Quo vadis, quantum crystallography?

An overview is given by the guest editors of the papers published in the virtual special issue on quantum crystallography.

Interpreting the different emissive properties of cyclic tri­imidazole-based Cu^I and Ag^I coordination polymers: a QTAIM and IQA study

The different nature of the emissive states of Ag^I and Cu^I one- and three-dimensional coordination polymers is here explained by a higher covalent character of the Cu—N bond with respect to the Ag—N one.

On the transfer of theoretical multipole parameters for restoring static electron density and revealing and treating atomic anharmonic motion. Features of chemical bonding in crystals of an isocyanuric acid derivative

The transfer of theoretical multipole parameters to the experimental model was found to be helpful in separating the static electron density from the contaminant smearing effects emerging due to insufficiently accounted for atomic motion. A special refinement procedure resulted in significant improvement of the static electron density.

Ibuprofen and sila-ibuprofen: polarization effects in the crystal and enzyme environments

Intermolecular interactions of the new drug candidate sila-ibuprofen are investigated in the crystal and in the enzyme cyclooxygenase-II.

Can the results of quantum refinement be improved with a continuum-solvation model?

Quantum refinement has been shown to be a powerful approach to interpret and improve macromolecular crystal structures. Previous studies have shown that the results of quantum refinement can be improved if the charge of the quantum mechanical (QM) system is reduced by adding neutralizing groups. Here it is shown that a similar improvement can be obtained if the original highly charged QM system is instead immersed in a continuum solvent in the QM calculations.

Electronic structure of (MePh₃P)₂[Ni^II(bdtCl₂)₂]·2(CH₃)₂SO and (MePh₃P)[Ni^III(bdtCl₂)₂] (bdtCl₂ = 3,6-di­chloro­benzene-1,2-di­thiol­ate)

Differences between the formal oxidation states of two nickel(II/III) coordination compounds with a non-innocent ligand were studied. There are differences between the theoretical calculations as well as between various experimental results.

Mapping high-pressure crystallography in a structural chemistry landscape

The remarkable growth in volume of high-pressure crystal structural chemistry depositions in the Cambridge Structure Database has been reported in a recent paper by Kaźmierczak & Patyk-Kaźmierczak [Acta Cryst. (2021), B77, 1012–1020].

Crystallography meets orbital-free DFT – two-pronged approach towards chemical bonding characteristics in chemical bonding analysis

A promising route towards rational design of materials is the combination of quantum chemical calculations with experimental electron density-based techniques within the framework of quantum crystallography. A recent work by Bartashevich et al. [Acta Cryst. (2021), B77, 478–487] fits well under the umbrella of modern QCr methods and it focuses on Appel's salt where, apart from purely covalent bonds, a variety of non-trivial intermolecular interactions are present.

Deformation of polyiodides in Cs₂I₈ crystals at high pressure

Iodine triiodide contacts in zigzag octaiodide units in Cs₂I₈ undergo substantial compression, down to intramolecular distances typical for covalently bonded polyiodides. The polyiodide net twists out of its initial shape, allowing for even more expanded connectivity between caesium cations and negatively charged iodine layers.

Crystal structure of the high-P polymorph of Ca₂B₆O₆(OH)₁₀·2(H₂O) (meyerhofferite)

Meyerhofferite is known to undergo a pressure-mediated phase transition between 3.0 and 3.5 GPa. In this study, the crystal structure of the high-pressure polymorph of meyerhofferite and its pressure-induced deformation mechanisms are revealed, providing information aimed at improving knowledge of the high-pressure phase transitions in this class of hydrated borates.

Strategic synthon approach in obtaining cocrystals and cocrystal polymorphs of a high-Z′ system deferiprone – an anti-thalassemia drug

Eleven cocrystals along with a fourth polymorph of deferiprone are reported. The homo- and hetero-synthons preference amongst the cocrystal systems is discussed.

Investigation of polar crystalline materials containing hydro­chloro­thia­zide: electron density distribution and optical properties

A detailed examination of intermolecular interactions via topological analysis of electron density and optical properties has been made of two polar materials containing the hydro­chloro­thia­zide molecule.

Structural studies of N-(meth­oxy­salicyl­idene)-fluoroaniline, N-(meth­oxy­salicyl­idene)-chloro­aniline and N-(meth­oxy­salicyl­idene)-bromo­aniline derivatives

Structural studies of 27 N-(meth­oxy­salicyl­idene)-haloaniline (halo = F, Cl or Br) compounds are reported. New polymorphs were identified for two of the compounds and a phase transition was also identified for another. The link between their structures and visual observations of their thermochromic behaviour are made.

Structures and phase transitions in barium sodium niobate tungsten bronze (BNN)

From an analysis of published results, it is concluded that the room-temperature structure of Ba₂NaNb₅O₁₅ is that proposed in 1989 in the space group Bbm2 (Ama2 in standard setting). The structures and phase transitions at higher temperatures are also considered.

A previously unknown cyclic alkanolamine and molecular ranking using the pair distribution function

The crystal structure of a previously unknown small heterocyclic alkanolamine was determined by X-ray powder diffraction. Total scattering analysis of small organic molecules is shown to be useful to help disambiguate spectroscopic and elemental analyses.

Low-temperature phase transition and high-pressure phase stability of 1H-pyrazole-1-carboxamidine nitrate

The phase transition observed in a temperature-dependent experiment at 174 K is unachievable under high-pressure conditions. Negative thermal expansion for phase (II) and negative compressibility for phase (I) were observed.

K(Na,K)Na₂[Cu₂(SO₄)₄]: a new highly porous anhydrous sulfate and evaluation of possible ion migration pathways

Synthesis, crystal structure and evaluation of a possible ion migration pathways are reported for a novel highly porous Na-K-Cu anhydrous sulfate. The overall coordination polyhedra of Cu²⁺ in the new compound can be considered as `octahedra with one split vertex'.

Kilobytes of kilopascals: high-pressure depositions of the Cambridge Structural Database

An in-depth analysis of the high-pressure entries of the Cambridge Structural Database is presented.

Low-tem­per­ature investigation of natural iron-rich oxoborates vonsenite and hulsite: thermal deformations of crystal structure, strong negative thermal expansion and cascades of magnetic transitions

Vonsenite and hulsite were investigated by a set of in situ low-tem­per­ature methods. Cascades of magnetic transitions were revealed in both oxoborates and the critical tem­per­atures were determined. Anomalies in the unit-cell parameters and thermal expansion of the oxoborates at critical tem­per­atures were observed.

Crystal engineering of co-crystal of nicotinic acid and pyrogallol: an experimental and theoretical electron density analysis

Experimental and theoretical charge density analysis of a cocrystal of nicotinic acid and pyrogallol provides insight into the mechanism of nucleation and growth of the cocrystal.

Oxonium trans-bis­(oxalato)rhodate and related sodium salts: a rare example of crystalline complex acid

The polymorphism of Na[Rh(H₂O)₂(C₂O₄)₂]·2H₂O is noteworthy. The complex crystallizes with the formation of two different structures of the same water content. Oxalato-rhodic acid not only exists in solution, but, unlike most complex acids, it can also crystallize as a tetrahydrate.