The consequences of different treatments of H atoms in experimental charge-density studies are discussed. Geometric and topological parameters obtained after applying four different H-atom models in multipolar refinement on high-resolution X-ray data only were compared with the results obtained for a reference joint high-resolution X-ray/neutron refinement. The geometry and the topological critical point and integrated parameters closest to the reference values were obtained after a mixed refinement (high-order refinement of heavy atoms, low-angle refinement of H atoms and elongation of the X-H distance to the average neutron bond lengths) supplemented by an estimation of the anisotropic thermal motions of H atoms using the SHADE program. Such a procedure works very well even for strong hydrogen bonds. The worst fit to the reference results for both critical point and integrated parameters was obtained when only the standardization to the average neutron X-H distances was applied. The non-H-atom parameters are also systematically influenced by the H-atom modeling. In order to compare topological and integrated properties calculated for H and non-H atoms in multipolar refinement when there are no neutron data, the same treatment of H atoms (ideally the mixed refinement + estimated anisotropic atomic displacement parameters for H atoms) should be applied.

A comprehensive analysis of various properties derived from multiple high-resolution X-ray diffraction experiments is reported. A total of 13 charge-density-quality data sets of α-oxalic acid dihydrate (C2H2O4·2H2O) were subject to Hansen-Coppens-based modelling of electron density. The obtained parameters and properties were then statistically analysed yielding a clear picture of their variability across the different measurements. Additionally, a computational approach (CRYSTAL and PIXEL programs) was utilized to support and examine the experimental findings. The aim of the study was to show the real accuracy and interpretation limits of the charge-density-derived data. An investigation of raw intensities showed that most of the reflections (60-70%) fulfil the normality test and the lowest ratio is observed for weak reflections. It appeared that unit-cell parameters are determined to the order of 10(-3) Å (for cell edges) and 10(-2) ° (for angles), and compare well with the older studies of the same compound and with the new 100 K neutron diffraction data set. Fit discrepancy factors are determined within a 0.5% range, while the residual density extrema are about ±0.16 (3) e Å(-3). The geometry is very well reproducible between different data sets. Regarding the multipole model, the largest errors are present on the valence shell charge-transfer parameters. In addition, symmetry restrictions of multipolar parameters, with respect to local coordinate systems, are well preserved. Standard deviations for electron density are lowest at bond critical points, being especially small for the hydrogen-bonded contacts. The same is true for kinetic and potential energy densities. This is also the case for the electrostatic potential distribution, which is statistically most significant in the hydrogen-bonded regions. Standard deviations for the integrated atomic charges are equal to about 0.1 e. Dipole moments for the water molecule are comparable with the ones presented in various earlier studies. The electrostatic energies should be treated rather qualitatively. However, they are quite well correlated with the PIXEL results.

2-(4-Fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT) is a biologically active compound. It forms planar molecules and cations. Single crystals of the FABTH þ chloride grown from water solutions of different alcohols, such as methanol, propan-2-ol, and butanol, show structural changes induced mostly by hydrogen bond interactions with chloride anions and solvent molecules. For structures with the alcohol molecules built in the crystal lattice, the FABTH þ moiety takes the conformation with the o-OH (ortho position) group from the resorcin ring on the same side of the molecule as the sulfur atom in the thiadiazole ring, whereas, in the alcohol free crystals growth from a butanol-water mixture, this group is situated on the other side of the thiadiazole ring. The incorporation of the alcohol molecules into the crystal structures formed by FABTH þ cations strongly depends on their size, and it influences the properties of crystal lattices. In the case of the FABTH þ Clcrystallized from butanol, the crystal structure consists of columns of FABTH þ cations forming intermolecular channels containing two water molecules and two chloride anions related by centers of symmetry. The crystal structure of FABTH þ Clcrystallized from methanol is built of two separate layers consisting of FABTH þ cations and methanol and chloride anions repeating periodically. FABTH þ Clcrystallized from propan-2-ol forms a 3D structure with separate water and propan-2-ol molecules glued by chloride anions and layers of the FABTH þ cations. The Hirshfeld surface analysis is a very useful tool in identifying subtle differences between the solvates. The DFT computations allow us to estimate the energy difference between the two conformers to be 3.2 kcal/mol and the rotational barrier to be 12.6 kcal/mol.

A major update of the http://shade.ki.ku.dk (http://shade.ki.ku.dk) is presented. In addition to all of the previous options for estimating H‐atom anisotropic displacement parameters (ADPs) that were offered by SHADE2, the newest version offers two new methods. The first method combines the original translation–libration–screw analysis with input from periodic ab initio calculations. The second method allows the user to input experimental information from spectroscopic measurements or from neutron diffraction experiments on related structures and utilize this information to evaluate ADPs of H atoms. Tools are provided to set up the ab initio calculations and to derive the internal motion from the calculations. The new server was tested on a range of compounds where neutron diffraction data were available. In most cases, the results are significantly better than previous estimates, and for strong hydrogen bonds in proton sponges, the ab initio calculations become crucial.

A new crystallization method of N-iodoacetylamphotericin B derivative is reported. The crystallization process in the presence of different quantities of amphotericin B additives was extensively studied and its mechanism was proposed. It also resulted in good quality single crystals suitable for X-ray structure determination (100 K). The structural information obtained allowed for periodic and dimer single point computational studies at the B3LYP/6-31G** level of theory. These confirmed the proposed controlled crystallization mechanism and the stabilization of crystals formed by the iodoacetyl derivative and parent amphotericin B. The calculation results indicate the strength of different intermolecular interactions and reveal the great contribution of the solvent molecules to the crystal lattice formation, with the total energy gain of about 335 kJ•mol −1 , which almost doubles the cohesive energy value. Hirshfeld surface analysis shows the more efficient crystal packing of N-iodoacetylamphotericin versus amphotericin B and the effect of the iodoacetyl group on the intermolecular contacts. The generated electrostatic potential maps reveal the impact of the iodoacetyl substituent on the nitrogen atom basicity and thus confirm the stronger hydrogen bonding created via nitrogen atom in the case of N-iodoacetyl amphotericin B, and higher drug activity of amphotericin B related to the ability of the zwitterion formation.

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This study demonstrates and tests the refinement of a lattice-dynamical model derived from periodic ab initio calculations at the Γ point against elastic diffraction data (X-ray or neutron). Refinement of only a handful of parameters is sufficient to obtain a similar agreement with the data as the conventional crystallographic model using anisotropic displacement parameters. By refinement against X-ray data, H displacement parameters are obtained which compare favourably with those from neutron diffraction experiments. The approach opens the door for evaluating thermodynamic properties, and for refinement against multi-temperature data, against inelastic diffraction data, spectroscopic information and thermal diffuse scattering data.

Hydrogen atom positions can be obtained accurately from X-ray diffraction data of hydrogen maleate salts via Hirshfeld atom refinement.