Graphene nanoring as a tunable source of polarized electrons

We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.Comment: 12 pages, 7 figures, accepted in Nanotechnolog

A model for hand-over-hand motion of molecular motors

A simple flashing ratchet model in two dimensions is proposed to simulate the hand-over-hand motion of two head molecular motors like kinesin. Extensive Langevin simulations of the model are performed. Good qualitative agreement with the expected behavior is observed. We discuss different regimes of motion and efficiency depending of model parameters.Comment: 8 pages, Phys. Rev. E (in press

Building Fluorinated Hybrid Crystals: Understanding the Role of Noncovalent Interactions

Noncovalent interactions play a key role in functional materials. Metal-organofluorine interactions are of special interest because they directly affect the structure and reactivity of hybrid fluorinated materials. In-depth understanding and modulating of these interactions would enable the rational design of functional materials from fundamental chemical principles. In this work, we propose a computational approach that enables a comprehensive and quantitative characterization of noncovalent interactions (NCIs) in hybrid fluorinated crystals. Our approach couples dispersion-corrected density functional theory to NCI analysis. Additionally, we determine electron densities at bond critical points and identify electrostatic interactions using a simple electrostatic model. The versatility of this approach to probe a wide range of NCIs is demonstrated for a series of four bimetallic fluorinated crystals incorporating alkali-manganese(II) pairs and trifluoroacetato ligands. Noncovalent interactions in these hybrid crystals include metal-oxygen, metal-fluorine, hydrogen bonds, and van der Waals forces. Using K2Mn2(tfa)6(tfaH)2·H2O as an example, we demonstrate that its two-dimensional layered structure stems from a unique balance between these four NCIs. The computational approach presented herein should have general applicability to the quantitative study of NCIs in hybrid crystals, thereby serving as a guide for crystal engineering of novel hybrid materials

A bonding evolution theory study on the catalytic Noyori hydrogenation reaction

The electronic rearrangements involved in Noyori hydrogenation reactions with double bonds (ethene and formaldehyde) are analysed using the bonding evolution theory. The study and analysis of the changes on the electron localisation function topology along a given reaction path reveals fluxes of electron density, allowing to unambiguously identify the main chemical events happening along the chemical reactions. This analysis shows that the first hydrogen transfer (with hydride character) occurs before the transition state (TS), while the second hydrogen transfer (with proton character) takes places after having reached the TS. The lower energy barrier found for formaldehyde over ethene is explained by two reasons. First, the hydride transfer is favoured for the C = O bond over C = C due to the electrophilic character of the carbon atom. Second, a negatively charged CH3–X (X = CH2, O) hidden intermediate is formed in the proximities of the TS region. The oxygen atom is able to stabilise this negatively charged species more effectively than the CH2 group due to its higher electronegativity and the presence of V(O) lone pairs. The obtained analysis explains and rationalises catalyst chemoselectivity (C = O vs. C = C). Finally, a curly arrow representation diagram accounting for the electronic rearrangements is proposed on the basis of BET results

A first step towards quantum energy potentials of electron pairs

A first step towards the construction of a quantum force field for electron pairs in direct space is taken. Making use of topological tools (Interacting Quantum Atoms and the Electron Localisation Function), we have analysed the dependency of electron pairs electrostatic, kinetic and exchange-correlation energies upon bond stretching. Simple correlations were found, and can be explained with elementary models such as the homogeneous electron gas. The resulting energy model is applicable to various bonding regimes: from homopolar to highly polarized and even to non-conventional bonds. Overall, this is a fresh approach for developing real space-based force fields including an exchange-correlation term. It provides the relative weight of each of the contributions, showing that, in common Lewis structures, the exchange correlation contribution between electron pairs is negligible. However, our results reveal that classical approximations progressively fail for delocalised electrons, including lone pairs. This theoretical framework justifies the success of the classic Bond Charge Model (BCM) approach in solid state systems and sets the basis of its limits. Finally, this approach opens the door towards the development of quantitative rigorous energy models based on the ELF topology

Scores of hesitant fuzzy elements revisited: “Was sind und was sollen”

[EN] This paper revolves around the notion of score for hesitant fuzzy elements, the constituent parts of hesitant fuzzy sets. Scores allow us to reduce the level of uncertainty of hesitant fuzzy sets to classical fuzzy sets, or to rank alternatives characterized by hesitant fuzzy information. We propose a rigorous and normative definition capable of encapsulating the characteristics of the most important scores introduced in the literature. We systematically analyse different types of scores, with a focus on coherence properties based on cardinality and monotonicity. The hesitant fuzzy elements considered in this analysis are unrestricted. The inspection of the infinite case is especially novel. In particular, special attention will be paid to the analysis of hesitant fuzzy elements that are intervals

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) by functionalized NHC-based polynuclear catalysts: scope and mechanistic insights

