Mechanism of formation of shish kebab structures

To describe the characteristic crystalline structure of polyolefins, Pennings first proposed a model consisting of a combination of an extended chain crystal (a “shish”) and folded chain crystals (“kebabs”). In Pennings' model the “shish” forms first during a crystallization process under stress and then later the “kebabs” overgrow this “shish” structure epitaxially. Because we had some doubts about such a mechanism, we undertook a series of experimental studies on linear polyethylene, particularly in regard to the crystallization process from a solution under shear. Our conclusion is that the crystals grow first by a screw dislocation mechanism, like whiskers, and then later these are deformed by the shear stress to form the shish kebab structures.     

Luminescence Color‐Tuning through Polymorph Doping: Preparation of a White‐Emitting Solid from a Single Gold(I)–Isocyanide Complex by Simple Precipitation

We report the luminescent color tuning of a new complex, 2‐benzothiophenyl(4‐methoxyphenyl isocyanide)gold(I) ( 1 ), by using a new “polymorph doping” approach. The slow crystallization of the complex 1 afforded three different pure polymorphic crystals with blue, green, and orange emission under UV‐light irradiation. The crystal structures of pure polymorphs of 1 were investigated in detail by means of single‐crystal X‐ray analyses. Theoretical calculations based on the single‐crystal structures provided qualitative explanation of the difference in the excited energy‐levels of the three polymorphs of 1 . In sharp contrast, the rapid precipitation of 1 , with the optimized conditions reproducibly afforded homogeneous powder materials showing solid‐state white‐emission with Commission Internationale de l’Éclairage (CIE) 1931 chromaticity coordinates of (0.33, 0.35), which is similar to pure white. New “polymorphic doping” has been revealed to be critical to this white emission through spectroscopic and X‐ray diffraction analyses. The coexistence of the multiple polymorphs of 1 within the homogeneous powder materials and the ideal mixing of multiple luminescent colors gave its white emission accompanied with energy transfer from the predominant green‐emitting polymorph to the minor orange‐emitting polymorph.     

Structure of fibers drawn from polyethylene single crystals

Transmission electron microscopy and electron diffraction were used to study the molecular structure of fibers drawn from polyethylene single crystals at 77, 293, and 383°K. The results suggest that the formation of the fibers occurs by a two-step process. The first step is the breaking off of single blocks of folded chains from the single crystals so that a “string-of-pearls” structure is obtained. If the temperature is sufficiently high this process is followed by the thermally activated rearrangement of the molecules in the drawn fibers so that a “bamboo” structure results.     

“Surface water” and “strong-bonded water” in cyclodextrins: a Karl Fischer titration approach

Cyclodextrins are some of the most used carriers for bioactive compounds (as host–guest complex) and many factors influence the association–dissociation of this complex, some of them being related to hydrophobicity. In the solid state, cyclodextrins contain two types of water molecules: “surface” water molecules (especially close to the crystal surface) and “strong-bonded” water molecules (especially from the cyclodextrin cavity), but the classification is hard to do, and the concentration of these water molecules are relatively difficult to estimate by simple methods. In the present study we used the volumetric Karl Fischer titration to estimate these types of water molecules in cyclodextrins by means of the rate of water reaction (related to diffusion from cyclodextrin crystals). “Surface” water molecules are titrated with rates between 1.8–2.8 mM/s for α-cyclodextrin, while for β-cyclodextrin these rates are little bit higher (2.9–3.4 mM/s). The rates corresponding to “strong-bonded” water molecules are approximately tens fold lower (0.05–0.3 mM/s for α-cyclodextrin and 0.15–0.33 mM/s for β-cyclodextrin). The approximate ratio between “surface” and “strong-bonded” water molecules could also be estimated by this simple and rapid method.     

