Avoiding problems with hydrogen misplacement in reporting crystal structures

Intermolecular hydrogen bonding is an integral part of many crystal structures. Hydrogen bonding sometimes results in one‐, two‐ or three‐dimensional supramolecular assemblies, a common feature of which is positional disorder of H atoms related to space‐group symmetry. Yet some reported structures fail to include all possible donor–acceptor close contacts, or to seek H‐atom electron densities associated with apparent D—H...A trios, while some H‐atom positions violate principles of chemistry or crystal physics. Modern diffraction equipment and sophisticated computing systems provide high‐quality data; thus, failure to characterize and report fully an accurate, complete and physically correct hydrogen‐bonding model should not be acceptable. We illustrate the relevant issues with three published examples in the hope of slowing the proliferation of these problems, with the scientifically desirable goal of improving the accuracy of crystallographic models while also providing improved search keys for information retrieval.     

Two Connections Between Random Systems and Non-Gibbsian Measures

In this contribution we discuss the role disordered (or random) systems have played in the study of non-Gibbsian measures. This role has two main aspects, the distinction between which has not always been fully clear: 1) From disordered systems: Disordered systems can be used as a tool; analogies with, as well as results and methods from the study of random systems can be employed to investigate non-Gibbsian properties of a variety of measures of physical and mathematical interest. 2) Of disordered systems: Non-Gibbsianness is a property of various (joint) measures describing quenched disordered systems. We discuss and review this distinction and a number of results related to these issues. Moreover, we discuss the mean-field version of the non-Gibbsian property, and present some ideas how a Kac limit approach might connect the finite-range and the mean-field non-Gibbsian properties.     

Mechanical and dynamic aspects of voice production as related to voice therapy and phonosurgery

Laryngeal framework surgery can change the position and tensionof the vocal folds safely without direct surgical intervention in the vocal fold proper. Some 23 years of experience with phonosurgery have proved its usefulness in treating dysphonia related to unilateral vocal fold paralysis, vocal fold atrophy, and pitch-related dysphonias. Meanwhile, much information about the mechanism of voice production has been obtained through intraoperative findings of voice and fiberscopic examination of the larynx . Based on such knowledge together with information obtained through model experiments, the human vocal organ was reconsidered mainly from the mechanical view point, and the roles of voice therapy and singing pedagogy were discussed in relation to phonosurgery. The vocal organ may not be an ideal musical organ and is rather vulnerable, but its potential is enormous.     

Single-crystal 2H NMR of deuterated 1,10-decanedicarboxylic acid included in hydrogenated urea channels

The intergrowth of 1,10-decanedicarboxylic acid and urea give infinite hydrogen-bonded chains of the guest included in the hexagonal urea host. A deuterium high-resolution solid-state NMR study of the selectively deuterated intergrowth compound 1,10-decanedicarboxylic acid/hydrogenated urea at variable temperature in the range 90 < or = T < or = 300 K was performed on a single crystal. The analysis of the second moment as a function of temperature is shown to be compatible with the known phase transition occurring near T(c) = 203 K. Moreover, the spectra indicate that the orientational disorder is strong, and is compared to an axial uniform disorder. For this purpose, the general equation for the second moment of a system with uniform two-dimensional axial orientational disorder is given, and a method to take into account the non-uniform excitation of the pulse sequence is proposed.     

An unexpected tetragonal unit cell for N‐(L‐2‐aminobutyryl)‐L‐serine

The title dipeptide {systematic name: (S)‐2‐[(S)‐2‐azaniumylbutanamido]‐3‐hydroxypropanoate}, C₇H₁₄N₂O₄, was synthesized in the anticipation that it would form nanoporous crystals with hexagonal symmetry. Single‐crystal X‐ray diffraction analysis showed that it had instead adopted a unit cell in the space group I4, similar to L‐alanyl‐L‐alanine [Fletterick, Tsai & Hughes (1970). J. Phys. Chem. 75 , 918–922]. The resulting packing arrangement has a high density for a peptide (1.462 Mg m⁻³), which is rendered possible by extensive disorder over two positions for the ethyl side chain of the 2‐aminobutyric acid fragment and over three positions for the serine side chain.<!?tpb=17.5pt>     

