Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models VI. Square Lattice with Extra-Vertex Boundary Conditions

We study, using transfer-matrix methods, the partition-function zeros of the square-lattice q-state Potts antiferromagnet at zero temperature (= square-lattice chromatic polynomial) for the boundary conditions that are obtained from an m×n grid with free boundary conditions by adjoining one new vertex adjacent to all the sites in the leftmost column and a second new vertex adjacent to all the sites in the rightmost column. We provide numerical evidence that the partition-function zeros are becoming dense everywhere in the complex q-plane outside the limiting curve B_￥(sq)\mathcal{B}_{\infty}(\mathrm{sq}) for this model with ordinary (e.g. free or cylindrical) boundary conditions. Despite this, the infinite-volume free energy is perfectly analytic in this region.     

Aqueous Ring‐Opening Polymerization‐Induced Self‐Assembly (ROPISA) of N‐Carboxyanhydrides

Reported here is the first aqueous ring‐opening polymerization (ROP) of N‐carboxyanhydrides (NCAs) using α‐amino‐poly(ethylene oxide) as a macroinitiator to protect the NCA monomers from hydrolysis through spontaneous in situ self‐assembly (ISA). This ROPISA process affords well‐defined amphiphilic diblock copolymers that simultaneously form original needle‐like nanoparticles.     

On the wall slip of polymer blends

The slip flow of the polypropylene (PP)/poly[ethylene‐co‐(vinyl acetate)] (EVA) system was studied in a capillary rheometer for shear rates of 40–1000 s^?1 at four temperatures. Three dies made of aluminum with a length/diameter (L/D) ratio of 15 and diameters of 1.59, 1.19, and 0.79 mm provided the flow data. Calculations of the slip velocity by the Mooney method showed power‐law behavior with the stress. Blends were prepared at various proportions of PP and EVA for observation of the variation of the slip velocity for different compositions and temperatures. Direct microscopic observations of the slip layer on extruded samples showed domains of the dispersed phase unevenly distributed between the slip layer and the core and provided estimates of the thickness of the layer adjacent to the capillary wall. Results showed that the viscosity in the slip layer was 10–100 times lower than that in the bulk for the same value of the shear stress. In terms of the extrapolation length, the development of the slip layer was the result of different disentanglement dynamics of the molecules in the slip layer in comparison with those in the bulk. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 303–316, 2002     

Polymorphic coordination networks responsive to CO2, moisture, and thermal stimuli: porous cobalt(II) and zinc(II) fluoropyrimidinolates

The novel porous [{M(F-pymo)(2)}(n)]2.5n H(2)O coordination networks (M=Co, Zn; F-pymo=5-fluoropyrimidin-2-olate), possessing sodalitic topology, have been synthesised and structurally characterised by means of powder diffraction methods. Thermodiffractometry demonstrated their plasticity: when heated up to 363 K, they reversibly transform into three-dimensional dehydrated [{M(F-pymo)(2)}(n)] species, with significantly different lattice parameters. Further heating induces irreversible polymorphic transformations into layered phases, in which the original MN(4) coordination sphere changes into an MN(3)O one. A mixed-metal phase, [{Co(x)Zn(1-x)(F-pymo)(2)}(n)]2.5n H(2)O, was also prepared, showing that zinc is preferentially inserted, when starting from a Co/Zn reagent ratio of 1:1. The solid-gas adsorption properties of the anhydrous 3D frameworks have been explored towards N(2), H(2) (77 K) and CH(4), CO(2) (273 K). These results show that these materials permit the diffusion of CO(2) molecules only. Remarkably, the CO(2) adsorption process for the [{Co(F-pymo)(2)}(n)] network proceeds in two steps: the first step takes place at low pressures (<600 kPa) and the second one above a threshold pressure of 600 kPa. By contrast, the [{Zn(F-pymo)(2)}(n)] network only permits CO(2) diffusion by applying pressures above 900 kPa. This type of behaviour is typical of porous networks with gated channels. The high CO(2) selectivity of these systems over the rest of the essayed probe gases is explained in terms of flexibility and polarity of the porous network. Finally, the magnetic studies on the Co(II) systems reveal that the as synthesised [{Co(F-pymo)(2)}(n)]2.5n H(2)O material behaves as an antiferromagnet with a T(N) of about 29 K. At variance, the [{Co(F-pymo)(2)}(n)] layered phase shows an unusually weak ferromagnetic ordering below 17 K, arising from a spin-canting phenomenon.     