Volume Effect or Paddle-Wheel Mechanism—Fast Alkali-Metal Ionic Conduction in Solids with Rotationally Disordered Complex Anions

Transport phenomena in the solid state are of equal importance in basic research and practice. In the past two decades particular interest has been directed towards so-called “fast” or “super” ionic conductors because of their attractive potential applications. We have synthesized highly conductive alkali-metal ionic conductors based on ionic crystals in which, on the one hand, high concentrations of the charge carriers can be realized by doping (point defects in the cation substructure) and, on the other, the activation energy of the interchange of sites is decreased by translationally fixed but rotationally mobile complex anions. Mixed crystals with Na₃PO₄ or Na₃AlF₆ structures have proven especially suitable in this connection. With the object of establishing a broader experimental foundation for clarifying the controversially discussed question of whether the higher free transport volume or the rotational motion of the anions is responsible for the high cation mobility in these rotary phases we have systematically varied the type of anions and concentration of defects and monitored the resulting changes in conductivity. Although the macroscopical characteristics investigated are not suitable for explaining mechanisms in detail at the atomic level, the results afford clear support for the assumption of a “paddle wheel mechanism”; but also effects of the enlarged transport volume are not to be disputed. Both these effects enhancing the cationic conductivity are concomitantly operative in amounts varying from system to system; they cannot be totally separated from each other. Seen in this light, the alternative, “volume effect” or “paddle wheel mechanism,” is not as sharply defined as was previously discussed.     

PHYSICAL-CHEMICAL BEHAVIOR OF CONFECTIONERY FATS

Confectionery coatings employ hard butter fat systems made from both lauric and non-lauric source oils. These oils are routinely modified by a combination of treatments including hydrogenation, fractionation and interesterification to achieve desired physical properties. Such processing methods create heterogeneous triglyceride mixtures consisting of a variety of compositional and positional isomers. Published phase diagrams of “simple” binary triglyceride mixtures of closely related molecules are complex, and suggest that innumberable unique liquid and solid phases may co-exist at any given temperature ( and pressure ) in vastly more complex triglyceride mixtures such as confectionery hard butters. Thus we may view confectionery fat systems as multiphasic mixtures (liquid, solid and compositional) with a propensity to undergo liquid content fluctuations and crystal size/morphology changes in response to slight changes in temperature. A true equilibrium among all phases may indeed never be attained, and a potential for movement of certain components in response to temperature change is probably constant. Surface growth of long needle-like fat crystals, “fat bloom”, most likely results from this non-equilibrium condition and serves to reduce the system's free energy. The ever present, ever changing liquid phase(s) is viewed as the vehicle for free energy minimization via triglyceride migration and ongoing crystal growth.     

A study on the mechanism of dissolution of the cellulose/NH3/NH4SCN system. I

Ammonia/ammonium thiocyanate (NH₃/NH₄SCN) is an excellent swelling agent and solvent for cellulose, even at a high degree of polymerization. Because polymorphic conversion in cellulose has been a long-standing, perplexing, troublesome problem, we have undertaken to study that mechanism. Solid state CP/MAS ¹³C-NMR and X-ray analysis proved to be very useful analytical techniques for the task. It appears that during temperature cycling, specific cellulosic inter- and intramolecular hydrogen-bonds are broken as polymorphic conversion proceeds sequentially from the polymorph I to III, and finally at total solvation to amorphous. This proceeds correspondingly via transformation of the polymorph conformations of CH₂OH from trans-gauche, “tg,” to gauche-trans, “gt,” to gauche-gauche, “gg.” © 1994 John Wiley & Sons, Inc.     

The existence of mosaic block structures in polymer single crystals

X-ray line broadening measurements were used to determine the apparent “mosaic block” sizes of randomly oriented polyethylene and polyoxymethylene single crystals. Both dried-down and uncollapsed crystals were examined. PE lamellae were grown at 80, 85, and 90°C by isothermal crystallization from a 0.1% solution in xylene. POM crystals were grown at 125°C by self seeding from an 0.05% solution of the polymer in o-dichlorobenzene. A given preparation was split into two parts. One part was dried down in the usual manner, the other was exchanged to paraffin oil and the crystals never permitted to dry down. Previously reported studies used dried-down crystals and gave crystallite sizes of approximately 300 Å. More recently, using electron microscopy, it has been postulated that PE single crystals are free of “mosaic block” over regions of several thousand angstroms. It is evident from this present study that crystallite sizes in uncollapsed lamellae are significantly larger than those observed for the same crystals dried down. In the case of uncollapsed lamellae, one can explain the observed crystallite sizes solely on the basis of chain obliquity rather than by invoking the “mosaic block” model. It has also been determined that there is an upper limit to the crystallite sizes that can be observed in PE and POM crystals using wide-angle x-ray techniques. This limit may account for discrepancies between x-ray and electron diffraction.     

