Pressure-Dependent Structural and Luminescence Properties of 1-(Pyren-1-yl)but-2-yn-1-one

The crystal structure of 1-(pyren-1-yl)but-2-yn-1-one ( 1 a , a polynuclear aromatic hydrocarbon displaying enhanced luminescence in the solid state, has been re-determined at several pressures ranging from atmospheric up to 3 GPa using a Diamond Anvil Cell (DAC). These experiments were augmented by periodic DFT calculations at pressures up to 4.4 GPa. UV-Vis fluorescence of 1 a at non-ambient pressures has also been investigated. The crystal structure consists of infinite π -stacks of anti-parallel 1 a molecules with discernible dimers, which may exemplify aggregates formed by pyrene derivatives in solution and thin films, and is predominantly stabilized by dispersion. The average inter-planar distance between individual molecules within π -stacks decreases with pressure in the investigated range. This results in piezochromic properties of 1 a : a red-shift of sample color, as well as a bathochromic shift of fluorescence with pressure (by ca. 100 nm at 3.5 GPa). Two-component fluorescence spectra support the hypothesis that at least two types of excimers are involved in the electronic excitation processes in crystalline 1 a

Refinement of Organic Crystal Structure with Multipolar Electron Scattering Factors

A revolution in resolution is occurring now in electron microscopy arising from the development of methods for imaging single particles at cryogenic temperatures and obtaining electron diffraction data from nanocrystals of small organic molecules or macromolecules. Near- atomic or even atomic resolution of molecular structures can be achieved. The basis of these methods is the scattering of an electron beam due to the electrostatic potential of the sample. To analyze this high-quality experimental data, it is necessary to use appropriate atomic scattering factors. The independent atomic model (IAM) is commonly used although various more advanced models, already known from X-ray diffraction, can also be applied to enhance the analysis.In this study we present a comparison of IAM and TAAM (Transferable Aspherical Atom Model), the latter with the parameters of the Hansen-Coppens multipole model transferred from the University at Buffalo Databank (UBDB). By this method, TAAM takes into account the fact that atoms in molecules are partially charged and are not spherical. We performed structure refinements on a carbamazepine crystal using electron structure factor amplitudes determined experimentally (Jones et al., 2018) or modeled with theoretical quantum-mechanical methods. The results show the possibilities and limitations of the TAAM method when applied to electron diffraction. Among others, the method clearly improves model fitting statistics, when compared to IAM, and allows for reliable refinement of atomic thermal parameters. The improvements are more pronounced with poorer resolution of diffraction data.<br /

Linear MgCp*2 vs Bent CaCp*2 : London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions

In the family of metallocenes, MgCp*2 (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*2 is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp*2 suggests that the bending must have an intramolecular origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp2 and MCp*2 gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*2. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp*2 and CaCp*2 were determined which facilitated the identification and characterization of intermolecular pseudo-pregostic interactions, C–H···Ca, in the CaCp*2 crystal structure

Linear MgCp*₂ vs Bent CaCp*₂: London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions

In the family of metallocenes, MgCp*₂ (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*₂ is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp*₂ suggests that the bending must have an <i>intramolecular</i> origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp₂ and MCp*₂ gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*₂. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp*₂ and CaCp*₂ were determined which facilitated the identification and characterization of <i>intermolecular</i> pseudo-pregostic interactions, C–H···Ca, in the CaCp*₂ crystal structure

Linear MgCp*₂ vs Bent CaCp*₂: London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions

In the family of metallocenes, MgCp*₂ (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*₂ is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp*₂ suggests that the bending must have an <i>intramolecular</i> origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp₂ and MCp*₂ gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*₂. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp*₂ and CaCp*₂ were determined which facilitated the identification and characterization of <i>intermolecular</i> pseudo-pregostic interactions, C–H···Ca, in the CaCp*₂ crystal structure

Linear MgCp*₂ vs Bent CaCp*₂: London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions

In the family of metallocenes, MgCp*₂ (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*₂ is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp*₂ suggests that the bending must have an <i>intramolecular</i> origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp₂ and MCp*₂ gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*₂. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp*₂ and CaCp*₂ were determined which facilitated the identification and characterization of <i>intermolecular</i> pseudo-pregostic interactions, C–H···Ca, in the CaCp*₂ crystal structure