Carbonic acid: from polyamorphism to polymorphism

Layers of glassy methanolic (aqueous) solutions of KHCO3 and HCl were deposited sequentially at 78 K on a CsI window, and their reaction on heating in vacuo in steps from 78 to 230 K was followed by Fourier transform infrared (FTIR) spectroscopy. After removal of solvent and excess HCl, IR spectra revealed formation of two distinct states of amorphous carbonic acid (H2CO3), depending on whether KHCO3 and HCl had been dissolved in methanol or in water, and of their phase transition to either crystalline alpha- or beta-H2CO3. The main spectral features in the IR spectra of alpha- and beta-H2CO3 are observable already in those of the two amorphous H2CO3 forms. This indicates that H-bond connectivity or conformational state in the two crystalline phases is on the whole already developed in the two amorphous forms. The amorphous nature of the precursors to the two crystalline polymorphs is confirmed using powder X-ray diffraction. These diffractograms also show that alpha- and beta-amorphous H2CO3 are two distinct structural states. The variety of structural motifs found within a few kJ/mol in a computational search for possible crystal structures provides a plausible rationalization for (a) the observation of more than one amorphous form and (b) the retention of the motif observed in the amorphous form in the corresponding crystalline form. The polyamorphism inferred for carbonic acid from our FTIR spectroscopic and powder X-ray diffraction studies is special since two different crystalline states are linked to two distinct amorphous states. We surmise that the two amorphous states of H2CO3 are connected by a first-order-like phase transition.     

Enrichment of deuterium oxide at hydrophilic interfaces in aqueous solutions

The structure of water at aqueous interfaces is of the utmost importance in biology, chemistry, and geology. We use neutron reflectivity and quartz crystal microbalance to probe an interface between hydrophilic quartz and bulk liquid solutions of H2O/D2O mixtures. We find that near the interface the neutron scattering length density is larger than in the bulk solution and there is an excess adsorbed mass. We interpret this as showing that there is a region adjacent to the quartz that is enriched in D2O and extends 5-10 nm into the solution. This suggests caution when interpreting results where D2O is substituted for H2O in aqueous interfacial chemistry.     

The Resolved Mutual Information Function as a Structural Fingerprint of Biomolecular Sequences for Interpretable Machine Learning Classifiers

In the present article we propose the application of variants of the mutual information function as characteristic fingerprints of biomolecular sequences for classification analysis. In particular, we consider the resolved mutual information functions based on Shannon-, Rényi-, and Tsallis-entropy. In combination with interpretable machine learning classifier models based on generalized learning vector quantization, a powerful methodology for sequence classification is achieved which allows substantial knowledge extraction in addition to the high classification ability due to the model-inherent robustness. Any potential (slightly) inferior performance of the used classifier is compensated by the additional knowledge provided by interpretable models. This knowledge may assist the user in the analysis and understanding of the used data and considered task. After theoretical justification of the concepts, we demonstrate the approach for various example data sets covering different areas in biomolecular sequence analysis.     

Defect Structure in Te-doped GaAs single Crystals after Plastic Deformation (I). Twins and Stacking Faults

At temperatures above the brittle-to-ductile transition (490 °C) in Te-doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈ 550 °C) and (iii) dislocation cells (580 … 590 °C). In Part I quantitative details of the appearance of twins and stacking faults are given. Most frequently found were 30° partials in twins and stacking faults.     

Tip-enhanced Raman scattering: Influence of the tip-surface geometry on optical resonance and enhancement

The tip-sample distance (z) dependence of tip-enhanced Raman scattering (TERS) has been investigated. The intensities of both, the Raman lines and the broad TERS background, exhibit strong decays with increasing z, which are nearly complete within 10 nm withdrawal of the STM tip in z direction. Interestingly, the maximum of the broad Lorentzian-shaped TER background is substantially blue shifted in energy with z. This effect is ascribed to a corresponding blue shift of the energies of localized plasmon modes upon tip retraction. Both experimental results fit very well data of a simple theoretical near-field model.     

Fine-tuning catalytic activity and selectivity—[Rh(amino acid thioamide)] complexes for efficient ketone reduction

Amino acid-derived thioamides are prepared and evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. It is found that increasing the steric bulk at the C-terminus of the ligand had a positive impact on both activity and selectivity in the reduction reaction. In order to find the optimum catalyst, a study is performed on a series of thioamide ligands having substituents of varying size.