Metastable Self‐Assembly of Theta‐Shaped Colloids and Twinning of Their Crystal Phases

Anisotropic colloids self‐assemble into different crystal structures compared to spherical colloids. Exploring and understanding their self‐assembly behavior could lead to creation of new materials with hierarchical structures through a bottom‐up process. Herein, we report metastable self‐assembly of theta‐shaped SiO₂ colloids interacting with a depletion force in a quasi‐two‐dimensional space and we demonstrate that both a metastable “prone” crystal phase and a stable “standing” crystal phase can be formed, depending on the self‐assembly path. Path selection stems from an interplay between particle–particle interactions and particle–wall interactions. In particular, a twinning of the metastable crystals was observed and two twinning mirror axes were found. A variety of complex twinned crystals were formed by each individual mirror axis or their combinations.     

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg²⁺ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg²⁺ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.     

Thermal stability of shear-induced precursor structures in isotactic polypropylene by rheo-X-ray techniques with couette flow geometry

Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long-living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s⁻¹ and duration t_s = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s⁻¹ and duration, t_s = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two-phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006     

H?TiO oder TiB2?? – eine Korrektur

H? TiO or TiB₂? – A Correction Reinvestigations of “H? TiO” single crystals and the comparison with measurements of TiB₂ crystals have shown that “H? TiO” in fact is titanium diboride.     

Reply to comment on “H3O and OH, the real ions in aqueous acids and bases”

We have measured the quantum beats in the decay of pulse-excited “prompt” fluorescence in tetracene crystals as a function of magnetic field. We have demonstrated that counting the beats is a very simple and reliable method to study a possible influence of triplet-triplet interactions. In all so far performed experiments in tetracene crystals no influence has been found.     

Growth and morphology of single crystals of linear aliphatic polyesters

In order to elucidate the relations between morphological habits and chemical structure of polymers, poly(ethylene sebacate), poly(hexamethylene sebacate) and poly(decamethylene 1,16-hexadecanedicarboxylate) were crystallized from dilute solutions in n-hexanol, isoamyl acetate etc., and were studied with the electron microscopy and x-ray diffraction. The crystal structure of these polyesters are tentatively determined. Morphological “regularity” and “simplicity” of the single crystals are correlated with the chemical structure of the polymers. The crystallization conditions under which “regular” and “simple” single crystals are obtained are relaxed with increase of methylene sequence length in chemical repeat unit. The Bragg extinction bands in the single crystals of poly(hexamethylene sebacate) and poly(decamethylene 1,16-hexadecanedicarboxylate) suggest nonplanar nature of these crystals. The molecular chains in the poly(ethylene sebacate) single crystals are inclined from the normal of the basal plane; the fold surface corresponds to the (001) plane.     

“Sponge Crystal”: a novel class of microporous single crystals formed by self-assembly of polyoxometalate (NH4)3PW12O40 nanocrystallites

In this account, a new concept of “sponge crystals” is presented on the basis of new interpretation of our previous results of porous heteropolyacids, that is, porous aggregates of self-assembled (NH₄)₃PW₁₂O₄₀ nanocrystallites (Ito, Inumaru, and Misono, J. Phys. Chem. B 101 (1997) 9958; Chem. Mater. 13 (2001) 824) “Sponge crystals” are defined as single crystals having continuous voids within them, but unlike zeolites, no intrinsic structural pores. This new category includes molecular single crystals having continuous voids originating from series of neighboring vacancies (≥1 nm) of the constituent large molecules, affording nanospaces in the crystals. A typical example of “sponge crystals” is (NH₄)₃PW₁₂O₄₀, which is formed via the dropwise addition of ammonium hydrogen carbonate into an H₃PW₁₂O₄₀ aqueous solution (titration method) at 368 K. The resulting (NH₄)₃PW₁₂O₄₀ nanocrystallites (ca. 6–8 nm) then self-assemble with the same crystal orientation to form porous dodecahedral aggregates in the solution. During the formation process, necks grow epitaxially between the surfaces of the nanocrystallites (“Epitaxial Self-Assembly”) to form aggregates of which each aggregate has an ordered structure as a whole single crystal. Although the crystal structure of (NH₄)₃PW₁₂O₄₀ has no intrinsic structural(“built-in”) pores, X-ray diffraction, electron diffraction and gas adsorption experiments all reveal that each (NH₄)₃PW₁₂O₄₀ aggregate is comprised of a single crystal bearing many micropores. These pores are considered to be continuous spaces as neighboring vacancies of the molecules (anions and cations) originating from the residual spaces between the self-assembled nanocrystallites. The porous (NH₄)₃PW₁₂O₄₀ single crystals are considered a special case of “mesocrystals,” as was recently discussed by Cölfen and Antonietti (Angew. Chem. Int. Ed. 44 (2005) 5576). In contrast to most “mesocrystals,” which are generally polycrystalline in nature, each aggregate of (NH₄)₃PW₁₂O₄₀ is a characteristic porous single crystal. Furthermore, the micropores of (NH₄)₃PW₁₂O₄₀ are totally different from those found in other microporous crystals: zeolites have “built-in” pores defined by their crystal structure, while the (NH₄)₃PW₁₂O₄₀ nanocrystallites have none. Since (NH₄)₃PW₁₂O₄₀ micropores are continuous spaces as neighboring vacancies of the molecules, the shapes of the (NH₄)₃PW₁₂O₄₀ pores can in principle, assume various connectivities or networks within the crystal, and as such, are subsequently termed: “sponge crystals.”     

