derivatives.     

A novel method for the preparation of alpha,alpha'-difluoroesters and acids using BrF3

Alkyl-, haloalkyl-, and ketoalkyl-2-ethoxycarbonyl-1,3-dithianes were easily made from the appropriate primary or secondary alkyl bromides, 1,3-dithiane, and ethyl chloroformate. They were reacted with BrF(3) to form the corresponding alpha,alpha-difluoro esters in 65-75% yield. Reaction conditions are very mild (1-2 min, 0 degrees C). The two sulfur atoms of the dithiane are essential for the reaction.     

Epoxide-opening and group-transfer reactions mediated by monomeric zirconium imido complexes

N-tert-Butylimidozirconocene (Cp2Zr=Nt-Bu) and its analogue (ebthi)Zr=NAr (ebthi = ethylenebis(tetrahydroindenyl), Ar = 2,6-dimethylphenyl) undergo overall cycloaddition reactions at ambient temperature with epoxides that lack accessible beta-hydrogens. The process results in addition of the Zr=N bond to an epoxide C-O bond, leading to azazirconacyclopentanes. The regio- and stereochemistry of the products implicate a stepwise mechanism, with the intermediacy of zwitterions having substantial carbocation character at the substituted carbon of the ring-opened epoxide-derived fragment. The azametallacycles undergo facile cleavage to beta-amino alcohols upon addition of mild acid.     

Dynamics of propylene glycol and its oligomers confined in clay

The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with (about 70 monomers), confined in a Na-vermiculite clay have been investigated by quasi-elastic neutron scattering and dielectric spectroscopy. The liquids are confined to a single molecular layer between the clay platelets, thus giving a true 2D liquid. The results show that the average relaxation time , deduced from neutron scattering at a momentum transfer Q of about , is in perfect agreement with the dielectric -relaxation time, although neutron scattering does not only probe the main ( -) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature is proportional to Q ², indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that (or the dielectric -relaxation time) is almost unaffected by the 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Only the 7-mer has a significantly slower segmental translational diffusion in the clay. The results suggest that the interactions to the clay surfaces are weak and that the present 2D confinement has a very small influence on the time scale of all our observed relaxation processes, except the normal-mode relaxation.Received: 1 January 2003, Published online: 8 October 2003PACS: 61.25.Em Molecular liquids - 68.35.Ja Surface and interface dynamics and vibrations - 61.12.-q Neutron diffraction and scattering     

Dielectric relaxation studies of poly(propylene glycol) confined in vermiculite clay

The molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (M(w)=1200, 2000 and 4000 g/mol) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy. In addition to the alpha-relaxation, the normal mode relaxation process was studied for all samples both in bulk and confinement. For the normal mode process the relaxation rate in the clay is drastically shifted to lower frequencies compared to that of the bulk material in contrast to the alpha-process whose relaxation time is only slightly affected by the confinement. Also the temperature dependence of the relaxation time for the normal mode process is strongly affected by the confinement. Moreover, in the clay the intensity of the normal mode is stronger than that of the alpha-process, in contrast to the bulk samples where the opposite is observed.     

Does confined water exhibit a fragile-to-strong transition?

The dynamics of supercooled confined water has recently been shown to have a pronounced, apparent fragile-to-strong transition (FST). Here we use broadband dielectric spectroscopy (10^-2–10⁹ Hz) to study the dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21.4 and 36.1 ?, respectively. The local dynamics of water molecules and the dynamics of the hydroxyl groups on the inner wall of the pores are followed up to over 240 K. We argue that the reported FST for confined water is due to the vanishing of the cooperative α relaxation, which implies that it should not be interpreted as a true FST.