From orientation disordered to ordered—An ab initio simulation on ammonia borane phase transition within van der Waals corrections

In this work, we report a detailed theoretical investigation of the phase transition of ammonia borane (NH₃BH₃; AB), from a tetragonal I4mm ( ) phase with disordered orientation of hydrogen to an orthorhombic phase with Pmn2₁ ( ) symmetry, as a function of temperature based on Density Functional Theory calculations with semiempirical dispersion potential correction. We define a series of substructures with the NH₃BH₃ moiety always in C_3v symmetry and the partially occupied high temperature state can be described as a continuous transformation between these substructures. To understand the role of the van der Waals corrections to the physical properties, we use the empirical Grimme's dispersion potential correction (PBE‐D2). Both Perdew–Burke–Emzerhof (PBE) and PBE‐D2 functional yield almost the same energy sequence along the transition path. However, PBE‐D2 functional shows obvious advantage in describing the lattice parameters of AB. The rigid rotor harmonic oscillator approximation is used to compute the free energy and the entropies contribution along the transition pathway. With knowledge of free energy surfaces along rotations of the ? [NH₃] and ? [BH₃] groups, complete transformation paths are mapped out. The phase transition is found to follow the sequence of partially occupied tetragonal system (I4mm) of a mixture of states with monoclinic (Cc), (CM) and orthorhombic (Pmn2₁) symmetries to fully occupied quasitetragonal system (the intermediate phase, Pmn2₁) to fully occupied orthorhombic system (Pmn2₁). © 2014 Wiley Periodicals, Inc.     

Theoretical Studies of the Reactions CFxH3−xCOOR+Cl and CF3COOCH3+OH

The mechanism and kinetics of the reactions of CF₃COOCH₂CH₃, CF₂HCOOCH₃, and CF₃COOCH₃ with Cl and OH radicals are studied using the B3LYP, MP2, BHandHLYP, and M06‐2X methods with the 6‐311G(d,p) basis set. The study is further refined by using the CCSD(T) and QCISD(T)/6‐311++G(d,p) methods. Seven hydrogen‐abstraction channels are found. All the rate constants, computed by a dual‐level direct method with a small‐curvature tunneling correction, are in good agreement with the experimental data. The tunneling effect is found to be important for the calculated rate constants in the low‐temperature range. For the reaction of CF₃COOCH₂CH₃+Cl, H‐abstraction from the CH₂ group is found to be the dominant reaction channel. The standard enthalpies of formation for the species are also calculated. The Arrhenius expressions are fitted within 200–1000 K as k_T(1)=8.4×10^?20T ^2.63exp(381.28/T), k_T(2)=2.95×10^?21T ^3.13exp(?103.21/T), k_T(3)=1.25×10^?23T ^3.37exp(791.98/T), and k_T(4)=4.53×10^?22T ^3.07exp(465.00/T).     

The Transition States for CO2 Capture by Substituted Ethanolamines

Quantum chemical studies are used to understand the electronic and steric effects on the mechanisms of the reaction of substituted ethanolamines with CO₂. SCS‐MP2/6‐311+G(2d,2p) calculations are used to obtain the activation energy barriers and reaction energies for both the carbamate and bicarbonate formation. Implicit solvent effects are included with the universal solvation model SMD. Carbamate formation is more favorable than bicarbonate formation for monoethanolamine (MEA) both kinetically and thermodynamically. Increase of the steric hindrance on the C atoms around the N atom in substituted ethanolamines favors bicarbonate formation over carbamate formation with lower activation barriers and thereby higher reaction rates. In contrast, substitution by an N‐methyl or N‐ethyl group on MEA leads to a lower activation barrier for both carbamate formation and bicarbonate formation. As a result, higher reaction rates are expected as compared to MEA, and therefore these compounds have significant potential as industrial CO₂ capturing solvents.     

A Ratiometric Luminescent Thermometer Co‐doped with Lanthanide and Transition Metals

Herein, we report the fabrication of a sensitive ratiometric and colorimetric luminescent thermometer with a wide operating‐temperature range, from cryogenic temperatures up to high temperatures, through the combination of lanthanide and transition metal complexes. Benefiting from the transition metal complex as a self‐reference, the lanthanide content in the mixed‐coordination complex, Eu_0.05(Mebip‐mim bromine)_0.15Zn_0.95(Mebip‐mim bromine)_1.9, was lowered to 5 %.     

