Monomeric Building Units Research Articles

The HfFe2Si2 type silicides ScT 2Si2 (T = Ru, Rh, Os) – structure and solid-state 29Si/45Sc NMR spectroscopy

Abstract The silicides ScT 2Si2 (T = Ru, Rh, Os) were synthesized by arc-melting of the elements and subsequent annealing in sealed silica ampoules. They crystallize with the rarely observed HfFe2Si2 type structure, space group Pbcm. The structures of ScRu2Si2 (a = 761.64(4), b = 730.70(6), c = 521.07(6) pm, wR = 0.0314, 633 F 2 values, 31 variables) and ScOs2Si2 (a = 771.10(8), b = 736.68(7), c = 521.88(5) pm, wR = 0.0479, 623 F 2 values, 31 variables) were refined from single crystal X-ray diffractometer data. The refinements showed small degrees of Ru/Si respectively Os/Si mixing on one 4c site, leading to the refined compositions ScRu1.96(1)Si2.04(1) and ScOs1.91(1)Si2.09(1). The monomeric building units in both structures are two slightly distorted, crystallographically independent RuSi5 respectively OsSi5 square pyramids, which are condensed via common edges. The resulting densely packed [Ru2Si2] and [Os2Si2] networks leave voids for the scandium atoms with coordination number 18: Sc@Si8Ru8Sc2 and Sc@Si8Os8Sc2. Temperature dependent magnetic susceptibility measurements of ScRu2Si2 and ScOs2Si2 indicate Pauli paramagnetism. Solid-state 29Si and 45Sc static and MAS NMR spectroscopy reveal significant Knight shifts and a strong influence of the T/Si mixing on the experimental NMR line shapes. A larger overall 29Si magnetic shift for ScRu2Si2 compared to ScOs2Si2, although with a negative sign for one of the 29Si resonances, suggests stronger paramagnetic effects for ScRu2Si2, in agreement with the magnetic susceptibility measurements.

A novel chemical approach for synthesizing highly porous graphene analogue and its composite with Ag nanoparticles for efficient electrochemical oxygen reduction

Single-step phase transformation from amorphous to crystalline material avoiding drastic reaction condition is a novel approach in the synthetic and applied materials chemistry research. Both conjugated microporous polymers (CMPs) and graphene/graphene analogues are very demanding materials in the energy research. Depending upon the nature of the graphene material like amorphous, crystalline, functionalized, single-layered and multi-layered, its property and potential application can be largely tuned. In this perspective, 2D carbon allotropes like graphene and its analogues have attracted a huge attention due to their unique electrical, thermal, mechanical, and physico-chemical properties. On the other hand, CMPs are another class of porous nanomaterial synthesized through the polycondensation/coupling of reactive monomeric building units via bottom-up chemistry approach, where extended π-conjugation prevails in the entire organic framework. To the best of our knowledge, herein we first demonstrated a novel methodology for amorphous CMP to crystalline graphene analogue through single step chemical transformation avoiding drastic reaction conditions. For this purpose, we have synthesized a new pyrene-naphthalene based spherical amorphous conjugated microporous polymer, designated as pyrene-naphthalene CMP (PNC), which upon chemical treatment (oxidative C-C coupling reaction) transforms into a highly porous crystalline multilayer porous nanographene sheet-like material pyrene-naphthalene-graphene (PNG). The BET surface area has been drastically enhanced from 600 m2/g for the CMP to 1414 m2/g for this graphene analogue PNG. As analysed by scanning tunnelling spectroscopy (STS), the PNG material has showed atomic level graphitic images with almost zero bandgap, which suggested highly conductive nature for this crystalline graphene analogue. This material has been further explored in the electrochemical oxygen reduction reaction (ORR) by grafting silver nanoparticles (AgNPs) onto the interlayer nanosheets of PNG. The ORR results displayed an admirable half-wave potential (E1/2) and onset potential of 0.78 V and 0.984 V, respectively. The electrocatalyst achieved a very high current density of −0.53 mA cm−2 under the basic electrolyte medium (0.1 M KOH). An efficient ORR catalyst is very demanding towards the fabrication of metal-air batteries as well as fuel cells, which are promising for clean and green energy production.