Polymorphism of N,N'-diacetylbiuret studied by solid-state 13C and 15N NMR spectroscopy, DFT calculations, and X-ray diffraction

The molecular configuration and crystal structure of solid polycrystalline N,N′′‐diacetylbiuret (DAB), a potential nitrogen‐rich fertilizer, have been analyzed by a combination of solid‐ and liquid‐state NMR spectroscopy, X‐ray diffraction, and DFT calculations. Initially a pure NMR study (“NMR crystallography”) was performed as available single crystals of DAB were not suitable for X‐ray diffraction. Solid‐state ¹³C NMR spectra revealed the unexpected existence of two polymorphic modifications (α‐ and β‐DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X‐ray diffraction. The X‐ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two‐fold screw in the case of α‐DAB and a c‐glide plane in the case of β‐DAB, lead to a more symmetric (α‐DAB) or asymmetric (β‐DAB) intermolecular hydrogen‐bonding pattern for each molecule.     

Variational Properties of Adiabatic Molecular Wave Functions and Their Generalizations for Collision States

Born-Oppenheimer wave functions are shown to be variationally stable with respect to all other approximations to unbound-state molecular wave functions that are products of an electronic factor, and a function of nuclear coordinates only. The validity of this result is verified for functions having an “outgoing wave” or “ingoing wave” or “standing wave” behaviour. In the case of either of the first two classes of wave functions we use a variational principle associated with the transition matrix to derive our conclusion, whereas in the treatment of the third class, of “standing waves”, a variational principle for the reaction operator is employed. We then extend our investigations to the set of wave functions that are a finite sum of terms, each of which is a product of an electronic factor and an arbitrary function of nuclear coordinates. The result that emerges is that the variationally stable functions of this set are those whose nuclear functions satisfy Born's set of coupled equations for nuclear motions. On démontre que les fonctions d'onde de type Born-Oppenheimer sont stables du point de vue variationnel, par rapport à toute autre approximation d'une fonction d'onde moléculaire pour un état non lié, qui est le produit d'un facteur électronique et d'une fonction des coordonnées nucléaires seules. On vérifie ce résultat-ci pour des fonctions de type “onde sortante”, “onde incidente” ou “onde stationnaire”. Dans le cas d'une des deux premières classes de fonctions d'onde on déduit ce résultat d'un principe de variation associé à la matrice de transition, tandis que pour la troisième classe le même résultat est obtenu d'un principe de variation pour l'opérateur de réaction. Enfin on considère des fonctions d'onde, qui peuvent être écrites comme une somme finie de produits d'un facteur électronique et d'une fonction arbitraire des coordonnées nucléaires. On en déduit une condition pour les fonctions de cette classe-ci, qui sont stables du point de vue variationnel: ce sont celles dont les fonctions nucléaires satisfont au système d'équations couplées de Born pour les mouvements nucléaires. Es wird bewiesen, dass Born-Oppenheimer Wellenfunktionen von variationaler Standpunkte aus stabil sind mit Rücksicht auf alle andere Approximationen einer molekularen Wellenfunktion für einen nicht-gebundenen Zustand, die Produkte von einem elektronischen Faktor und einer Funktion der Kernkoordinaten sind. Dieses Resultat wurde für Funktionen, die sich als “ausgehende Wellen”, “einfallende Wellen” oder “stehende Wellen” verhalten, bestätigt. Für die zwei ersten Klassen wurde dabei ein Variationsprinzip für die Ubergangsmatrize, für die dritte Klasse eines für den Reaktionsoperator, angewendet. Schliesslich wurden solche Wellenfunktionen betrachtet, die eine endliche Summe von Produkten eines elektronischen Faktors und einer willkürlichen Funktion der Kernkoordinaten sind. Davon wurde eine Bedingung für von variationaler Standpunkte aus stabile Funktionen dieser Klasse hergeleitet: solche Funktionen müssen Lösungen von Born's gekoppelten Gleichungen für Kernbewegungen sein.     

