HP-Ca2Si5N8--a new high-pressure nitridosilicate: synthesis, structure, luminescence, and DFT calculations

HP-Ca(2)Si(5)N(8) was obtained by means of high-pressure high-temperature synthesis utilizing the multianvil technique (6 to 12 GPa, 900 to 1200 degrees C) starting from the ambient-pressure phase Ca(2)Si(5)N(8). HP-Ca(2)Si(5)N(8) crystallizes in the orthorhombic crystal system (Pbca (no. 61), a=1058.4(2), b=965.2(2), c=1366.3(3) pm, V=1395.7(7)x10(6) pm(3), Z=8, R1=0.1191). The HP-Ca(2)Si(5)N(8) structure is built up by a three-dimensional, highly condensed nitridosilicate framework with N([2]) as well as N([3]) bridging. Corrugated layers of corner-sharing SiN(4) tetrahedra are interconnected by further SiN(4) units. The Ca(2+) ions are situated between these layers with coordination numbers 6+1 and 7+1, respectively. HP-Ca(2)Si(5)N(8) as well as hypothetical orthorhombic o-Ca(2)Si(5)N(8) (isostructural to the ambient-pressure modifications of Sr(2)Si(5)N(8) and Ba(2)Si(5)N(8)) were studied as high-pressure phases of Ca(2)Si(5)N(8) up to 100 GPa by using density functional calculations. The transition pressure into HP-Ca(2)Si(5)N(8) was calculated to 1.7 GPa, whereas o-Ca(2)Si(5)N(8) will not be adopted as a high-pressure phase. Two different decomposition pathways of Ca(2)Si(5)N(8) (into Ca(3)N(2) and Si(3)N(4) or into CaSiN(2) and Si(3)N(4)) and their pressure dependence were examined. It was found that a pressure-induced decomposition of Ca(2)Si(5)N(8) into CaSiN(2) and Si(3)N(4) is preferred and that Ca(2)Si(5)N(8) is no longer thermodynamically stable under pressures exceeding 15 GPa. Luminescence investigations (excitation at 365 nm) of HP-Ca(2)Si(5)N(8):Eu(2+) reveal a broadband emission peaking at 627 nm (FWHM=97 nm), similar to the ambient-pressure phase Ca(2)Si(5)N(8):Eu(2+).     

Binary Compounds of Boron and Beryllium: A Rich Structural Arena with Space for Predictions

We explore ground‐state structures and stoichiometries of the Be? B system in the static limit, with Be atom concentrations of 20 % or greater, and from P=1 atm up to 320 GPa. At P=1 atm, predictions are offered for several known compounds, the structures of which have not yet been determined experimentally. Specifically, at 1 atm, we predict a structure of R$\bar 3$ m symmetry for the compound Be₂B₃, seen experimentally at high temperatures, which contains interesting BeBBBBe rods; and for the compound BeB₄ we calculate metastability with respect to the elements with a structure similar to MgB₄, which is quickly replaced as the pressure is elevated by a Cmcm structure that features 6‐ and 4‐membered rings in B cages, with Be interstitials. For another high‐temperature compound, Be₂B, we confirm the CaF₂ structure, but find a competitive and actually slightly more stable ground‐state structure of C2/m symmetry that features B₂ pairs. In the case of BeB₂, a material for which the stoichiometry has been the subject of debate, we have a clear prediction of a stable F$\bar 4$ 3m structure at P=1 atm. It has a diamondoid structure that is based on cubic (lower P) or hexagonal (higher P) diamond networks of B, but with Be in the interstices. This Zintl structure is a semiconductor at low and intermediate pressures. At higher pressures, BeB₂ dominates the phase diagram. In general, the Zintl–Klemm concept of effective electron transfer from the more electropositive ion and bond formation among the resulting anions has proven useful in analyzing the structural preferences of many compositions in the Be? B system at P=1 atm and at elevated pressures. An unusual feature of this binary system is that the 1:1 BeB stoichiometry never appears to reach stability in the static limit, although it comes close, as does Be₁₇B₁₂. Also stable at high pressures are stoichiometries BeB₃, BeB₄, and Be₅B₂.     

Gas‐Phase Chemistry of Ethynylamine, ‐Phosphine and ‐Arsine. Structure and Stability of their Cu+ and Ni+ Complexes

The Cu⁺ and Ni⁺ binding energies of ethynylamine, ethynylphosphine and ethynylarsine have been calculated at the B3LYP/6‐311+G(2df,2p)//B3LYP/6‐311G(d,p) level of theory. Significant differences between nitrogen‐containing and phosphorus‐ or arsenic‐containing compounds have been found regarding structural effects upon metal cation association. While for ethynylamine the global minimum of the potential energy surface corresponds to the complex in which the metal cation binds to the β‐carbon, for ethynylphosphine the most favourable process corresponds to phosphorus attachment. For ethynylarsine, the conventional π‐complex is the most stable one. This behavior resembles that found for the corresponding vinyl analogues, with the only exception being the arsenic derivative. The calculated Cu⁺ and Ni⁺ binding energies for attachment to the heteroatom follow a different trend, P>As>N, to that predicted for the corresponding proton affinities, P>N>As. Cu⁺ and Ni⁺ binding energies are almost identical when the metal cation binds to the heteroatom. However, Ni⁺ binding energies are slightly larger than Cu⁺ binding energies when the metal cation interacts with the C?C bond.     

