The crystal structures of three polymorphs found for the addition complex of urea and barbituric acid are described and compared. Two polymorphs are monoclinic, space groups P2(1)/c and Cc, whereas the third is triclinic, P1. The displacement of electron density towards the mesomeric forms, corresponding to the tautomeric forms of higher stability, of the barbituric acid molecule seem to influence the type of hydrogen bonds formed, which in turn determines the different packing topology in the polymorphs. While the polymorphic forms can be easily differentiated at the first-level graph-set analysis of their hydrogen-bonding patterns, a higher-level analysis enables important features of the mutual spatial arrangement of the structural components to be revealed.

Analysis of the spectral properties and structural differences of two turn-on ratiometric fluorescent receptors for Zn(2+) and Cd(2+) ions, derivatives of pyrrolo[2,3-b]quinoxaline (2), and earlier published 3 (Ostrowska et al. CrystEngComm 2015, 17, 498-502) was performed. Both ligands are E/Z push-pull olefins interconverting at room temperature, with barriers to rotation about enamine double bonds, from E to Z isomers of 19.3 ± 0.1 and 16.9 ± 0.3 kcal/mol and from Z to E of 16.9 ± 0.3 and 15.7 ± 0.2 kcal/mol, respectively. Diastereoisomers (E)-2 and (Z)-2 were isolated and characterized by X-ray structural analysis. The formation of complexes by (E/Z)-2 with acetates and acetylacetonates of Zn(2+) and Cd(2+) was monitored by UV-vis, fluorescence, and (1)H NMR titrations in acetonitrile, respectively. X-ray structural analysis for isolated [(E)-2]2Zn in relation to earlier published (E)-3-ZnOAc revealed the formation of a six-coordinated zinc ion with six- and four-membered bis-chelate rings by (E)-2. The chelate effect increases the ligand affinity for Zn(2+) (log β12 = 12.45) and causes the elongation of nitrogen-metal bonds. Extension of the coordination cavity size allows coordination of a cadmium ion. The introduction of a flexible ethylene linker between the fluorophore and ionophore pyridyl groups in 3 significantly affects the selectivity of zinc-ion recognition. The distorted tetrahedral geometry of (E)-3-ZnOAc with a four-coordinated zinc ion appears to be the most preferred because of the short donor-zinc distance with a 1:1 binding mode. The formation of the small coordination cavity size with six-membered bis-chelate rings provides an effective overlap of zinc and donor orbitals, precluding the coordination of a cadmium ion in the same manner as zinc.

New nontoxic and biocompatible ferroelectric materials are a subject undergoing intense study. One of the most promising research branches is focused on H-bonded organic or hybrid ferroelectrics. The engineering of these materials is based on mimicking the phase transition mechanisms of the well-known inorganic ferroelectrics. In our study, a coupled experimental and theoretical methodology was used for a precise investigation of the ferroelectric phase transition mechanism in ammonium sulfate (AS). A series of single-crystal X-ray diffraction measurements were performed in the temperature range between 273 and 163 K. The detailed inspection of the obtained static structural data, in the above-mentioned temperature range, allowed us to reveal dynamical effects at the ferroelectric phase transition. Accurate analysis of all geometrical features within the obtained crystal structures was carried out. The results were discussed in the view of previously discovered physical properties. X-ray studies were complemented by the use of quantum theory of atoms in molecules calculations and Hirshfeld surface analysis. Valence shell charge concentration analysis allowed us to find the subtle changes between charge density distribution within SO in para- and ferroelectric phases. H-bond interactions, geometrically classified in both AS phases, were all confirmed by the appropriate critical points. The interaction energies were estimated for the structures at 273, 233, 213, 183, and 163 K. Correlation between the geometrical approach and the results of theoretical calculations enabled us to discover the differences in interaction equilibrium between the AS phases. The mechanism of the phase transition originates from the disruption of the vibrational lattice mode between sulfate anions. Our studies resolved the problem, which was under discussion for more than 60 years.

High-resolution single-crystal X-ray diffraction measurements at 100 K were performed for the two polymorphs of urea-barbituric acid co-crystals: (I) P2(1)/c and (II) Cc. Experimental and theoretical charge density and its properties were analysed for (I) and (II) in order to confirm the previous observation that in the polymorphs studied the barbituric acid molecules adopt different mesomeric forms, leading to different hydrogen-bond systems. Koch and Popelier criteria were applied to distinguish between hydrogen bonds and van der Waals interactions in the structures presented.

The accurate electron density and the linear optical properties of L-histidinium hydrogen oxalate are discussed in this paper. Two high resolution single crystal X-ray diffraction experiments were performed and compared with density functional calculations in the solid state as well as in the gas phase. The crystal packing and the hydrogen bond network are accurately investigated using topological analysis based on Quantum Theory of Atoms in Molecules, Hirshfeld surface analysis and electrostatic potential mapping. This compound also offers the possibility to test the electron density building block approach for material science and different refinement schemes for accurate positions and displacement parameters of hydrogen atoms, in the absence of neutron diffraction data.

The melamine barbital co-crystal is a product of crystal engineering of non-linear optical materials composed of pharmaceutically active ingredients. The resulting crystal phase shows a non-linear effect higher than that of KDP. The material was characterized by means of X-ray diffraction and optical property measurements and calculations.

 max /  min / rms / eÅ 3 0.517/-0.286/0.061 Multipolar Refinement (XDLSM) on F 2 and finally on F No. of data in refinement [I>3σ(I)] 3751 No. of refined parameters 318 R[I>3σ(I)] / Rw(I) / S 0.028/0.023/0.955 Max shift / esd. in last cycle <10 -3 Weighting scheme 1/[s^2^(Fo)]  max /  min / rms / eÅ 3 0.

Technological innovation enforces a revolutionized approach towards materials chemistry. In this paper a new methodology towards crystal engineering of polar materials for possible applications in linear or non-linear optics (NLO), as well as ferroelectric, pyroelectric or piezoelectric crystals is presented. The necessity to fulfil several criteria concerning symmetry, electron properties of the building blocks, and also mechanical and optical stability was achieved by fusion of a pharmaceutical molecule and an NLO-phore. Co-crystals of 2-amino-5-nitropyridine barbital, presented in this manuscript, show cutting-edge optical performance. Large second harmonic generation (SHG) efficiency (40 times better than potassium dihydrogen phosphate, KDP), extreme birefringence (2.7 times higher than for calcite), simplicity in preparation, and optical and mechanical stability of the product proves that in fact a new generation of smart materials was obtained.