Structural,magnetic, and vibrational properties of stoichiometric clusters of CrN

We performed density‐functional‐theoretic calculations to investigate the structural, magnetic and vibrational properties of the stoichiometric clusters (CrN)_n ( ). We show that the building block of the ground‐state structures of these clusters is a square CrNCrN unit; the only exception with n > 2 occurs for (CrN)₃, but this cluster has an isomer not very far in energy from the ground state consisting of a pair of CrNCrN squares sharing a CrN bond. In the smaller CrN, (CrN)₂, and (CrN)₃ clusters the magnetic moments of the N atoms are non‐negligible and antiparallel to those of the Cr atoms, but for the larger species (CrN)₄, (CrN)₅, (CrN)₆, and (CrN)₉ the cluster magnetic moments are almost entirely due to the Cr atoms. Lack of imaginary vibrational frequencies in the predicted ground‐state structures of (CrN)_n ( ) confirms that they are mechanically stable equilibrium states. © 2015 Wiley Periodicals, Inc.     

Fluoride‐Promoted Esterification with Imidazolide‐Activated Compounds: A Modular and Sustainable Approach to Dendrimers

Based on the growing demand for facile and sustainable synthetic methods to structurally perfect polymers, we herein describe a significant improvement of esterification reactions capitalizing on 1,1′‐carbonyldiimidazole (CDI). Cesium fluoride was shown to be an essential catalyst for these reactions to reach completion. This approach was successfully applied to the synthesis of structurally flawless and highly functional polyester dendrimers employing traditional and accelerated growth strategies. A sixth generation bis‐MPA dendrimer with a molecular weight of 22.080 Da and 192 peripheral hydroxy groups was isolated in less than one day of total reaction time. Large quantities of dendrimerswere obtained in high yields (>90 %) using simple purification steps under sustainable conditions. The fluoride‐promoted esterification (FPE) via imidazolide‐activated compounds is wide in scope and constitutes a potentially new approach toward functional polymers and other materials.     

Gas chromatographic determination of 29 organic acids in foodstuffs after continuous solid-phase extraction

A simple and expeditious method based on continuous solid-phase extraction and gas chromatography-mass spectrometry (GC-MS) was reported for the direct determination of 29 organic acids in food and beverages. A sorbent column packed with 80 mg of LiChrolut EN-Supelclean ENVI-18 (1:1) was employed for extraction and clean-up purposes. After elution with 200 μL of methanol, the methanolic extract was directly injected into the GC-MS without prior derivatization. The method provided good linearity (0.5-1000 μg kg⁻¹) and fairly good precision for all compounds (RSD lower than 6.2%). The recoveries of the organic acids from diluted samples that were spiked at three different concentrations (10, 40 and 100 μg kg⁻¹) ranged from 93 to 98%. The applicability of the method was demonstrated by analyzing the target compounds in a wide variety of foodstuffs including beer, wine, fruit juice, soy sauce, soya milk and honey samples.     

Unexpected multiple bond cleavage and rearrangement of organosulfide ligands in the presence of Cu(II) assisted by solvothermal and solvothermal-microwave conditions

An unprecedented in situ multiple bond cleavage of S-S, S-C(sp(2)) and C-N in the pyrimidinedisulfide (pym(2)S(2)) ligand is observed by the reaction of CuCl(2)·2H(2)O with this ligand under solvothermal and solvothermal-microwave conditions. In this process the formation of the compound [Cu(II)(μ-Cl)(Cl)L](2), where L represents the new ligand (L = 2-(pyrimidin-2-ylamino)-1,3-thiazole-4-carbaldehyde), is observed. This ligand has been further isolated and X-ray characterized. The similar reaction carried out under solvothermal-microwave conditions gives, in addition to the latter compound, the complex {9·[Cu(pym(2)S(3))(μ-Cl)(Cl)](2)·[Cu(pym(2)S(2))(μ-Cl)(Cl)](2)}. Coordination of a pyrimidinetrisulfide ligand (pym(2)S(3)) is reported for the first time. This work represents an illustrative example of the novel synthetic perspectives attainable via solvothermal-microwave procedures.     

A Proposal for a Low-Frequency Axion Search in the 1–2 μ eV Range and Below with the BabyIAXO Magnet

In the near future BabyIAXO will be the most powerful axion helioscope, relying on a custom-made magnet of two bores of 70 cm diameter and 10 m long, with a total available magnetic volume of more than 7 m³. In this document, it proposes and describe the implementation of low-frequency axion haloscope setups suitable for operation inside the BabyIAXO magnet. The RADES proposal has a potential sensitivity to the axion-photon coupling $g_{a\gamma }$ down to values corresponding to the KSVZ model, in the (currently unexplored) mass range between 1 and 2 $\mu$ eV, after a total effective exposure of 440 days. This mass range is covered by the use of four differently dimensioned 5-meter-long cavities, equipped with a tuning mechanism based on inner turning plates. A setup like the one proposed will also allow an exploration of the same mass range for hidden photons coupled to photons. An additional complementary apparatus is proposed using LC circuits and exploring the low energy range ( $\approx {10}^{-4}-{10}^{-1}\mu$ eV). The setup includes a cryostat and cooling system to cool down the BabyIAXO bore down to about 5 K, as well as an appropriate low-noise signal amplification and detection chain.     