New isostructural transition metal complexes with a non‐innocent dithiolate ligand

Three new complexes with 3,6‐dichlorobenzene‐1,2‐dithiol (bdtCl₂), namely methyltriphenylphosphonium bis(3,6‐dichlorobenzene‐1,2‐dithiolato‐κ²S,S′)cobaltate(1−), (C₁₉H₁₈P)[Co(C₆H₂Cl₂S₂)₂], (I), bis(methyltriphenylphosphonium) bis(3,6‐dichlorobenzene‐1,2‐dithiolato‐κ²S,S′)cuprate(2−) dimethyl sulfoxide disolvate, (C₁₉H₁₈P)₂[Cu(C₆H₂Cl₂S₂)₂]·2C₂H₆OS, (II), and methyltriphenylphosphonium bis(3,6‐dichlorobenzene‐1,2‐dithiolato‐κ²S,S′)cuprate(1−), (C₁₉H₁₈P)[Cu(C₆H₂Cl₂S₂)₂], (III), have been synthesized and characterized by single‐crystal X‐ray diffraction. The X‐ray structure analyses of all three complexes confirm that the four donor S atoms form a slightly distorted square‐planar coordination arrangement around the central metal atom. An interesting finding for both the Cu^II and Cu^III complexes, i.e. (II) and (III), respectively, is that the coordination polyhedra are principally the same and differ only slightly with respect to the interatomic distances.     

A three‐dimensional anionic metal–dicyanamide network in poly[ethyltriphenylphosphonium [tri‐μ2‐dicyanamidato‐cadmium(II)]]

The title compound, (C₂₀H₂₀P)[Cd(C₂N₃)₃], consists of ethyltriphenylphosphonium (EtPh₃P⁺) cations filling voids in a three‐dimensional anionic cadmium dicyanamide network. In the structure, each Cd^II atom is connected to six neighbouring Cd^II atoms through six separate dicyanamide ligands, forming cube‐shaped cages. The three‐dimensional anionic network encloses a solvent‐accessible void space of 1851 ų, amounting to 69.3% of the unit‐cell volume. Each cage accommodates only one EtPh₃P⁺ cation.     

Recent developments in CVD growth and applications of 2D transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with fascinating electronic energy band structures, rich valley physical properties and strong spin–orbit coupling have attracted tremendous interest, and show great potential in electronic, optoelectronic, spintronic and valleytronic fields. Stacking 2D TMDs have provided unprecedented opportunities for constructing artificial functional structures. Due to the low cost, high yield and industrial compatibility, chemical vapor deposition (CVD) is regarded as one of the most promising growth strategies to obtain high-quality and large-area 2D TMDs and heterostructures. Here, state-of-the-art strategies for preparing TMDs details of growth control and related heterostructures construction via CVD method are reviewed and discussed, including wafer-scale synthesis, phase transition, doping, alloy and stacking engineering. Meanwhile, recent progress on the application of multi-functional devices is highlighted based on 2D TMDs. Finally, challenges and prospects are proposed for the practical device applications of 2D TMDs.     

Spin waves and phase transition on a magnetically frustrated square lattice with long-range interactions

We investigate the effects of long-range interactions on the spin wave spectra and the competition between magnetic phases on a frustrated square lattice with large spin S. Applying the spin wave theory and assisted with symmetry analysis, we obtain analytical expressions for spin wave spectra of competing Neel and (π, 0) stripe states of systems containing any-order long-range interactions. In the specific case of long-range interactions with power-law decay, we find surprisingly that the staggered long-range interaction suppresses quantum fluctuation and enlarges the ordered moment, especially in the Neel state, and thus extends its phase boundary to the stripe state. Our findings illustrate the rich possibilities of the roles of long-range interactions, and advocate future investigations in other magnetic systems with different structures of interactions.     

狄尔斯-阿尔德反应催化剂

狄尔斯-阿尔德(Diels-Alder)可以构建结构丰富的有机化合物,被认为是现代有机化学中的基石反应之一。自1928年被发现以来,Diels-Alder (D-A)反应得到了深入发展,这主要是由于该反应能够产生六元环结构,可以一步反应得到具有四个立体中心的产物,从而大大增加分子的复杂性。这种特殊的转化已广泛应用于复杂天然产物的合成、药物化学以及材料科学等领域。近十年,大量天然生物酶(如SpnF、MaDA等)被发现可用于体外单独催化D-A反应,同时,大量的非酶催化剂(如路易斯酸、过渡金属与配体复合物等)也被应用于催化D-A反应。本文主要从D-A反应催化剂的类型分类,对近年来天然酶、酸、过渡金属、电催化等参与的D-A反应进行简要概述,同时对催化剂所存在的问题和局限性进行总结,并对今后发展做了展望。     

Atomic Layer Deposited Ti2O3 Thin Films

Ti₂O₃ thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti₂O₃ thin films, the insulator-metal transition is observed at ∼80 K, with nearly 3–4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depending on the crystallographic c/a ratio leads to a change in electrical resistivity as a function of temperature.