Spectral parameters for quantitative mobility contrast in NMR imaging of solid polymers

Different procedures based on parameters of the wideline NMR absorption spectrum are presented to obtain localized molecular mobility contrast for imaging of solid polymers. For this purpose a ¹H-NMR imaging technique with magic sandwich echoes is used for acquiring localized wideline spectra. With samples composed of polystyrene and high impact strength polystyrene, and polycarbonate and low density polyethylene a spatial difference in NMR absorption spectrum lineshape and linewidth is displayed. Furthermore, the spatial distribution of rigid and mobile domains in a heterogeneous polymer can be derived from the NMR spectral components. It is demonstrated that a van Vleck moment analysis can be performed from spatially resolved magic echo decays. The second (M₂) and fourth (M₄) moments of the rigid components show considerable variation with the spatial composition of the investigated samples.     

Half-sandwich ruthenium complexes of pentafluorobenzenethiolato ligands

A synthetic study of ruthenium complexes containing pentafluorobenzenethiolato ligand is presented. The bis(triphenylphosphine) complex CpRu(PPh₃)₂SC₆F₅ (1) is prepared from CpRu(PPh₃)₂Cl and C₆F₅S⁻ in high yield. This complex is readily reacted with CO gas to give the mixed carbonyl-phosphine complex CpRu(PPh₃)(CO)SC₆F₅ (2) and with NOBF₄ at room temperature to give [CpRu(PPh₃)(NO)SC₆F₅]BF₄ (3). The one-pot reaction of CpRu(PPh₃)₂Cl, dppa ligands, and C₆F₅S⁻ produces CpRu(dppa)SC₆F₅ [dppa = bis(diphenylphosphino)methane: dppm (4); bis(diphenylphosphino)ethane: dppe (5)]. Complexes (1)–(5) have been characterized by spectroscopic techniques (i.r., ¹H-n.m.r., ³¹P-n.m.r.) and by elemental analysis. The X-ray structural analysis of (5) is reported. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hexaphenylhexaphosphinane benzene solvate

In the molecular crystal of hexaphenylhexaphosphinane benzene solvate, C₃₆H₃₀P₆·C₆H₆, representing the trigonal form of phosphobenzene as a solvate, the six‐membered ring of P atoms adopts a chair conformation wherein the six phenyl groups are located in equatorial positions. The molecules [molecular symmetry (C_3i)] are stacked infinitely along the c‐axial direction.     

Visible-Light-Triggered Photoswitching of Diphosphene Complexes

Although diphosphene transition metal complexes are known to undergo E to Z isomerization upon irradiation with UV light, their potential for photoswitching has remained poorly explored. In this study, we present diphosphene complexes capable of reversible photoisomerizations through haptotropic rearrangements. The compounds [( 2 -κ²P,κ⁶C)Mo(CO)₂][OTf] ( 3 a [OTf]), [( 2 -κ²P,κ⁶C)Fe(CO)][OTf] ( 3 b [OTf]), and [( 2 -κ²P)Fe(CO)₄][OTf] ( 4 [OTf]) were prepared using the triflate salt [(L_C)P=P(Dipp)][OTf] ( 2 [OTf) as a precursor (L_C=4,5-dichloro-1,3-bis(2,6-diisiopropylphenyl)-imidazolin-2-yl; Dipp=2,6-diisiopropylphenyl, OTf=triflate). Upon exposure to blue or UV light (λ=400 nm, 470 nm), the initially red-colored η²-diphosphene complexes 3 a , b [OTf] readily undergo isomerization to form blue-colored η¹-complexes [( 2 -κ¹P,κ⁶C)M(CO)_n][OTf] ( 5 a , b [OTf]; a : M=Mo, n=2; b : M=Fe, n=1). This haptotropic rearrangement is reversible, and the (κ²P,κ⁶C)-coordination mode gradually reverts back upon dissolution in coordinating solvents or more rapidly upon exposure to yellow or red irradiation (λ=590 nm, 630 nm). The electronic reasons for the reversible visible-light-induced photoswitching observed for 3 a , b [OTf] are elucidated by DFT calculations. These calculations indicate that the photochromic isomerization originates from the S₁ excited state and proceeds through a conical intersection.     

Sequences—Basic elements for discrete mathematics

Sequences are fundamental mathematical objects with a long history in mathematics. Sequences are also tools for the development of other concepts (e. g. the limit concept), as well as tools for the mathematization of real-life situations (e. g. growth processes). But, sequences are also interesting objects in themselves, with lots of surprising properties (e. g. Fibonacci sequence, sequence of prime numbers, sequences of polygonal numbers). Nowadays, new technologies provide the possibility to generate sequences, to create symbolic, numerical and graphical representations, to change between these different representations. Examples of some empirical investigation are given, how students worked with sequences in a computer-supported environment.     

3,5‐Di­methyl‐1,3,5‐oxadi­azane‐2,4,6‐trione: short intermolecular contacts determining the crystal packing

In the title compound, C₅H₆N₂O₄, the mol­ecules lie across a crystallographic mirror plane. The compound lacks traditional hydrogen‐bond donors, and hence crystals are held together by unusual C=O⋯O, O⋯C and weak C—H⋯O interactions, forming layers. Adjacent layers are arranged in an antiparallel manner, yielding an ABA layer sequence. The intermolecular contacts are quite short; a topological analysis of charge density based on density‐functional‐theory calculations was used for consideration of these short contacts and indicated a strong attractive bonding closed‐shell interaction between these atoms in the crystal structure.     

Freezing in forced convection flows inside ducts: A review

The present work gives an overview about the recent developments in research on freezing phenomena in forced convection flows inside ducts. Emphasis is given to the fundamental aspects of the phenomena observed in the solidification processes as well as on the analytical and numerical modelling aspects for this kind of problems. The paper deals with solidification problems inside tubes, parallel plate channels, curved rectangular channels and in diverging rectangular channels. Additionally, some new experimental and numerical results on solidification in duct flows are shown from the current research program in Darmstadt on freezing phenomena.