Ba3P5N10Br:Eu2+: A Natural‐White‐Light Single Emitter with a Zeolite Structure Type

Illumination sources based on phosphor‐converted light emitting diode (pcLED) technology are nowadays of great relevance. In particular, illumination‐grade pcLEDs are attracting increasing attention. Regarding this, the application of a single warm‐white‐emitting phosphor could be of great advantage. Herein, we report the synthesis of a novel nitridophosphate zeolite Ba₃P₅N₁₀Br:Eu²⁺. Upon excitation by near‐UV light, natural‐white‐light luminescence was detected. The synthesis of Ba₃P₅N₁₀Br:Eu²⁺ was carried out using the multianvil technique. The crystal structure of Ba₃P₅N₁₀Br:Eu²⁺ was solved and refined by single‐crystal X‐ray diffraction analysis and confirmed by Rietveld refinement and FTIR spectroscopy. Furthermore, spectroscopic luminescence measurements were performed. Through the synthesis of Ba₃P₅N₁₀Br:Eu²⁺, we have shown the great potential of nitridophosphate zeolites to serve as high‐performance luminescence materials.     

4,10‐Dibromoanthanthrone as a New Building Block for p‐Type,n‐Type,and Ambipolar π‐Conjugated Materials

New p‐type, n‐type, and ambipolar molecules were synthesized from commercially available 4,10‐dibromoanthanthrone dye. Substitution at the 4,10‐ and 6,12‐positions with different electron‐rich and electron‐poor units allowed the modulation of the optoelectronic properties of the molecules. A bis(dicyanovinylene)‐functionalized compound was also prepared with a reduction potential as low as ?50 mV versus Ag⁺ with a crystalline two‐dimensional lamellar packing arrangement. These characteristics are important prerequisites for air‐stable n‐type organic field‐effect transistor applications.     

Structural phase transitions in heavy alkali metals under pressure.

We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs III and Rb III phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.     

A QM/MM Investigation of the Catalytic Mechanism of Metal‐Ion‐Independent Core 2 β1,6‐N‐Acetylglucosaminyltransferase

β1,6‐GlcNAc‐transferase (C2GnT) is an important controlling factor of biological functions for many glycoproteins and its activity has been found to be altered in breast, colon, and lung cancer cells, in leukemia cells, in the lymhomonocytes of multiple sclerosis patients, leukocytes from diabetes patients, and in conditions causing an immune deficiency. The result of the action of C2GnT is the core 2 structure that is essential for the further elongation of the carbohydrate chains of O‐glycans. The catalytic mechanism of this metal‐ion‐independent glycosyltransferase is of paramount importance and is investigated here by using quantum mechanical (QM) (density functional theory (DFT))/molecular modeling (MM) methods with different levels of theory. The structural model of the reaction site used in this report is based on the crystal structures of C2GnT. The entire enzyme–substrate system was subdivided into two different subsystems: the QM subsystem containing 206 atoms and the MM region containing 5914 atoms. Three predefined reaction coordinates were employed to investigate the catalytic mechanism. The calculated potential energy surfaces discovered the existence of a concerted S_N2‐like mechanism. In this mechanism, a nucleophilic attack by O6 facilitated by proton transfer to the catalytic base and the separation of the leaving group all occur almost simultaneously. The transition state for the proposed reaction mechanism at the M06‐2X/6‐31G** (with diffuse functions on the O1′, O5′, O_Glu, and O6 atoms) level was located at C1? O6=1.74 Å and C1? O1=2.86 Å. The activation energy for this mechanism was estimated to be between 20 and 29 kcal mol^?1, depending on the method used. These calculations also identified a low‐barrier hydrogen bond between the nucleophile O6H and the catalytic base Glu320, and a hydrogen bond between the N‐acetamino group and the glycosidic oxygen of the donor in the TS. It is proposed that these interactions contribute to a stabilization of TS and participate in the catalytic mechanism.     

