The Relevance of Experimental Charge Density Analysis in Unraveling Noncovalent Interactions in Molecular Crystals

The work carried out by our research group over the last couple of decades in the context of quantitative crystal engineering involves the analysis of intermolecular interactions such as carbon (tetrel) bonding, pnicogen bonding, chalcogen bonding, and halogen bonding using experimental charge density methodology is reviewed. The focus is to extract electron density distribution in the intermolecular space and to obtain guidelines to evaluate the strength and directionality of such interactions towards the design of molecular crystals with desired properties. Following the early studies on halogen bonding interactions, several "sigma-hole" interaction types with similar electrostatic origins have been explored in recent times for their strength, origin, and structural consequences. These include interactions such as carbon (tetrel) bonding, pnicogen bonding, chalcogen bonding, and halogen bonding. Experimental X-ray charge density analysis has proved to be a powerful tool in unraveling the strength and electronic origin of such interactions, providing insights beyond the theoretical estimates from gas-phase molecular dimer calculations. In this mini-review, we outline some selected contributions from the X-ray charge density studies to the field of non-covalent interactions (NCIs) involving elements of the groups 14-17 of the periodic table. Quantitative insights into the nature of these interactions obtained from the experimental electron density distribution and subsequent topological analysis by the quantum theory of atoms in molecules (QTAIM) have been discussed. A few notable examples of weak interactions have been presented in terms of their experimental charge density features. These examples reveal not only the strength and beauty of X-ray charge density multipole modeling as an advanced structural chemistry tool but also its utility in providing experimental benchmarks for the theoretical studies of weak interactions in crystals.

lamaGOET: an interface for quantum crystallography

In quantum crystallography, theoretical calculations and crystallographic refinements are closely intertwined. This means that the employed software must be able to perform both quantum-mechanical calculations and crystallographic least-squares refinements. So far, the program Tonto is the only one able to do that. The lamaGOET interface described herein deals with this issue since it interfaces dedicated quantum-chemical software (the widely used Gaussian package and the specialized ELMOdb program) with the refinement capabilities of Tonto. Three different flavours of quantum-crystallographic refinements of the dipetide glycyl-l-threonine dihydrate are presented to showcase the capabilities of lamaGOET: Hirshfeld atom refinement (HAR), HAR-ELMO, namely HAR coupled with extremely localized molecular orbitals, and X-ray constrained wavefunction fitting.

Contributions of charge-density research to medicinal chemistry

This article reviews efforts in accurate experimental charge-density studies with relevance to medicinal chemistry. Initially, classical charge-density studies that measure electron density distribution via least-squares refinement of aspherical-atom population parameters are summarized. Next, interaction density is discussed as an idealized situation resembling drug-receptor interactions. Scattering-factor databases play an increasing role in charge-density research, and they can be applied both to small-molecule and macromolecular structures in refinement and analysis; software development facilitates their use. Therefore combining both of these complementary branches of X-ray crystallography is recommended, and examples are given where such a combination already proved useful. On the side of the experiment, new pixel detectors are allowing rapid measurements, thereby enabling both high-throughput small-molecule studies and macromolecular structure determination to higher resolutions. Currently, the most ambitious studies compute intermolecular interaction energies of drug-receptor complexes, and it is recommended that future studies benefit from recent method developments. Selected new developments in theoretical charge-density studies are discussed with emphasis on its symbiotic relation to crystallography.

Verification of structural and electrostatic properties obtained by the use of different pseudoatom databases

The existing pseudoatom databases (ELMAM, Invariom, UBDB and ELMAM2) enable structure refinement to be performed with the use of aspherical scattering factors computed from the transferable aspherical atom model (TAAM) as an alternative to independent atom model refinement. In addition, electrostatic properties can be estimated with the help of the databases. The quality of the structural and electrostatic properties obtained from the individual databases was tested. On the basis of a 100 K high-resolution single-crystal X-ray diffraction experiment on L-His-L-Ala dihydrate and 23 K high-resolution data for L-Ala [Destro & Marsh (1988). J. Phys. Chem. 92, 966-973], the structural properties, electron-density distributions and molecular electrostatic potentials obtained from different TAAMs were compared to each other and to reference models. Experimental multipolar models and theoretical models refined against theoretical structure factors computed from periodic density functional theory (DFT) calculations were compared to the TAAMs in order to determine which model best describes the crystal-field effect. Unperturbed wavefunctions based on the MP2 and DFT calculations and properties obtained directly from these were used as a reference to judge how properly the databases reproduce the properties of isolated molecules. For Gly-L-His dihydrate, D,L-His and the above-mentioned two crystal structures, deviations of the molecular dipole moments and Coulombic intermolecular interaction energies from the reference values were examined. Root-mean-square deviations (RMSDs) and correlation coefficients were used as a quantitative measure of the quality of the analysed properties. TAAM refinements reproduce X-H bond lengths optimized in theoretical periodic calculations. Structural properties obtained from different database models are similar to each other. The anisotropic displacement parameters from TAAMs are similar to the results of experimental multipolar refinement; differences are about 0.5 and 2.5% for high-resolution and low-resolution data, respectively. Differences in dipole-moment magnitudes calculated from database models are about 5%, and directions differ by up to 30°. The values of electrostatic interaction energies estimated from the individual TAAMs differ greatly from each other and from the reference values. RMSDs are about 9-15 and 22-33 kJ mol(-1) for UBDB and the other database models, respectively.