Finding the transition state of quasi-barrierless reactions by a growing string method for newton trajectories: application to the dissociation of methylenecyclopropene and cyclopropane

A method for finding a transition state (TS) between a reactant minimum and a quasi-flat, high dissociation plateau on the potential energy surface is described. The method is based on the search of a growing string (GS) along reaction pathways defined by different Newton trajectories (NT). Searches with the GS-NT method always make it possible to identify the TS region because monotonically increasing NTs cross at the TS or, if not monotonically increasing, possess turning points that are located in the TS region. The GS-NT method is applied to quasi-barrierless and truly barrierless chemical reactions. Examples are the dissociation of methylenecyclopropene to acetylene and vinylidene, for which a small barrier far out in the exit channel is found, and the cycloaddition of singlet methylene and ethene, which is barrierless for a broad reaction channel with Cs-symmetry reminiscent of a mountain cirque formed by a glacier.     

Pyridylthiazoles: highly luminescent heterocyclic compounds

Absorption, fluorescence, and fluorescence excitation spectra of two substituted [(5-methyl-2-pyridine-2'-yl-1,3-thiazole-4-yl)oxy]acetic acid and its methyl ester (2,2'-pyridylthiazoles) are studied at various pH values in aqueous solution. The acid exhibits pKa(1)=2.10+/-0.07 and pKa(2)=3.45+/-0.03, whereas the ester pKa=1.93+/-0.03. The protonation site is the pyridyl-nitrogen. When protonated, the cisoid conformer is the most stable; however, the transoid conformer is more stable in the deprotonated form. Fluorescence quantum yields close to unity are found. Large Stokes shift values are explained by the shortening of the inter-ring bond in the excited state. These compounds may be useful for metal sensing and as laser dyes.     

Mimicking biological cation-transport based on sphere-surface supramolecular chemistry: simultaneous interaction of porous capsules with molecular plugs and passing cations

Spherical capsules of the type [{(Mo)Mo(5)}(12){Mo(2)(ligand)}(30)](n-) exhibiting 20 {Mo(9)O(9)} pores with crown ether functions allowed us to perform a sophisticated study of the title phenomenon based on synthetic work as well as NMR spectroscopy. The pores of the host system interact in solution specifically with guests that can be noncovalently bonded, such as formamidinium and acetamidinium cations, while having different affinities to the pores. The exchange between the guest species present in solution and in the pores was investigated, including, besides the extreme scenarios of complete pore closing and complete opening, that of stepwise pore plugging. Because of this option it was possible to model for the first time passive transmembrane cation transport based on gated pores/channels. These have the appropriate dimensions and can even adopt different structure flexibilities in response to different cations. The present investigation is based on related syntheses as well as on numerous detailed (7)Li NMR studies of Li(+) transport/exchange equilibria in dependence of the pore environment/guest situations. One compound containing capsules with sulfate ligands (2) could be obtained in which all the pores are plugged with formamidinium cations and another corresponding one was obtained with additionally encapsulated Ca(2+) ions (3); these were taken up after temporary release of some of the formamidinium plugs/guests upon short heating of the related solution.     

Double-stranded cycles: toward C84's belt region

The reactivity of the double-stranded hydrocarbon cycle with two ether bridges (1) toward iodotrimethylsilane (TMSI) was investigated in some detail. The carbon skeleton of cycle 1 resembles the belt region of a C84 fullerene which makes it a potential precursor to the long sought after fully aromatic derivative. Upon exposure to TMSI, cycle 1 undergoes a cascade of reactions which involve different states of iodination/reduction which ultimately lead to the hydrogenated cycle 5a, whose structure was proven by single-crystal X-ray analysis. A deeper insight into mechanistic aspects of this sequence of conversions was gained by performing the reaction under dry and wet conditions, whereby the latter involved both normal and deuterated water. With the help of detailed NMR correlation studies and DFT computations, all important aspects were clarified including an unexpected selective H/D exchange at the naphthalenic moieties.     

Phosphinines as ligands in homogeneous catalysis: recent developments, concepts and perspectives

Phosphinines belong to an intriguing class of phosphorus-containing heterocycles with interesting steric, electronic and coordinating properties. This article focuses on recent developments, concepts and perspectives in the field of phosphinines and phosphinine-based ligands and their application as ligands in homogeneous catalytic reactions.     

Analytical Investigations of Bacterial Cellulose

The cultivation of the bacterium Acetobacter xylinus AX 5 was carried out in the common Hestrin-Schramm medium containing D -glucose as C-source and citric acid as buffer component. HPLC studies proved to be convenient methods to investigate the stability and interactions of these constituents in the starting culture liquid. Within the initial sterilization step and limited by the citric acid, up to 6% of the D -glucose was partially isomerized to D-fructose and degraded to dark-yellow products. In static culture, A. xylinus AX 5 produces cellulose extracellularly on the surface of this medium. Solid-state NMR spectroscopy represents a suitable analytical method to characterize the supramolecular structure of the bacterial cellulose in never-dried, air-dried, and freeze-dried states. It could be demonstrated that the drying process reduces the degree of crystallinity in the range of about 12% without changes in the Iα/β ratio of these cellulose modifications.     

Modeling of the Phase Separation Behavior of Polydisperse Semi‐Flexible Diblock Copolymers

Summary: A modified random phase approximation method with a cumulant expansion for the semi‐flexible structure factor of diblock copolymers was exercised to describe the phase separation behavior of semi‐flexible and polydisperse diblock copolymers. Scattering curves and spinodal diagrams were calculated applying monomer specific input parameters. The influence of polydispersity was included applying basic concepts of mathematical statistics utilizing several probability density distributions in the case of the two single blocks. In contrast to semi‐flexibility, the main effect of polydispersity was found to shift the spinodal up, thus to enlarge the range of existence of the homogeneous phase.

Twofold Schultz‐Zimm distribution of diblock copolymers.     

Modeling of the dynamics of multi-section waveguide lasers

A dynamic model for the design of multi-section semiconductor lasers is developed based on the Transmission Line Method. The different building blocks of this model are presented and the model is evaluated for a tunable two-section laser device.