New magnetic copper(II) coordination polymers with the polynitrile ligand (C[C(CN)2]3)2- and N-donor co-ligands

Reactions between CuCl2 and K2tcpd (tcpd2- = [C10N6]2- = (C[C(CN)2]3)2-) in the presence of neutral co-ligands (bpym = 2,2'-bipyrimidine, and tn = 1,3-diaminopropane) in aqueous solution yield the new compounds [Cu2(bpym)(tcpd)2(H2O)4] x 2H2O (1), [Cu(tn)(tcpd)] (2), and [Cu(tn)2(tcpd)] x H2O (3), which are characterized by X-ray crystallography and magnetic measurements. Compound 1 displays a one-dimensional structure in which the bpym ligand, acting with a bis-chelating coordination mode, leads to [Cu2(bpym)]4+ dinuclear units which are connected by two mu2-tcpd2- bridging ligands. Compound 2 consists of a three-dimensional structure generated by [Cu(tn)]2+ units connected by a mu4-tcpd2- ligand. The structure of 3 is made up of centrosymmetric planar [Cu(tn)]2+ units connected by a mu2-tcpd2- ligand leading to infinite zigzag chains. In compounds 1 and 3, the bridging coordination mode of the tcpd2- unit involves only two nitrogen atoms of one C(CN)2 wing, while in 2, this ligand acts via four nitrogen atoms of two C(CN)2 wings. Despite this difference, the structural features of the tcpd2- units in 1-3 are essentially similar. Magnetic measurements for compound 1 exhibit a maximum in the chi(m) vs T plot (at approximately 150 K) which is characteristic of strong antiferromagnetic exchange interactions between the Cu(II) metal ions dominated by the magnetic exchange through the bis-chelating bpym. The fit of the magnetic data to a dimer model gives J and g values of -90.0 cm(-1) and 2.12, respectively. For compounds 2 and 3 the thermal variations of the magnetic susceptibility show weak antiferromagnetic interactions between the Cu(II) metal ions that can be well reproduced with an antiferromagnetic regular S = 1/2 chain model that gives J values of -0.07(2) and -0.18(1) cm(-1) with g values of 2.12(1) and 2.13(1) for compounds 2 and 3, respectively (the Hamiltonian is written in all the cases as H = -2JS(a)S(b)).     

Drift-kinetic Alfvén waves observed near a reconnection X line in the earth's magnetopause

We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth's magnetopause. The dispersive properties of these waves have been determined using wavelet interferometric techniques applied to multipoint observations from the Cluster spacecraft. Comparison of the observed wave dispersion with that expected for drift-kinetic Alfvén waves shows close agreement. The waves propagate outwards from the X line suggesting that reconnection is a kinetic Alfvén wave source. Energetic O+ ions observed in these waves indicate that reconnection is a driver of auroral ion outflow.     

Centrality dependence of charged hadron transverse momentum spectra in Au+Au collisions from sqrt[s(NN)]=62.4 to 200 GeV

We have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt[s(NN)]=62.4 GeV. The spectra are presented for transverse momenta 0.25相似文献   

All-fiber comb filter with tunable free spectral range

We demonstrate a new class of all-fiber frequency comb filter that exhibits unprecedented capabilities for tuning the comb's free spectral range (FSR). The filter exploits a spectral Talbot-like effect in a sampled chirped fiber Bragg grating. The FSR is tailored by application of a linear strain gradient to modify the relative phase between the samples. The FSR can be tuned in discrete steps that correspond to the nominal value of the FSR of the sampled unchirped grating divided by integer factors. In this demonstration the FSR is varied from 51 to 3.9 GHz.     

Light collimation and focusing by a thin flat metallic slab

We present experimental and theoretical work showing that a flat metallic slab can collimate and focus light impinging on the slab from a point source. The effect is optimized when the radiation is around the bulk, not at the surface, plasma frequency. Also, the smaller the imaginary part of the permittivity is, the better the collimation. Experiments for Ag in the visible as well as calculations are presented. We also discuss the interesting case of Al, whose imaginary part of permittivity is very small at the plasma frequency in UV radiation. Generalization to other materials and radiations are also discussed.     

Spatial information transmission using orthogonal mutual coherence coding

We use the coherence of a light beam to encode spatial information. We apply this principle to obtain spatial superresolution in a limited aperture system. The method is based on shaping the mutual intensity function of the illumination beam in a set of orthogonal distributions, each one carrying the information for a different frequency bandpass or spatial region of the input object. The coherence coding is analogous to time multiplexing but with multiplexing time slots that are given by the coherence time of the illumination beam. Most images are static during times much longer than this coherence time, and thus the increase of resolution in our system is obtained without any noticeable cost.     

Characterization of a 20 kHz sonoreactor. Part I: analysis of mechanical effects by classical and numerical methods

Numerical simulations have been carried out in order to characterize the ultrasonic field propagation and to obtain the spatial distribution of the mechanical effect derived from it. The results have been compared with those obtained with different classical physical methods (calorimetry, aluminium foil erosion, thermal probes) and have given useful information about the influence of the presence of probes and auxiliary tools in the ultrasonic field. All these information have been used for the development of the Part II of this work: analysis of chemical effects, providing an accurate picture of the reaction environment in the sonoreactor used (20 kHz, 100 W supplied by Undatim) for further uses in sonoelectrochemical studies.     

(S,S)-(+)-pseudoephedrine as chiral auxiliary in asymmetric acetate aldol reactions

The asymmetric acetate-type aldol reaction using (S,S)-(+)-pseudoephedrine has been studied in detail. Experimental variables like the nature of the metal counterion of the enolate, the presence of additives and the structure of the aldehyde have been examined in order to reach to the highest possible yields and diastereoselectivities.     

Relaxations in poly(vinyl alcohol) and in poly(vinyl acetate) detected by fluorescence emission of 4-aminophthalimide and prodan

Steady-state and time-resolved emission spectroscopy (TRES) of the medium-sensitive probes 4-aminophthalimide (4-AP) and 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were performed at 77 and 298 K in vacuum-sealed thin films of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc). The two probes show similar red-edge effect in steady state emission and a red shift with time in TRES in PVA. In PVAc the red shifts are much smaller and the spectral shift for 4-AP is slower. 4-AP locates in highly polar environments in PVA, where H-bond interaction with the polymer is important. Prodan locates in less polar environments, as evidenced by the position of the emission maximum with respect to reference solvents. Consequently, the observed monoexponential spectral red shift with time of 4-AP in PVA and in PVAc is attributed to relaxation of the interaction of the probe with the hydroxy and acetate moieties, respectively. The more intense interaction of the lighter -OH moiety with the probes explains the greater and faster spectral shift observed in PVA compared to PVAc. The lifetime of this monoexponential spectral shift is independent of temperature in PVA and takes place with a highly negative activation entropy. This fact is attributed to a collective rearrangement of -OH groups to better interact with the excited state. This relaxation nevertheless does not account for the complete accommodation of the excited state. Prodan shows a linear variation of the spectral shift with time that can be explained by microheterogeneity. In PVA, the width at half-maximum of the emission spectra does not change with time for Prodan and it decays with a lifetime similar to the lifetime of the spectral shift in the case of 4-AP. The differences in the behavior of the probes are attributed to their different average location in the polymer matrix.