“All-in-one” covalent organic framework for photocatalytic CO2 reduction

Constructed by selecting appropriate building blocks, covalent organic frameworks (COFs) can be endowed with a variety of specific functions. Herein, we successfully synthesized an imine-linked H2PReBpy-COF with the CO2 reduction catalyst [ReI(bpy)(CO)3Cl] and the porphyrin photosensitizer as the monomeric building units. The light-harvesting properties of the porphyrin itself, augmented by the extended π-conjugated planar structure of 2D COF, enable H2PReBpy-COF the excellent light-harvesting capability, efficient charge separation, and fast interfacial charge transfer. In addition, a large amount of uniformly distributed [ReI(bpy)(CO)3Cl] units offer H2PReBpy-COF an excellent activity toward photocatalytic CO2 reduction with moderate selectivity and reusability. This study demonstrated a proof of concept in which the advantages of COFs and functional monomers are rationally integrated for photocatalytic solar fuel conversion.

Eutectogels as Matrices to Manipulate Supramolecular Chirality and Circularly Polarized Luminescence.

Solvent is regarded as a factor in tuning the supramolecular chirality of self-assemblies. Deep eutectic solvents (DESs) show diverse properties in contrast to other common solvents, which are emerging in fabricating functional aggregates and nanoarchitectures. Nevertheless, the emergence and manipulation of supramolecular chirality in DES still remain mysterious. Exploring supramolecular chirality in DES would produce tunable chiroptical materials considering their feasible preparation process and abundant hydrogen bonding sites. In this work, we explored the occurrence and manipulation of supramolecular chirality in DES. Transfer from inherent chiral DES to solutes in either aggregated or monomeric building units is blocked. However, the chiral assembly of π-conjugated amino acids was realized. Compared to aqueous media, self-assembly in DES hinders the spontaneous structural and chirality evolution that benefit from efficient solvation, where the π-conjugated amino acids were involved as hydrogen bonding donors. DES performs as a dye-friendly matrix to afford chiroptical eutectogels with tunable circularly polarized luminescence, whereby a large dissymmetry g-factor of up to 0.015 was realized. DES behaves as feasible and flexible solvents to fabricate and stabilize functional soft chiral self-assemblies with controllable chiroptical properties.

Cd2 and Co2 dumbbell formation in the yttrium-rich intermetallic compounds Y14Ni3Cd3 and Y6Co2Zn

Abstract The yttrium-rich intermetallic compounds Y14Ni3Cd3 and Y6Co2Zn were obtained by direct reactions of the elements in sealed tantalum tubes in an induction furnace. Both samples were characterized by X-ray powder diffraction and the structures were refined from single-crystal X-ray diffraction data: Lu14Co3In3 type, P42/nmc, a = 960.06(5), c = 2314.9(1) pm, wR2 = 0.0669, 2034 F 2 values, 63 parameters for Y14Ni3.16(2)Cd2.84(2) and Ho6Co2Ga type, Immm, a = 943.08(7), b = 950.08(7), c = 997.64(7) pm, wR2 = 0.0476, 981 F 2 values, 34 parameters for Y6Co2Zn. One cadmium site shows a small degree of Cd/Ni mixing, leading to the composition Y14Ni3.16(2)Cd2.84(2). Although both compounds crystallize with significantly different structure types, they show very similar monomeric building units: (i) transition metal centered trigonal prisms of yttrium, (ii) empty Y6 octahedra and (iii) icosahedral coordination of the cadmium respectively zinc atoms. The condensation patterns of these building units are discussed and for Y14Ni3.16(2)Cd2.84(2), we elaborate a crystal chemical building principle along with the rare earth metal-rich phases RE 15Rh5Cd2 (La15Rh5Cd2 type), RE 23 T 7 X 4 (Pr23Ir7Mg4 type), RE 4 TX (Gd4RhMg type) and RE 10 TX 3/RE 9 TX 4 (ordered Co2Al5 versions). The structures of Y14Ni3Cd3 and Y6Co2Zn show dumbbell formation: 306 pm Cd–Cd in Y14Ni3Cd3 and 224 pm Co1–Co1 and 246 pm Co2–Co2 in Y6Co2Zn.