Tetrahydrofuran-induced K and Li doping onto poly(furfuryl alcohol)-derived activated carbon (PFAC): influence on microstructure and H2 sorption properties

We have doped poly(furfuryl alcohol)-derived activated carbon (PFAC) with two alkali metals, potassium (K) and lithium (Li), by previously reacting the metals with naphthalene in the presence of tetrahydrofuran (THF), followed by introducing them to pristine PFAC. The THF molecule causes a minor alteration of the microstructure of PFAC as confirmed by Raman spectra, X-ray diffraction, and pore textural analysis. Raman spectra and X-ray diffraction indicated a slight localized ordering toward the stacking defects of disordered carbon, as in PFAC, which can be attributed to the movement of THF molecules within the internal planes of graphene sheets. Pore textural analysis confirmed the lowering of the specific surface area and pore volume of both K- and Li-doped PFACs (BET SSA, 1378 m(2)/g (PFAC); 1252 m(2)/g (K-PFAC), 1081 m(2)/g (Li-PFAC)). Volumetric hydrogen adsorption measurements at temperatures of 298, 288, 273, and 77 K and pressures of up to 1 bar indicated the enhanced adsorption potential imposed by the presence of alkali metals, which can be reconfirmed by the elevated heats of adsorption of metal-doped PFACs (Li-PFAC, -(10-11) kJ/mol; K-PFAC, -(16-19) kJ/mol) compared to that of pristine PFAC (-9.6 kJ/mol).     

Facile Access to Silicon‐Functionalized Bis‐Silylene Titanium(II) Complexes

A series of unprecedented bis‐silylene titanium(II) complexes of the type [(η⁵‐C₅H₅)₂Ti(LSiX)₂] (L=PhC(NtBu)₂; X=Cl, CH_3, H) has been prepared using a phosphane elimination strategy. Treatment of the [(η⁵‐C₅H₅)₂Ti(PMe₃)₂] precursor ( 1 ) with two molar equivalents of the N‐heterocyclic chlorosilylene LSiCl ( 2 ), results in [(η⁵‐C₅H₅)₂Ti(LSiCl)₂] ( 3 ) with concomitant PMe₃ elimination. The presence of a Si? Cl bond in 3 enabled further functionalization at the silicon(II) center. Accordingly, a salt metathesis reaction of 3 with two equivalents of MeLi results in [(η⁵‐C₅H₅)₂Ti(LSiMe)₂] ( 4 ). Similarly, the reaction of 3 with two equivalents of LiBHEt₃ results in [(η⁵‐C₅H₅)₂Ti(LSiH)₂] ( 5 ), which represents the first example of a bis‐(hydridosilylene) metal complex. All complexes were fully characterized and the structures of 3 and 4 elucidated by single‐crystal X‐ray diffraction analysis. DFT calculations of complexes 3 – 5 were also carried out to assess the nature of the titanium–silicon bonds. Two σ and one π‐type molecular orbital, delocalized over the Si‐Ti‐Si framework, are observed.     

The Structural Diversity Triggered by Intermolecular Interactions between AuIS2 Groups: Aurophilia and Beyond

The present study is aimed at elucidating the factors that direct the assembly of a specific family of Au^I species. The assembly of Au^I centers and dithiocarboxylato or xanthato ligands results in a surprising structural diversity observed by single‐crystal X‐ray diffraction. However, in solution, just evidences for discrete bimetallic [Au₂L₂] species have been observed. Interestingly, when dithiocarboxylato ligands have been used, a reversible supramolecular assembly has been observed forming the supramolecules of formulae [Au₂L₂]₂ and [Au₂L₂]₃. Initial studies on luminescent properties have been carried out at variable temperature. All the compounds show red emissions in the solid state at very similar energies, suggesting that the intramolecular interactions play a more relevant role in the luminescent properties than the intermolecular ones. The computational studies indicate that not only Au???Au interactions, but also Au???S and S???S ones play a role in the structure and energetic of the supramolecular species, as well as for the choice between supramolecular association or intramolecular oligomerization.     

Accuracy analysis of simplified and rigorous numerical methods applied to binary nanopatterning gratings in non-paraxial domain

A set of simplified and rigorous electromagnetic vector theories is used for analyzing the transmittance characteristics of diffraction phase gratings. The scalar diffraction theory and the effective medium theory are validated with the exact results obtained via the rigorous coupled-wave theory and the finite-difference time-domain method. The effects of surface profile parameters and also the angle of incidence is demonstrated to be a limiting factor in the accuracy of these theories. Therefore, the error of both simplified theories is also analyzed in non-paraxial domain with the intention of establishing a specific range of validity for both simplified theories.     

Heuristics and metaheuristics for the maximum diversity problem

This paper presents extensive computational experiments to compare 10 heuristics and 20 metaheuristics for the maximum diversity problem (MDP). This problem consists of selecting a subset of maximum diversity from a given set of elements. It arises in a wide range of real-world settings and we can find a large number of studies, in which heuristic and metaheuristic methods are proposed. However, probably due to the fact that this problem has been referenced under different names, we have only found limited comparisons with a few methods on some sets of instances. This paper reviews all the heuristics and metaheuristics for finding near-optimal solutions for the MDP. We present the new benchmark library MDPLIB, which includes most instances previously used for this problem, as well as new ones, giving a total of 315. We also present an exhaustive computational comparison of the 30 methods on the MDPLIB. Non-parametric statistical tests are reported in our study to draw significant conclusions.