Preface

Philip Coppens describes this special issue on charge density, photocrystallography and time-resolved crystallography.

Introduction

This article introduces a special issue dedicated to a lifetime of outstanding scientific achievements by Professor Philip Coppens.

Accurate charge densities from powder X-ray diffraction – a new version of the Aarhus vacuum imaging-plate diffractometer

The present state of X-ray electron-density determination from powder-diffraction data is briefly reviewed together with the first results from a new large-diameter in-vacuum diffractometer.

Experimental charge-density studies: data reduction and model quality: the more the better?

In experimental charge-density investigations, it is essential to reach the highest possible quality of data and the most resilient model. Recent developments concerning data and model quality are reviewed.

When combined X-ray and polarized neutron diffraction data challenge high-level calculations: spin-resolved electron density of an organic radical

Experimental joint refinement against X-ray and polarized neutron diffraction data has been performed to simultaneously model the charge and spin density of a free radical; the resulting model challenges density functional theory (DFT) calculations, but is in fair agreement with ab initio complete active-space self-consistent field (CASSCF) calculations.

Multi-temperature study of potassium uridine-5′-mono­phosphate: electron density distribution and anharmonic motion modelling

A multi-temperature high-resolution diffraction study of a biologically important salt potassium uridine monophosphate, K(UMPH), is reported. Anharmonic motion in the uracil moiety was detected and verified by He-temperature X-ray diffraction experiments. The charge density distribution was modelled within the Hansen–Coppens formalism, while anharmonicity was taken into account using Gram–Charlier parameters.

Insights on spin delocalization and spin polarization mechanisms in crystals of azido copper(II) dinuclear complexes through the electron spin density Source Function

Electron spin density at a point may be seen as determined by a local Source Function for such density, operating at all other points of space. Integration of the local source over Bader's quantum atoms measures their contribution in determining the spin density features at any system's location. This novel tool is able to provide interesting insights on the electron spin delocalization and polarization mechanisms along with their dependence from the spin density quality.

Exploring charge density analysis in crystals at high pressure: data collection, data analysis and advanced modelling

This paper reports on the determination of charge density distributions from crystals compressed at high pressure. Although much less accurate than traditional charge density analysis, these studies provide valuable information because the electronic states of molecules confined in highly condensed space are not easily predictable by theory and therefore experimental validation is a must.

Interplay of point multipole moments and charge penetration for intermolecular electrostatic interaction energies from the University at Buffalo pseudoatom databank model of electron density

University at Buffalo DataBank (UBDB)-derived electrostatic energies () closely approach those computed using quantum mechanics mainly because the UBDB allows for a direct account of charge penetration effects. For interactions between polar molecules, penetration constitutes up to 50% of E_es, but for nonpolar molecules, it may reach 100% of E_es at the equilibrium distance.

A theoretical-electron-density databank using a model of real and virtual spherical atoms

A spherical-electron-density database with additional spherical scattering centres located on bonds and lone-pair sites is created for charge-density modelling.

Exploring the rare S—H⋯S hydrogen bond using charge density analysis in isomers of mercapto­benzoic acid

The appearance of an elusive S—H⋯S hydrogen bond with the S atom acting as a donor in 2-, 3-mercapto­benzoic acids has been analysed based on charge density calculations. The importance of the locking potential provided by an intramolecular S⋯O chalcogen bond is highlighted.

Charge density studies of 3d metal (Ni/Cu) complexes with a non-innocent ligand

The redox characteristics of pairs of square planar complexes M(mnt)₂²⁻/M(mnt)₂⁻, where mnt is the non-innocent maleo­nitrile­dithiol­ate ligand, are established through charge density analysis and X-ray absorption spectroscopy: the oxidation is ligand based in Ni complexes and are metal based for the Cu case.

Experimental observation of charge-shift bond in fluorite CaF₂

F⁻⋯F⁻ interactions in fluorite have properties associated with the recently recognized type of interaction referred to as `charge-shift' bonding. This finding is supported by the topology of the electron localization function and the analysis of quantum theory of atoms in molecules and crystals. These interactions are attractive in character, which is additionally indicated by the topology of non-covalent interactions based on the reduced density gradient.