Chitosan: Gels and Interfacial Properties

Chitosan is a unique biopolymer in the respect that it is abundant, cationic, low-toxic, non-immunogenic and biodegradable. The relative occurrence of the two monomeric building units (N-acetyl-glucosamine and d-glucosamine) is crucial to whether chitosan is predominantly an ampholyte or predominantly a polyelectrolyte at acidic pH-values. The chemical composition is not only crucial to its surface activity properties, but also to whether and why chitosan can undergo a sol–gel transition. This review gives an overview of chitosan hydrogels and their biomedical applications, e.g., in tissue engineering and drug delivery, as well as the chitosan’s surface activity and its role in emulsion formation, stabilization and destabilization. Previously unpublished original data where chitosan acts as an emulsifier and flocculant are presented and discussed, showing that highly-acetylated chitosans can act both as an emulsifier and as a flocculant.

Distribution and Quantification of Flavan-3-ols and Procyanidins with Low Degree of Polymerization in Nuts, Cereals, and Legumes

The monomeric flavan-3-ols catechin and epicatechin as well as procyanidins are of great interest due to their potential beneficial health effects observed in epidemiological studies. However, the occurrence and concentration of these compounds is not well-known due to the fact that reference compounds are not commercially available. In this study we determined the pattern and concentration of catechin, epicatechin, and different dimeric and trimeric procyanidins in 38 food samples (nuts, cereals, legumes) using a reversed phase high-performance liquid chromatography electrospray ionization tandem mass spectrometry (RP-HPLC-ESI-MS/MS) approach based on isolated authentic reference compounds. Of the analyzed food samples 21 were found to contain dimeric and trimeric procyanidins and their monomeric building units catechin and epicatechin. Mainly the monitored nut samples contained the analyzed procyanidins as well as catechin and epicatechin whereas only 3 cereals were identified as sources of the analyzed compounds. The concentration ranged from 148 μg/100 g in macadamia nut to 55 mg/100 g in pinto bean. Catechin and procyanidin B3 were found to be the most abundant analytes. The only A-type procyanidin that could be identified was procyanidin A2, which was found in peanut. The achieved data could be used for authenticity control and furthermore in combination with dietary studies to calculate the daily intake of monomeric flavan-3-ols and procyanidins. To our knowledge this is the first detailed study quantifying monomeric flavan-3-ols and dimeric and trimeric procyanidins in various nuts, cereals, and legumes.

Secondary Structure of Short β-Peptides as the Chiral Expression of Monomeric Building Units: A Rational and Predictive Model

Chirality of the monomeric residues controls and determines the prevalent folding of small oligopeptides (from di- to tetramers) composed of 2-aminocyclobutane-1-carboxylic acid (ACBA) derivatives with the same or different absolute and relative configuration. The cis-form of the monomeric ACBA gives rise to two conformers, namely, Z6 and Z8, while the trans-form manifests uniquely as an H8 structure. By combining these subunits in oligo- and polypeptides, their local structural preference remains, thus allowing the rational design of new short foldamers. A lego-type molecular architecture evolves; the overall look depends only on the conformational properties of the structural building units. A versatile and efficient method to predict the backbone folds of designed cyclobutane β-peptides is based on QM calculations. Predictions are corroborated by high-resolution NMR studies on selected stereoisomers, most of them being new foldamers that have been synthesized and characterized for the first time. Thus, the chiral expression of monomeric building units results in the defined secondary structures of small oligomers. As a result of this study, a new set of chirality controlled foldamers is provided to probe as biocompatible biopolymers.

Conformational Behavior of Basic Monomeric Building Units of Glycosaminoglycans: Isolated Systems and Solvent Effect

