On the internal nucleation of melting

Crystals of quartz have been superheated by some 450°C and crystals of albite by some 185°C above their respective melting points. In all cases, melting took place by the nucleation of liquid at the external surfaces (and internal boundaries as well in the case of albite). No evidence for the internal nucleation of liquid was found at any superheat for either material.The results of quartz indicate an exceptionallu large barrier to the internal nucleation of liquid. It is suggested that this large nucleation barrier is associated with the strain energy of forming a liquid nucleus within the crystalline phase.It is also indicated that the nucleation of liquid at the external surfaces of crystals at negligible superheats suggests that the free surfaces of liquids do not per se serve as preferred nucleating sites for crystallization — and that the crystal nucleation often observed at external surfaces or internal surfaces is in fact associated with condensed second-phase impurities.     

Homogeneous versus heterogeneous crystal nucleation in Li2O · 2SiO2 glass

When crystal nucleation occurs in glasses, it is difficult to ascertain whether the crystallization has initiated homogeneously or by a heterogeneous mechanism. Here we suggest a method which might enable one to make this choice, and it is applied to the bulk crystal nucleation observed in Li₂O · 2SiO₂. It is concluded that crystal nucleation in the latter system most probably occurs homogeneously.     

Fraktometrische Analyse von Deuterium im Wasserstoffstrom

Summary A universally usable, quantitative assay for deuterium-labeling work demands a stoichiometrical transformation of all samples into elementary hydrogen. For the detector, it is the best to connect directly a fractometer, that works with a catharometer cell. Tests with several carrier gases and an attempt with a palladium-black column failed; a molecular sieve with ordinary hydrogen as carrier was appropriate. The fractogram, written by a compensograph, shows a well gaugeable peak which is linearly proportional to the deuterium content of the sample. The error at full deflection amounts to ±0,5%. Protium deuteride, HD, is acting as if it were 1/2 D₂, a fact that was confirmed by mass-spectrometric measurements.

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Für Ratschläge danken wir Herrn Dr. Kienitz, Leiter der physikalisch-analytischen Gruppe im Ammoniaklaboratorium der BASF. Herr Dr. Kraus machte die massenspektrometrischen Aufnahmen.     

Counterexamples in chemical ring perception

Ring information is a large part of the structural topology used to identify and characterize molecular structures. It is hence of crucial importance to obtain this information for a variety of tasks in computational chemistry. Many different approaches for "ring perception", i.e., the extraction of cycles from a molecular graph, have been described. The chemistry literature on this topic, however, reports a surprisingly large number of incorrect statements about the properties of chemically relevant ring sets and, in particular, about the mutual relationships of different sets of cycles in a graph. In part these problems seem to have arisen from a sometimes rather idiosyncratic terminology for notions that are fairly standard in graph theory. In this contribution we translate the definitions of concepts such as the Smallest Set of Smallest Rings, Essential Set of Essential Rings, Extended Set of Smallest Rings, Set of Smallest Cycles at Edges, Set of Elementary Rings, K-rings, and beta-rings into a more widely used mathematical language. We then outline the basic properties of different cycle sets and provide numerous counterexamples to incorrect claims in the published literature. These counterexamples may have a serious practical impact because at least some of them are molecular graphs of well-known molecules. As a consequence, we propose a catalog of desirable properties for chemically useful sets of rings.     

Chiral induction effects in ruthenium(II) amino alcohol catalysed asymmetric transfer hydrogenation of ketones: an experimental and theoretical approach

The enantioselective outcome of transfer hydrogenation reactions that are catalysed by ruthenium(II) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substituent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reaction to a large extent: enantioselectivities (ee values) of up to 95% were obtained for the reduction of acetophenone. The catalytic cycle of ruthenium(II) amino alcohol catalysed transfer hydrogenation was examined at the density functional theory level. The formation of a hydrogen bond between the carbonyl functionality of the substrate and the amine proton of the ligand, as well as the formation of an intramolecular H...H bond and a planar H-Ru-N-H moiety are crucially important for the reaction mechanism. The enantioselective outcome of the reaction can be illustrated with the aid of molecular modelling by the visualisation of the steric interactions between the ketone and the ligand backbone in the ruthenium(II) catalysts.     

On positive functionals on algebras of test functions for quantum fields

It is shown that the *-algebraR of test-functions for a quantum field is reduced, i.e. for eachbR,b0, there exists a positive continuous linear functionalW(a) onR withW(b)0.On leave of absence from Mathematisches Institut, Karl-Marx-Universität Leipzig.     

Surface stresses and crystal twisting in hippuric acid and in polymers

An approximate model is developed which relates the twisting seen in the spherulitic crystallization of many materials to surface stresses. Expressions are presented for the twist period as a function of the surface stress. These are compared with previous experimental data on polyethylene and data on hippuric acid obtained in the present investigation. In both cases the experimental data are in accord with the predictions of the model.