Untersuchungen im System LaI2/H2: Phasenverhältnisse und Fehlordnung

Investigation of the LaI₂/H₂ System: Phase Relations and Stacking Disorder Heating of LaI₂ under 1 bar hydrogen pressure to 650 °C leads to light gray LaI₂H_0.95(3), accompanied by a structural change from tetragonal to hexagonal. Sharp reflections in the XRD pattern can be indexed in P6₃/mmc with a = 4.2158(7)Å and c = 15.508(3)Å, however, diffuse reflections indicate the presence of stacking faults in the structure, which correspond to a polytypic intergrowth of MoS₂ and NbS₂ type structural fragments. Increasing the reaction temperature to 730 °C results in a better defined diffraction pattern with the peak positions close to those of the 2H‐NbS₂ structure type. An X‐ray powder study of the samples LaI₂H_n proved the miscibility gap between LaI₂ and LaI₂H_n (0 ≤ n ≤ 0.5) in agreement with previous results. With decreasing H‐content of the homogeneous phase the lattice parameters change in opposite direction, a increasing to 4.236(1)Å and c decreasing to 15.39(2)Å for the lower limit.     

Triple-Decker Complexes. 14. [1] A Model for Refinement of the End-for-End Disordered Cationic Triple-Decker Complex [Cp*Ru(μ-η5:η5-C4Me4P)FeCp*]+

The solid-state structure of the triple-decker salt [Cp*Fe(-⁵:⁵-C₄Me₄P)RuCp*] · CF₃SO₃ shows orientational disorder for the pseudosymmetric cations. A chemically related compound was used to define a restrained structure model. Comparison of different refinement strategies proves that this restrained model is superior to an unrestrained treatment.     

Long‐range order in physical networks of gel‐forming triblock copolymer solutions

Structural studies on physical gels, based on a triblock copolymer and a solvent selective for its midblocks, are presented. Gel formation in such systems arises due to interconnected microdomains that form the junctions in a three‐dimensional network. Small angle X‐ray scattering studies revealed that the physical networks show either short‐range order, described by an effective hard sphere model, or long‐range order, where junctions are arranged on a cubic lattice. The emerging morphology depends on the thermal conditions during preparation of such systems, which essentially reflects a competition among microdomain ordering and formation of the physical network. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1833–1840, 1999     

Replica Symmetry Breaking in Short-Range Spin Glasses: Theoretical Foundations and Numerical Evidences

We discuss replica symmetry breaking (RSB) in spin glasses. We update work in this area, from both the analytical and numerical points of view. We give particular attention to the difficulties stressed by Newman and Stein concerning the problem of constructing pure states in spin glass systems. We mainly discuss what happens in finite-dimensional, realistic spin glasses. Together with a detailed review of some of the most important features, facts, data, and phenomena, we present some new theoretical ideas and numerical results. We discuss among others the basic idea of the RSB theory, correlation functions, interfaces, overlaps, pure states, random field, and the dynamical approach. We present new numerical results for the behaviors of coupled replicas and about the numerical verification of sum rules, and we review some of the available numerical results that we consider of larger importance (for example, the determination of the phase transition point, the correlation functions, the window overlaps, and the dynamical behavior of the system).     

Orientational deconvolution of two-dimensional static disorder by a Tikhonov regularized method for 2H solid state NMR of nano-tubular-oriented structures

A general trend of supramolecular chemistry is the building of mesoscopic-oriented nanotubes. When the typical radius of the channel almost fits the mean supramolecular cross-sectional radius, the guest supramolecule is ultraconfined in the channel. In that case, only rotational disorder around the channel axis is allowed. For sufficiently low temperatures, we expect this 2D disorder to be static on the NMR timescale, and it should reflect the local symmetry of the channels. In this article, we show that experiments performed with deuterium single crystal solid-state NMR as a function of rotation around the channel axis may lead to important information concerning the static orientational disorder of the confined supramolecules. Using an inversion method with Tikhonov regularization and a positivity constraint, that also takes into account of the nonideal pulse sequence response, the orientational probability density can be obtained from 1D experiments performed at different angles, even for significant disorder. As a first step, the method is validated on four different theoretical distributions. It is then applied to ²H NMR single crystal experiments performed on an archetype of parallel channels intergrowth compounds: selectively deuterated 1,10-decanedicarboxylic diacids in hydrogenated urea channels. The diacids form one-dimensional infinite hydrogen-bonded chains ultraconfined in urea linear channels.