Amifostine protection against mitomycin-induced chromosomal breakage in fanconi anaemia lymphocytes

Fanconi anaemia (FA) is a rare genetic chromosomal instability syndrome caused by impairment of DNA repair and reactive oxygen species (ROS) imbalance. This disease is also related to bone marrow failure and cancer. Treatment of these complications with radiation and alkylating agents may enhance chromosomal breakage. We have evaluated the effect of amifostine (AMF) on basal and mitomycin C (MMC)-induced chromosomal breakage in FA blood cells using the micronucleus assay. The basal micronuclei count was higher among FA patients than healthy subjects. Pre-treatment with AMF significantly inhibited micronucleation induced by MMC in healthy subjects (23.4 +/- 4.0 - MMC vs 12.3 2.9 - AMF --> MMC) MN/1000CB, p < 0.01, one way ANOVA) as well as in FA patients (80.0 +/- 5.8 - MMC vs 40.1 +/- 5.8 - AMF --> MMC) MN/1000CB, p < 0.01, ANOVA). Release of ROS by peripheral blood mononuclear cells treated with AMF -> MMC and measured by chemoluminometry showed that AMF-protection was statistically higher among FA patients than in healthy individuals. Based on these results we suggest that AMF prevents chromosomal breakage induced by MMC, probably by its antioxidant effect.     

Effect of microwave heating in the asymmetric addition of dimethylzinc to aldehydes

Microwave-heated enantioselective additions of dimethylzinc to various aldehydes are reported. Dramatically reduced reaction times and lower catalyst loadings (5%), compared with conventionally used conditions, can be achieved, with excellent yields and just small loss of enantioselectivity (up to 83% enantioselectivity is achieved). In the reaction with aliphatic aldehydes the same enantioselectivity has been achieved for microwave-heated and conventional room temperature conditions.     

Highly Stable Double‐Stranded DNA Containing Sequential Silver(I)‐Mediated 7‐Deazaadenine/Thymine Watson–Crick Base Pairs

The oligonucleotide d(TX)₉, which consists of an octadecamer sequence with alternating non‐canonical 7‐deazaadenine (X) and canonical thymine (T) as the nucleobases, was synthesized and shown to hybridize into double‐stranded DNA through the formation of hydrogen‐bonded Watson–Crick base pairs. dsDNA with metal‐mediated base pairs was then obtained by selectively replacing W‐C hydrogen bonds by coordination bonds to central silver(I) ions. The oligonucleotide I adopts a duplex structure in the absence of Ag⁺ ions, and its stability is significantly enhanced in the presence of Ag⁺ ions while its double‐helix structure is retained. Temperature‐dependent UV spectroscopy, circular dichroism spectroscopy, and ESI mass spectrometry were used to confirm the selective formation of the silver(I)‐mediated base pairs. This strategy could become useful for preparing stable metallo‐DNA‐based nanostructures.     

Structural and magnetic properties of layered copper(II) coordination polymers intercalating s and f metal ions

The nanoporous coordination polymer [Cu(pyrimidin-2-olate-N1,N3)2]n (1C) of the sodalite zeotype sorbs a variety of metal nitrates [M(NO3)m, M = Na+, K+, Rb+, Tl+, Ca2+, Sr2+, Ba2+, Pb2+, La3+, Nd3+, Gd3+, Er3+] from H2O/MeOH solutions, with a concomitant structural change to a layered [Cu(pyrimidin-2-olate-N1,N3)2]n.[M(NO3)m]n/2 (MNO3@1L) coordination framework. Single-crystal X-ray diffraction analyses revealed that the layers are based on Cu4(pyrimidin-2-olate-N1,N3)4 square grids of copper(II) ions bridged by N1,N3 exobidentate ligands, displaying a structural motif of the metallacalix[4]arene type in pinched cone conformation. The interlayer space is occupied by the guest metal nitrates, each metal being coordinated by (at least) the four oxygen atoms of a metallacalix[4]arene. Magnetic measurements on the MNO3@1L series denoted a weak ferromagnetic ordering taking place below the Néel temperatures (typically close to 35 K), arising from spin-canting phenomena of the antiferromagnetically coupled copper centers. When M = Nd3+, Gd3+, or Er3+, additional magnetic ordering is observed at lower temperatures, which, on the basis of static and dynamic magnetic susceptibility measurements, can be attributed to copper- lanthanide interactions.