Chiral N,O‐Ligand/[Cu(OAc)2]‐Catalyzed Asymmetric Construction of 4‐Aminopyrrolidine Derivatives by 1,3‐Dipolar Cycloaddition of Azomethine Ylides with α‐Phthalimidoacrylates

A protocol to access useful 4‐aminopyrrolidine‐2,4‐dicarboxylate derivatives has been developed. A variety of chiral N,O‐ligands derived from 2,3‐dihydroimidazo[1,2‐a]pyridine motifs have been evaluated in the asymmetric 1,3‐dipolar cycloaddition of azomethine ylides to α‐phthalimidoacrylates. Reactions catalyzed by copper in combination with ligand 7‐Cl‐DHIPOH provided the highest level of stereoselectivity for the 1,3‐dipolar cycloaddition reaction. The reaction tolerates both β‐substituted and β‐unsubstituted α‐phthalimidoacrylate as dipolarophiles, affording the corresponding quaternary 4‐aminopyrrolidine cycloadducts with excellent diastereo‐ (>98:2 d.r.) and enantioselectivities (up to 97 % ee). Removal of the phthalimido protecting group can be accomplished by a simple NaBH₄ reduction. Theoretical calculations employing DFT methods show this cycloaddition reaction is likely to proceed through a stepwise mechanism and the stereochemistry was also theoretically rationalized.     

A Continuous Flow Process Using a Sequence of Microreactors with In‐line IR Analysis for the Preparation of N,N‐Diethyl‐4‐(3‐fluorophenylpiperidin‐4‐ylidenemethyl)benzamide as a Potent and Highly Selective δ‐Opioid Receptor Agonist

This article describes the design, optimisation and development of a continuous flow synthesis of N,N‐diethyl‐4‐(3‐fluorophenylpiperidin‐4‐ylidenemethyl)benzamide, a potent δ‐opioid receptor agonist developed by AstraZeneca. The process employs a sequence of flow‐based microreactors, with integrated purification employing solid‐supported reagents and in‐line IR analytical protocols using a newly developed ReactIR flow cell. With this monitoring device, initiation of the fourth input flow stream can be precisely controlled during the synthesis.     

Heteronuclear NMR Spectroscopy as a Surface‐Selective Technique: A Unique Look at the Hydroxyl Groups of γ‐Alumina.

The surface hydroxyl groups of γ‐alumina dehydroxylated at 500 °C were studied by a combination of one‐ and two‐dimensional homo‐ and heteronuclear ¹H and ²⁷Al NMR spectroscopy at high magnetic field. In particular, by harnessing ¹H–²⁷Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the ¹H magic‐angle spinning (MAS) NMR spectrum was demonstrated thanks to ¹H–²⁷Al RESPDOR (resonance‐echo saturation‐pulse double‐resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {¹H}‐²⁷Al dipolar heteronuclear multiple quantum correlation (D‐HMQC), which was used to establish a first coordination map. Then, in combination with ¹H–¹H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO₂ reactivity with alumina.     

Identifying Protein β‐Turns with Vibrational Raman Optical Activity

β‐turns belong to the most important secondary structure elements in proteins. On the basis of density functional calculations, vibrational Raman optical activity signatures of different types of β‐turns are established and compared as well as related to other signatures proposed in the literature earlier. Our findings indicate that there are much more characteristic ROA signals of β‐turns than have been hitherto suggested. These suggested signatures are, however, found to be valid for the most important type of β‐turns. Moreover, we compare the influence of different amino acid side chains on these signatures and investigate the discrimination of β‐turns from other secondary structure elements, namely α‐ and 3₁₀‐helices.     

Dipyrenylcalix[4]arene—A Fluorescence‐Based Chemosensor for Trinitroaromatic Explosives

A new chemosensor‐based approach to the detection of nitroaromatics is described. It involves the analyte‐induced quenching of excimer emission of a dipyrenyl calix[4]arene ( L ). The chemical and photophysical properties of the complexes formed between L and mono‐, di‐, and trinitrobenzene, and di‐ and trinitrotoluene were studied in acetonitrile and chloroform by using ¹H NMR, UV/Vis, and fluorescence spectroscopy. Fluorescence spectroscopy revealed that the trinitroaromatics engendered the largest response among the various substrates tested, with the sensitivity for these analytes being correspondingly high. Quantitative analysis of the fluorescence titration profile generated from the titration of L with TNT provided evidence that this particular functionalized calix[4]arene receptor allows for the detection of TNT down to the low ppb level in CH₃CN. A single‐crystal X‐ray diffraction analysis revealed that in the solid state the complex L? TNT consists of a supramolecular crystalline polymeric structure, the formation of which appears to be driven by intermolecular π–π interactions between two pyrene units and a TNT molecule held at a distance of 3.2–3.6 Å, as well as by intra‐ and intermolecular hydrogen‐bonds among the amide linkages. Nevertheless, the changes in the ¹H NMR, UV/Vis, and fluorescence spectrum, including sharp color changes, are ascribed to a charge‐transfer interaction arising from complementary π–π overlap between the pyrene subunits and the bound trinitroaromatic substrates. A number of ab initio calculations were also carried out and, considered in concert, they provide further support for the proposed charge‐transfer interactions, particularly in the case of L? TNT.