Probing short length scales with restricted diffusion in a static gradient using the CPMG sequence

We experimentally verify a new method of extracting the surface-to-volume ratio (S/V) of porous media with diffusion NMR. In contrast to the widely used pulsed field gradient (PFG) technique, which employs the stimulated echo coherence pathway, we use here the direct Carr-Purcell-Meiboom-Gill (CPMG) path. Even for high echoes, which exhibit ample attenuation due to diffusion in the field gradient, the relevant ruler length for the direct pathway is fixed by the diffusion length during a single inter-pulse spacing. The direct path, therefore, is well suited for probing shorter length scales than is possible with the conventional approach. In our experiments in a low-field static-gradient system, the direct CPMG pathway was found to be sensitive to structure an order of magnitude smaller than accessible with the stimulated-echo pathway.     

A numerical model of multimode fiber PON frequency response

Frequency response of passive optical network (PON) based on multimode fibers is investigated. The network comprises fibers, connectors and splitters/couplers. It is shown that due to mode filtering at splitters, the frequency response is different for different network nodes in otherwise symmetrical network.     

Experimental and computational study on the reactivity of 2,3-bis[(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile, a key intermediate for the synthesis of tribenzoporphyrazine bearing peripheral methyl(3-pyridylmethyl)amino substituents

Abstract  

An earlier developed alkylating path leading to tetraalkylated diaminomaleonitrile derivatives was explored. Attempts to explain the reactivity of the representative dialkylated diaminomaleonitrile 2,3-bis[(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile during the alkylation reaction were performed using X-ray and density functional theory (DFT) studies. The condensed Fukui functions accompanied by softness indices were found to be useful in explaining its reactivity observed during the reaction. The values of the Fukui functions and condensed softness for electrophilic attack calculated from Mulliken, L?wdin, and natural population analyses closely corresponded to the experimental observations. When 2,3-bis[(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile disodium salt was treated with dimethyl sulfate at lower temperatures the alkylation reaction prevailed, whereas at higher temperatures the alkylating agent acted as a hydride anion acceptor, which favored the elimination reaction. The tetraalkylated dinitrile 2,3-bis[methyl(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile was used in the synthesis of tribenzoporphyrazine bearing methyl(3-pyridylmethyl)amino groups, which was subsequently subjected to solvatochromic and metallation studies. The changes observed during metallation seem to result from the coordination of the 3-pyridyl group by a palladium ion. This could influence the configuration of the methyl(3-pyridylmethyl)amino moiety, causing more effective donation of a lone pair of electrons from peripheral nitrogen to the macrocyclic ring.     

Desymmetrization of malonamides via an enantioselective intramolecular Buchwald-Hartwig reaction

A new form of enantioselective nitrogen arylation reaction is described. Beginning with symmetrical α-(2-bromobenzyl)malonamides, intramolecular palladium-catalyzed cross-coupling using a catalyst system including 3.3 mol % Pd(OAc)₂ and 6.6 mol % of the chiral biaryl monophosphine (R)-MOP, desymmetrized quinolinone products are obtained in nearly quantitative yields in enantiomeric ratios up to 88:12. This Letter represents a rare example of enantioselective Buchwald-Hartwig reaction.     

Studies on the uptake of different arsenic forms and the influence of sample pretreatment on arsenic speciation in White mustard (Sinapis alba)

The concentration and speciation of arsenic incorporated by plants grown in the presence of different arsenic compounds was compared, and the influence of plant sample pretreatment and extraction procedures on the recovery and reliability of speciation analyses was studied. It was concluded that sample pretreatment greatly affected the extraction efficiency, but did not change arsenic speciation. The most suitable extraction procedure involved dried plant material without the use of liquid nitrogen. To assess the ability of White mustard to uptake arsenic in different forms, samples were cultivated in nutrient solutions containing either As(III), As(V), monomethylarsonic acid (MMA) or dimethylarsinic acid (DMA). The translocation factor was the highest (0.70) when DMA was added to the nutrient solution, however the overall As concentration in plant tissues was the lowest in this case. Only inorganic As was found in plant tissues when either As(III) or As(V) was added to the nutrient solution. When organic arsenic was present in the nutrient medium, however, it was partially transformed by the plants into inorganic forms.     

Boryl (Hetero)aryne Precursors as Versatile Arylation Reagents: Synthesis through CH Activation and Orthogonal Reactivity

(Pinacolato)boryl ortho‐silyl(hetero)aryl triflates are presented as a new class of building blocks for arylation. They demonstrate unique versatility by delivering boronate or (hetero)aryne reactivity chemoselectively in a broad range of transformations. This approach enables the unprecedented postfunctionalization of fluoride‐activated (hetero)aryne precursors, for example, as substrates in transition‐metal catalysis, and offers valuable new possibilities for aryl boronate postfunctionalization without the use of specialized protecting groups.     

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture

A combination of atomic force microscopy (AFM), high‐resolution scanning electron microscopy (HR‐SEM), focused‐ion‐beam scanning electron microscopy (FIB‐SEM), X‐ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron‐based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM‐5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol‐to‐olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM‐5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM‐5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5–50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.     

The quest for modular nanocages: tbo-MOF as an archetype for mutual substitution, functionalization, and expansion of quadrangular pillar building blocks

A new blueprint network for the design and synthesis of porous, functional 3D metal-organic frameworks (MOFs) has been identified, namely, the tbo net. Accordingly, tbo-MOFs based on this unique (3,4)-connected net can be exclusively constructed utilizing a combination of well-known and readily targeted [M(R-BDC)](n) MOF layers [i.e., supermolecular building layers (SBLs)] based on the edge-transitive 4,4 square lattice (sql) (i.e., 2D four-building units) and a novel pillaring strategy based on four proximal isophthalate ligands from neighboring SBL membered rings (i.e., two pairs from each layer) covalently cross-linked through an organic quadrangular core (e.g., tetrasubstituted benzene). Our strategy permits the rational design and synthesis of isoreticular structures, functionalized and/or expanded, that possess extra-large nanocapsule-like cages, high porosity, and potential for gas separation and storage, among others. Thus, tbo-MOF serves as an archetypal tunable, isoreticular MOF platform for targeting desired applications.