Hydrogen-Bonding Networks from First-Principles: Exploring the Guanidine Crystal

Hydrogen bonding is among the most important interactions in molecular crystals, and examples are abundant. As a consequence of such interactions, many molecules crystallize in complex but intriguing structures, in contrast to the relatively simple packing principles of metallic or ionic solids. In this work, we present a computational approach based on plane-wave density-functional theory (DFT) and supercell techniques, aiming to understand and quantify hydrogen-bonded networks in the solid state and in two-, one-, and zero-dimensional fragments derived from the molecular crystal. With such methodology at hand, we investigate guanidine, a fitting example of a molecular crystal and an important compound for inorganic and organic chemistry alike. On the basis of our computations, we discuss the initially proposed layered structure of guanidine and identify both stabilizing and destabilizing cooperative interactions in the three crystalline dimensions

Accurate Hydrogen Positions in Organic Crystals: Assessing a Quantum-Chemical Aide

Organic molecules crystallize in manifold structures. The last few decades have seen the rise of high-resolution X-ray diffraction techniques that make the structures of even the most complex crystals easily accessible. Still, an intrinsic challenge lies in assigning hydrogen atoms’ positions from X-ray experiments alone. Quantum chemistry plays a fruitful, complementary role here, and so ab initio optimization techniques for organic crystals are on the rise as well. In this context, we review and evaluate a popular ab initio strategy based on plane-wave density-functional computations, namely, selectively relaxing H positions in an otherwise fixed cell. Our data show that such-optimized C–H, N–H, O–H, and B–H bond lengths coincide well with results from neutron diffractionthe experimental technique that sets the “gold standard” for H positions in molecular crystals but which is far less easily available. We have thus justified the use of a quantum-chemical aide with a broad variety of possible applications

Colloidal Synthesis of 1T-WS₂ and 2H-WS₂ Nanosheets: Applications for Photocatalytic Hydrogen Evolution

In recent years, a lot of attention has been devoted to monolayer materials, in particular to transition-metal dichalcogenides (TMDCs). While their growth on a substrate and their exfoliation are well developed, the colloidal synthesis of monolayers in solution remains challenging. This paper describes the development of synthetic protocols for producing colloidal WS₂ monolayers, presenting not only the usual semiconducting prismatic 2H-WS₂ structure but also the less common distorted octahedral 1T-WS₂ structure, which exhibits metallic behavior. Modifications of the synthesis method allow for control over the crystal phase, enabling the formation of either 1T-WS₂ or 2H-WS₂ nanostructures. We study the factors influencing the formation of the two WS₂ nanostructures, using X-ray diffraction, microscopy, and spectroscopy analytical tools to characterize them. Finally, we investigate the integration of these two WS₂ nanostructured polymorphs into an efficient photocatalytic hydrogen evolution system to compare their behavior