Structure of CdS/SiO2 Nanocomposites: Influence of the Precursor and Cd Concentration

The influence of using TMOS or TEOS in the formation of CdS quantum dots in a silica matrix have been studied by X-ray absorption spectroscopy (XAS). The amount of Cd-S bonds have been monitored as a function of the nominal Cd concentration. The relative amount of CdS crystals depends on the precursor. The use of TEOS is not recommended because it gives a poor yield, especially for high Cd concentration. A discussion of the influence of CdS concentration in matrices from TMOS is carried out from structural models created from their pore volume distribution. The mean pore size becomes smaller and the size distribution more uniform when CdS concentration increases but the nanocrystals of low CdS nominal content present a more efficient quantum confinement.     

Some probabilistic techniques in field theory

We study, in the context of the Markov hyerarchical fields (d=2, 3) the role of the Markov property, of formal renormalization and of formal positivity. We determine upper and lower bounds for the ground state energy and discuss their relation with the perturbation theory series.This work has been partially supported by I.N.F.N., G.N.F.M., and G.N.S.M.     

A first-order phase transition between crystal phases in the shift model

We describe rigorously a many-body model of interacting classical particles exhibiting the following behavior at zero temperature: as the pressure varies through a critical value, the system goes through a first-order phase transition between different crystal phases. Moreover, at the critical pressure the system is demonstrably a mixture of the two phases.Supported in part by NSF Grant No. MCS81-01596.     

Quantum Heisenberg groups and sklyanin algebras

New bialgebra structures on the Heisenberg-Lie algebra and their quantizations are introduced. Some of these quantizations give rise to new multiplications in homogeneous coordinate rings of Abelian varieties, via the well-known identification of theta functions with suitable matrix coefficients of the Stone-von Neumann representations.N.A.: Forschungsstipendiat der Alexander von Humboldt-Stiftung. J. D. and A. T.: Postdoctoral fellowship, ICTP. N. A. and A. T.: This work was also partially supported by CONICET, CONICOR and FAMAF, Argentina.     

Martensitic fcc-to-hcp transformations in solid xenon under pressure: a first-principles study

First-principles calculations reveal that the fcc-to-hcp pressure-induced transformation in solid xenon proceeds through two mechanisms between 5 and 70 GPa. The dynamics of the phase transition involves a sluggish stacking-disorder growth at lower pressures (path I) that changes to a path involving an orthorhombic distortion at higher pressures (path II). The switchover is governed by a delicate interplay of energetics (enthalpy of the system for the structural stability) and kinetics (energy barrier for the transition). The two types of martensitic transformations involved in this pressure-induced structural transformation are a twinned martensitic transition at lower pressures and a slipped martensitic transition at higher pressures.     

Theoretical study of the complexes of horminone with Mg2+ and Ca2+ ions and their relation with the bacteriostatic activity

The coordination of the horminone molecule with hydrated magnesium and calcium divalent ions was studied by means of the density functional theory. All-electron calculations were performed with the B3LYP/6-31G method. The first layer of the water molecules surrounding the metallic cations was included. It was found that the octahedral [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) complex is more stable than [Mg(H(2)O)(6)](2+). That is, horminone is able to displace two water units from the hexahydrated complex. This behavior does not occur for Ca(2+). Consistently, [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) has a greater metal-ligand binding energy than [horminone(O(a)-O(d))-Ca-(H(2)O)(4)](2+). The preference of horminone by Mg(2+) is enlightened by these results. Moreover, its electronic structure, as shown by huge changes in the atomic populations, is strongly perturbed by Mg(2+). Indeed, horminone, bonded to [Mg(H(2)O)(4)](2+), is able to cross the bacterial membrane cell. Once inside, [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) binds to rRNA phosphate groups yielding [horminone(O(a)-O(d))-Mg-(H(2)O)(PO(4)H(2))(PO(4)H(3))(2)](+). These results give insights into how horminone may inhibit the initial steps of protein synthesis. The stability of the studied systems is accounted for in terms of the calculated structural and electronic properties: Mg-O and Ca-O bond lengths, charge transfers, and binding energies.     

Fully Ramified Characters and Clifford Codes

In this article we study relations between groups and quantum error correcting codes. Groups of central type are used to construct quantum error correcting codes. Both stabilizer and Clifford codes can be derived from a construction involving this kind of groups. A more general construction of Clifford codes will be given and their correcting properties will be examined using group theoretical techniques.     

Central Limit Theorems for the Shrinking Target Problem

Suppose B _i :=B(p,r _i ) are nested balls of radius r _i about a point p in a dynamical system (T,X,μ). The question of whether T ⁱ x∈B _i infinitely often (i.o.) for μ a.e. x is often called the shrinking target problem. In many dynamical settings it has been shown that if $E_{n}:=\sum_{i=1}^{n} \mu(B_{i})$ diverges then there is a quantitative rate of entry and $\lim_{n\to\infty} \frac{1}{E_{n}} \sum_{j=1}^{n} 1_{B_{i}} (T^{i} x) \to1$ for μ a.e. x∈X. This is a self-norming type of strong law of large numbers. We establish self-norming central limit theorems (CLT) of the form $\lim_{ n\to\infty} \frac{1}{a_{n}} \sum_{i=1}^{n} [1_{B_{i}} (T^{i} x)-\mu(B_{i})] \to N(0,1)$ (in distribution) for a variety of hyperbolic and non-uniformly hyperbolic dynamical systems, the normalization constants are $a^{2}_{n} \sim E [\sum_{i=1}^{n} 1_{B_{i}} (T^{i} x)-\mu(B_{i})]^{2}$ . Dynamical systems to which our results apply include smooth expanding maps of the interval, Rychlik type maps, Gibbs-Markov maps, rational maps and, in higher dimensions, piecewise expanding maps. For such central limit theorems the main difficulty is to prove that the non-stationary variance has a limit in probability.     

N,N′-Ethylene-Bridged Bis-2-Aryl-Pyrrolinium Cations to E-Diaminoalkenes: Non-Identical Stepwise Reversible Double-Redox Coupled Bond Activation Reactions

This work presents a stepwise reversible two-electron transfer induced hydrogen shift leading to the conversion of a bis-pyrrolinium cation to an E-diaminoalkene and vice versa. Remarkably, the forward and the reverse reaction, which are both reversible, follow two completely different reaction pathways. Establishing such unprecedented property in this type of processes was possible by developing a novel synthetic route towards the starting dication. All intermediates involved in both the forward and the backward reactions were comprehensively characterized by a combination of spectroscopic, crystallographic, electrochemical, spectroelectrochemical, and theoretical methods. The presented synthetic route opens up new possibilities for the generation of multi-pyrrolinium cation scaffold-based organic redox systems, which constitute decidedly sought-after molecules in contemporary chemistry.