On the hydrolysis of the dioxouranium(VI) ion in sulfate solutions

The formation of ternary UO2(2+)-(OH-)-SO4(2-) complexes has been studied at 25 degrees C in 3 M NaClO4 ionic medium by measurements with a glass electrode. The solutions had uranium concentrations between 0.3 and 30 mM, sulfate between 20 and 200 mM, and 1.66 < or = [SO4(2-)]/[U(VI)] < or = 300. The hydrogen ion concentration ranged from 10(-3) M to incipient precipitation of basic sulfates. This occurred, depending on the metal concentration, at [H+] between 10(-4) and 10(-5.3) M. In the interpretation of the data the stabilities of binary complexes were assumed from independent sources. The data could be explained with the mixed complexes and equilibria (beta(pqr)(3sigma) refers to pUO2(2+) + qH2O + rSO4(2-) <==> (UO2)p(OH)q(SO4)r(2p-q-2r) + qH+): logbeta222 = -2.94 +/- 0.03, logbeta341 = -9.82 +/- 0.06, logbeta211 = -0.30 +/- 0.09, logbeta212 = 1.09 +/- 0.09, logbeta351 = -15.04 +/- 0.09 and logbeta462 = -14.40 +/- 0.06. The fit could be improved by including UO2OH+ with logbeta110 = -5.1 +/- 0.1. The identity of the minor species remains, however, an open question.     

Synthèse de spiro-pyrazolines par cycloaddition du diazométhane sur des sucres insaturés ramifiés. Influence de l'isomérie géométrique sur l'orientation de la réaction Communication préliminaire

The orientation of the cycloaddition of diazomethane on unsaturated branchedchain sugars has been studied. For 3-C-cyanomethylidene-3-deoxy-1,2-O-isopropylidene-α-D-glycero-tetrofuranose the orientation was ‘normal’ and did not depend on the configuration at the double bond. The same situation prevailed with derivatives of 3-deoxy-1,2:5,6-di-O-isopropylidene-3-C-methylidene-α-D-xylo-hexofuranose. For the 3-C-acylmethylidene- and the 3-C-cyanomethylidene-3-deoxy-1,2:5,6-di-O-isopropylidene-α-D-ribo-hexofuranoses, the trans-(H–C(3′)–C(2))-isomer gave the ‘normal’ cycloadduct whereas the cis-isomer gave predominantly the αabnormal spiro-pyrazoline. This observation represents the first instance where the regioselectivity of a cycloaddition reaction is affected by the geometrical isomerism of the dipolarophile. The most probable explanation of the phenomenon is the conformational perturbation about the C(4)--C(5) bond of the unsaturated sugars induced by a change in the configuration at C(3). The consequence of that ‘conformational transmission’ of a difference in configuration at C(3) is that the steric crowding on the cis- than in the trans-isomer. Several novel examples of a new series of C-glycosylidenic derivatives, the spiro-pyrazolines, are described.     

Nucleation activity of nanosized CaCO3 on crystallization of isotactic polypropylene, in dependence on crystal modification, particle shape, and coating

A detailed analysis of the effect of calcium carbonate nanoparticles on crystallization of isotactic polypropylene (iPP) is reported in this contribution. CaCO₃ nanoparticles with different crystal modifications (calcite and aragonite) and particle shape were added in small percentages to iPP. The nanoparticles were coated with two types of compatibilizer (either polypropylene-g-maleic anhydride copolymer, or fatty acids) to improve dispersion and adhesion with the polymer matrix.It was found that the type of coating agent used largely affects the nucleating ability of calcium carbonate towards formation of polypropylene crystals. CaCO₃ nanoparticles coated with maleated polypropylene can successfully promote nucleation of iPP crystals, whereas the addition of nanosized calcium carbonate coated with fatty acids delays crystallization of iPP, the effect being mainly ascribed to the physical state of the coating in the investigated temperature range for crystallization of iPP, as well as to possible dissolution by fatty acids of heterogeneities originally present in the polypropylene matrix.     

Conservative logic

Conservative logic is a comprehensive model of computation which explicitly reflects a number of fundamental principles of physics, such as the reversibility of the dynamical laws and the conservation of certainadditive quantities (among which energy plays a distinguished role). Because it more closely mirrors physics than traditional models of computation, conservative logic is in a better position to provide indications concerning the realization of high-performance computing systems, i.e., of systems that make very efficient use of the computing resources actually offered by nature. In particular, conservative logic shows that it is ideally possible to build sequential circuits with zero internal power dissipation. After establishing a general framework, we discuss two specific models of computation. The first uses binary variables and is the conservative-logic counterpart of switching theory; this model proves that universal computing capabilities are compatible with the reversibility and conservation constraints. The second model, which is a refinement of the first, constitutes a substantial breakthrough in establishing a correspondence between computation and physics. In fact, this model is based on elastic collisions of identical balls, and thus is formally identical with the atomic model that underlies the (classical) kinetic theory of perfect gases. Quite literally, the functional behavior of a general-purpose digital computer can be reproduced by a perfect gas placed in a suitably shaped container and given appropriate initial conditions.This research was supported by the Defense Advanced Research Projects Agency and was monitored by the Office of Naval Research under Contract No. N000 14-75-C-0661.     

Supramolecular microcontact printing and dip-pen nanolithography on molecular printboards

The transfer of functional molecules onto self-assembled monolayers (SAMs) by means of soft and scanning-probe lithographic techniques-microcontact printing (muCP) and dip-pen nanolithography (DPN), respectively-and the stability of the molecular patterns during competitive rinsing conditions were examined. A series of guests with different valencies were transferred onto beta-cyclodextrin- (beta-CD-) terminated SAMs and onto reference hydroxy-terminated SAMs. Although physical contact was sufficient to generate patterns on both types of SAMs, only molecular patterns of multivalent guests transferred onto the beta-CD SAMs were stable under the rinsing conditions that caused the removal of the same guests from the reference SAMs. The formation of kinetically stable molecular patterns by supramolecular DPN with a lateral resolution of 60 nm exemplifies the use of beta-CD-terminated SAMs as molecular printboards for the selective immobilization of printboard-compatible guests on the nanometer scale through the use of specific, multivalent supramolecular interactions. Electroless deposition of copper on the printboard was shown to occur selectively on the areas patterned with dendrimer-stabilized gold nanoparticles.     

Adsorption of d-alaninol on Cu(1 0 0)

Adsorption of d-alaninol on Cu(1 0 0) at room temperature has been investigated by photoelectron spectroscopy in the soft X-ray and VUV energy range and low energy electron diffraction (LEED). d-Alaninol was found by LEED to self-assemble at full coverage; core and valence photoemission spectra are presented at low and full coverage. Chemisorption occurs at room temperature. The bonding at low coverage takes place at the hydroxylic group; at full coverage there is evidence of bonding for both hydroxylic and amino groups.     

Stringy Chern classes of singular varieties

Motivic integration [M. Kontsevich, Motivic integration, Lecture at Orsay, 1995] and MacPherson's transformation [R. MacPherson, Chern classes for singular varieties, Ann. of Math. 100 (1974) 423-432] are combined in this paper to construct a theory of “stringy” Chern classes for singular varieties. These classes enjoy strong birational invariance properties, and their definition encodes data coming from resolution of singularities. The singularities allowed in the theory are those typical of the minimal model program; examples are given by quotients of manifolds by finite groups. For the latter an explicit formula is proven, assuming that the canonical line bundle of the manifold descends to the quotient. This gives an expression of the stringy Chern class of the quotient in terms of Chern-Schwartz-MacPherson classes of the fixed-point set data.