Copper(I) Cu2(µ-Br)2(tBuImCH2pyCH2L)]n (L = OMe, NEt2, NHtBu) compounds supported by flexible functionalized NHC-based polydentate ligands have been prepared in a one-pot procedure by reacting the corresponding imidazolium salt with an excess of copper powder and Ag2O. An X-ray diffraction analysis has revealed that Cu2(µ-Br)2(tBuImCH2pyCH2NEt2)]n is a linear coordination polymer formed by bimetallic Cu(µ-Br)]2 units linked by the lutidine-based NHC-py-NEt2 ligand, which acts as a heteroditopic ligand with a 1¿C-2¿2N, N' coordination mode. We propose that the polymeric compounds break down in the solution into more compact tetranuclear Cu2(µ-Br)2(tBuImCH2pyCH2L)]2 compounds with a coordination mode identical to the functionalized NHC ligands. These compounds have been found to exhibit high catalytic activity in the Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. In particular, Cu2(µ-Br)2(tBuImCH2pyCH2NEt2)]2 efficiently catalyzes the click reaction of a range of azides and alkynes, under an inert atmosphere at room temperature in neat conditions at a very low catalyst loading, to quantitatively afford the corresponding 1, 4-disubstituted 1, 2, 3-triazole derivatives in a few minutes. The cycloaddition reaction of benzyl azide to phenylacetylene can be performed at 25-50 ppm catalyst loading by increasing the reaction time and/or temperature. Reactivity studies have shown that the activation of the polynuclear catalyst precursor involves the alkyne deprotonation by the NHC moiety of the polydentate ligand to afford a copper(I)-alkynyl species bearing a functionalized imidazolium ligand. DFT calculations support the participation of the dinuclear species (CuBr)2(µ-tBuImCH2pyCH2NEt2)], resulting from the fragmentation of the tetranuclear compound, as the catalytically active species. The proposed reaction pathway proceeds through zwitterionic dinuclear intermediates and entails the active participation of both copper atoms, as well as the NHC moiety as an internal base, which activates the reacting alkyne via deprotonation

Efficient Rhodium-catalyzed multicomponent reaction for the synthesis of novel propargylamines

[{Rh(μ-Cl)(H)2(IPr)}2] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazole-2-ylidene) was found to be an efficient catalyst for the synthesis of novel propargylamines by a one-pot three-component reaction between primary arylamines, aliphatic aldehydes, and triisopropylsilylacetylene. This methodology offers an efficient synthetic pathway for the preparation of secondary propargylamines derived from aliphatic aldehydes. The reactivity of [{Rh(μ-Cl)(H)2(IPr)}2] with amines and aldehydes was studied, leading to the identification of complexes [RhCl(CO)IPr(MesNH2)] (MesNH2 = 2,4,6-trimethylaniline) and [RhCl(CO)2IPr]. The latter shows a very low catalytic activity while the former brought about reaction rates similar to those obtained with [{Rh(μ-Cl)(H)2(IPr)}2]. Besides, complex [RhCl(CO)IPr(MesNH2)] reacts with an excess of amine and aldehyde to give [RhCl(CO)IPr{MesN[DOUBLE BOND]CHCH2CH(CH3)2}], which was postulated as the active species. A mechanism that clarifies the scarcely studied catalytic cycle of A3-coupling reactions is proposed based on reactivity studies and DFT calculations.This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593, CTQ2012-35665 and CTQ2013-42532-P projects) and the DGA/FSE-E07. The support from KFUPM-University of Zaragoza research agreement and the Centre of Research Excellence in Petroleum Refining & KFUPM is gratefully acknowledged. V. P. thankfully acknowledges the resources from the supercomputer >Memento>, technical expertise and assistance provided by BIFI-ZCAM (Universidad de Zaragoza). L.R.-P thanks to CONACyT for a postdoctoral fellowship (204033).Peer Reviewe

An insight into transfer hydrogenation reactions catalysed by iridium(III) bis-N-heterocyclic carbenes

A variety of [M(L)2(L′)2{κC,C′-bis(NHC)}]BF4 complexes (M = Rh or Ir; L = CH3CN or wingtip group; L′ = I– or CF3COO–; NHC=N-heterocyclic carbene) have been tested as pre-catalysts for the transfer hydrogenation of ketones and imines. The conversions and TOF's obtained are closely related to the nature of the ligand system and metal centre, more strongly coordinating wingtip groups yielding more active and recyclable catalysts. Theoretical calculations at the DFT level support a classic stepwise metal-hydride pathway against the concerted Meerwein–Ponndorf–Verley (MPV) mechanism. The calculated catalytic cycle involves a series of ligand rearrangements due to the high trans effect of the carbene and hydrido ligands, which are more stable when situated in mutual cis positions. The reaction profiles obtained for the complexes featuring an iodide or a trifluoroacetate in one of the apical positions agree well with the relative activity observed for both catalysts.The authors would like to acknowledge the support by the Ministry of Higher Education, Saudi Arabia, in establishment of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at KFUPM (KACST-funded project ART-32-68). The support under the KFUPM–University of Zaragoza research agreement is also highly appreciated. This work was further supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593 and CTQ2012-35665 projects) and the Diputación General de Aragón (DGA/FSE-E07).Peer Reviewe