Revisiting the charge density analysis of 2,5-di­chloro-1,4-benzo­quinone at 20 K

The charge density of 2,5-di­chloro-1,4-benzo­quinone has been redetermined at 20 K. Two different multipole refinement models lead to similar topological properties of the total electron density.

Formation of local spin-state concentration waves during the relaxation from a photoinduced state in a spin-crossover polymer

Broken symmetry and spin-state concentration waves compete during the relaxation from the photoinduced high-spin to the ground low-spin phase of a two-dimensional spin-crossover polymer.

Solvent exchange in a metal–organic framework single crystal monitored by dynamic in situ X-ray diffraction

Dynamic in situ single crystal X-ray diffraction revealed unique and important information about the structural reorganization that occurs upon guest exchange in a cobalt-based metal organic framework.

The index of dispersion as a metric of quanta – unravelling the Fano factor

The need for high-quality assessments of data motivates appreciation of shot noise as a data resource. The index of dispersion is a critical but often overlooked metric in this regard. When properly understood and carefully used it reveals the energies of quanta, in both monochromatic and polychromatic situations. To accomplish this, and in due course its wider application, its conceptual origins and quantitative and physical bounds are identified. Situations are examined that involve large numbers of quanta, but in which individual quanta are below measurement noise threshold. Practical applications are extremely diverse, so relevant considerations and how to use the metric are shown in broad contexts.

In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector

We report a new experimental methodology for time-resolved X-ray diffraction after pulsed laser excitation using a gated hybrid pixel X-ray detector with dedicated pump probe mode, and implemented on a laboratory diffractometer.

Nanoscale symmetry fluctuations in ferroelectric barium titanate, BaTiO₃

The crystal symmetry of ferroelectric BaTiO₃ is determined by scanning convergent-beam electron diffraction. The results show inhomogeneous symmetry with nanometre-sized regions of almost perfect symmetry interspersed in regions of lower symmetry.

Crystal structure and luminescent properties of novel coordination polymers constructed with bi­furandi­carboxylic acid

Four novel coordination polymers (CPs) assembled with [2,2′-bifuran]-5,5′-dicarboxylic acid (H₂L) and metal ions Zn^II, Cd^II or Co^II have been successfully synthesized and characterized. It was found that both the metal ion and the crystal structure play important roles for the luminescent properties of CPs.

Anharmonic motions versus dynamic disorder at the Mg ion from the charge densities in pyrope (Mg₃Al₂Si₃O₁₂) crystals at 30 K: six of one, half a dozen of the other

A very accurate single-crystal X-ray diffraction study of synthetic pyrope (Mg₃Al₂Si₃O₁₂, ) at 30 K reveals that residual density features around Mg²⁺ ions, previously attributed to static disorder, can be better accounted for by anharmonicity/dynamic disorder.

Synthesis, structure and non-linear optical properties of new isostructural β-D-fructo­pyran­ose alkaline halide metal–organic frameworks: a theoretical and an experimental study

Theoretical and experimental investigations on non-linear optical properties of four metal–organic framework isomorphs, based on fructose and alkali-earth halogenides, show that both the intrinsic nature of the anion and the induced dissociation of the cation and anion by fructose play a role in the second-harmonic generating properties of such compounds.

Local structure and stacking disorder of chloro(phthalocyaninato)aluminium

In chloro­(phthalocyaninato)aluminium, every second double layer is disordered over two positions. The disorder is analysed and explained by order–disorder theory and lattice-energy minimizations using dispersion-corrected density functional theory.

Validation of missed space-group symmetry in X-ray powder diffraction structures with dispersion-corrected density functional theory

Two hundred crystal structures with missed symmetry were investigated by dispersion-corrected density functional theory. In 98.5% of the cases the correct space group is found.

Crystal structures of η′′-Cu_3+xSi and η′′′-Cu_3+xSi

The incommensurately modulated structures of η′′-Cu_3+xSi and η′′′-Cu_3+xSi are described in the superstructure approximation. The structures are compared.