Our work reports in detail the results of systematic large-scale theoretical investigations of the glycosaminoglycan building units 1-OMe DeltaIdoA-2SNa2 (1; 2H1 and 1H2 forms), 1-OMe GlcN-S6SNa2 (2), 1,4-DiOMe GlcNa (3), 1,4-DiOMe GlcN-S3S6SNa3 (4), 1,4-DiOMe IdoA-2SNa2 (5; 4C1, 1C4, and 2So conformations), and 1,4-DiOMe GlcN-S6SNa2 (6) at the BP86/TZ2P level of the density functional theory. The optimized geometries indicate that the most stable structure of these monomeric units in the neutral state is stabilized via "multifurcated" sodium bonds. The equilibrium structure of the biologically active anionic forms of the glycosaminoglycans studied changed considerably in a water solution. The computed interaction energies, DeltaE, of sodium coordinated systems 1-6 are negative and span a rather broad energy interval (from -130 to -590 kcal mol-1). Computations that include the effect of solvation indicated that in water the relative stability of Na+...ligand ionic bonds is considerably diminished. The computed interaction energy in water is small (from -20 to -53 kcal mol-1) and negative, that is, stabilizing.

ChemInform Abstract: Nucleotides. Part 68. Acetals as New 2′‐O‐Protecting Functions for the Synthesis of Oligoribonucleotides: Synthesis of Monomeric Building Units and Oligoribonucleotides.

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Nucleotides, Part LXVIII, Acetals as New 2′-O-Protecting Functions for the Synthesis of Oligoribonucleotides: Synthesis of Monomeric Building Units and Oligoribonucleotides

For the efficient synthesis of oligoribonucleotides by the 5′-O-(4,4′-dimethoxytrityl) phosphoramidite approach, the 2′-O-[1-(benzyloxy)ethyl]acetals 56 – 67 were investigated. Studies with the 2′-O-[1-(benzyloxy)ethyl]-5′-O-(dimethoxytrityl)ribonucleoside 3′-phosphoramidites 56 – 59 gave, however, only reasonable results. The oligoribonucleotides obtained showed some impurities since the acid stabilities of the acetal and dimethoxytrityl functions are too close to guarantee a high selectivity. A combination of new acid-labile protected 2′-O-protecting groups with the 2-(4-nitrophenyl)ethyl/[2-(4-nitrophenyl)ethoxy]carbonyl (npe/npeoc) strategy for base protection was more successful. The synthesis and physical properties of the monomeric building units and their intermediates 8 – 67 and the conditions for the automated generation of homo- and mixed oligoribonucleotides is described. The new 2′-acetal protecting group could be cleaved off in a two step procedure and was designed for levelling their stability with regard to the attached nucleobase as well. Therefore, we used the 1-{{3-fluoro-4-{{[2-(4-nitrophenyl)ethoxy]carbonyl}oxy}benzyl}oxy}ethyl (fnebe) moiety for the protection of 2′-OH of uridine, and for that of 2′-OH of A, C, and G, the 1-{{4-{{[2-(4-nitrophenyl)ethoxy]carbonyl}oxy}benzyl}oxy}ethyl (nebe) residue. After selective deprotection by β-elimination induced by a strong organic base like DBU, the remaining activated acetal was hydrolyzed under very mild acidic protic conditions, which reduced 2′-3′ isomerization and chain cleavage. Also storage, handling, and purification of the chemically and enzymatically sensitive oligomers was simplified by this approach.

Estimates of the Rate Coefficients for Chain Initiation, Propagation, and Termination during Fischer-Tropsch Synthesis over Ru/TiO2

Transient response isotopic tracer experiments have been used to study chain growth during Fischer-Tropsch synthesis over an Ru/TiO2 catalyst. This involves observation of the incorporation of 13C into reaction products after an abrupt switch from 12CO/D2 to 13CO/D2 in the feed. Values for the rate coefficients for initiation, propagation, and termination are determined by fitting theoretically generated model curves to the observed transient responses. The rate coefficient for chain initiation is independent of temperature and D2/CO ratio. The rate coefficients for propagation and termination increase with temperature. The rate constant for propagation is not affected by the D2/CO ratio. The rate coefficient for termination increases linearly with increasing D2/CO ratio. The activation energy for chain propagation is 8 kcal/mol, whereas the activation energy for chain termination is 20 kcal/mol. The relative values of these two activation energies explains the observed decrease in chain growth probability, α, with increasing temperature. Coverages by reaction intermediates are also estimated. The dominant species are monomeric building units which occupy 0.3-0.6 ML. Alkyl species that are the direct hydrocarbon product precursors occupy < 0.2 ML and adsorbed CO covers 0.7 ML.