Microporous Organically Pillared Layered Silicates (MOPS): A Versatile Class of Functional Porous Materials

The design of microporous hybrid materials, tailored for diverse applications, is a key to address our modern society's imperative of sustainable technologies. Prerequisites are flexible customization of host–guest interactions by incorporating various types of functionality and by adjusting the pore structure. On that score, metal–organic frameworks (MOFs) have been the reference in the past decades. More recently, a new class of microporous hybrid materials emerged, microporous organically pillared layered silicates (MOPS). MOPS are synthesized by simple ion exchange of organic or metal complex cations in synthetic layered silicates. MOFs and MOPSs share the features of “component modularity” and “functional porosity”. While both, MOFs and MOPS maintain the intrinsic characteristics of their building blocks, new distinctive properties arise from their assemblage. MOPS are unique since allowing for simultaneous and continuous tuning of micropores in the sub-Ångström range. Consequently, with MOPS the adsorbent recognition may be optimized without the need to explore different framework topologies. Similar to the third generation of MOFs (also termed soft porous crystals), MOPS are structurally ordered, permanently microporous solids that may also show a reversible structural flexibility above a distinct threshold pressure of certain adsorbents. This structural dynamism of MOPS can be utilized by meticulously adjusting the charge density of the silicate layers to the polarizability of the adsorbent leading to different gate opening mechanisms. The potential of MOPS is far from being fully explored. This Concept article highlights the main features of MOPS and illustrates promising directions for further research.     

Reconfigurable Photonic Crystal Reversibly Exhibiting Single and Double Structural Colors

Unlike absorption-based colors of dyes and pigments, reflection-based colors of photonic crystals, so called “structural colors”, are responsive to external stimuli, but can remain unfaded for over ten million years, and therefore regarded as a next-generation coloring mechanism. However, it is a challenge to rationally design the spectra of structural colors, where one structure gives only one reflection peak defined by Bragg's law, unlike those of absorption-based colors. Here, we report a reconfigurable photonic crystal that exhibits single-peak and double-peak structural colors. This photonic crystal is composed of a colloidal nanosheet in water, which spontaneously adopts a layered structure with single periodicity (407 nm). After a temperature-gradient treatment, the photonic crystal segregates into two regions with shrunken (385 nm) and expanded (448 nm) periodicities, and thus exhibits double reflection peaks that are blue- and red-shifted from the original one, respectively. Notably, the transition between the single-peak and double-peak states is reversible.     

Quadratisch,praktisch, natürlich?

In 2013 the consumer group Stiftung Warentest conducted tests on nut containing chocolates and detected piperonal (heliotropine) in Ritter‐Sport's “Voll‐Nuss” (whole hazelnut). Stiftung Warentest concluded that piperonal could not be obtained naturally and that Ritter‐Sport misled consumers by falsely labelling it as “natural flavor.” Ritter said the allegations were “baseless” and went to court. During the court case, Ritter's flavor supplier Symrise AG guaranteed that the “natural” piperonal was manufactured “naturally” according to the European flavor regulations. Ritter‐Sport had “won” the “German chocolate battle.” Although the full manufacturing process has not been revealed during the court case, there are many hints in the scientific and patent literature that piperonal can be indeed produced naturally in accordance with the European flavor regulations.