Electron density: historical remarks with a focus on the Berlin school

The history of electron density (ED) research is surveyed and its historical highlights in the 20th century are summarized. The progress, reached at the beginnig of the 21st century, is described including contributions from the Berlin school. Key experimental advances concern X-ray sources and synchrotron radiation, measuring reflection data at very low temperatures (down to ~10 K), as well as the introduction of area detectors. Methodological progress was reached by new software systems for the refinement and analysis of ED data sets. This allowed to obtain quantitative ED properties, making use of Bader’s Quantum Theory of Atoms in Molecules (QTAIM). More recently methods, which combine experimental X-ray data and quantum-chemically derived wave functions, established the rapidly developing new field of “Quantum Crystallography.”

The truth is out there: the metal-π interactions in crystal of Cr(CO)3(pcp) as revealed by the study of vibrational smearing of electron density

The vibrational smearing of electron density was studied in the crystal of complex of Cr(CO)3 with [2.2]paracyclophane. The combination of theoretical and experimental methods, including periodic calculations and screening of DFT calculated and multipole-decomposed electron densities, was utilized to reveal the vibrational smearing of electron density and its influence on the multipole-constructed electron density. The multipole model, commonly used to treat the high-resolution X-ray diffraction data, was shown to be rather inaccurate in description of electron density and its vibrational smearing in metal-π complex where the interchange between diatomic interactions can occur. Namely, some bond critical points can be hidden while analyzing multipole-decomposed electron density with proved effects of vibrational smearing even if the deconvolution problem is overcome by using the invariom approach. On the contrary, the recently proposed “clouds of critical point variation” (CCPV) approach is demonstrated as the route to gather all reasonable bonding trends and to reconstruct static electron density pattern in metal-π complexes.

Transferable Aspherical Atom Modeling of Electron Density in Highly Symmetric Crystals: A Case Study of Alkali-Metal Nitrates

A comparative electron density study (from X-ray diffraction and periodic quantum chemistry) of sodium and potassium nitrates is performed to test the performance of a transferrable aspherical atom model, which is based on the invarioms, to describe chemical bonding features of ions occurring in sites of different symmetry typical of inorganic salts and in different crystal environments. Relying on tabulated entries for the isolated ions (although tailor-made to account for different site symmetries), it takes the same time to employ as the spherical atom model routinely used in X-ray diffraction studies but provides an electron density distribution that faithfully reveals all the interionic interactions-even the weakest ones (such as between the nitrate anions or a K···N interaction found in the metastable form of KNO₃) yet important for properties of inorganic materials-as if obtained from high-resolution X-ray diffraction data.

Invariom approach to electron density studies of open-shell compounds: the case of an organic nitroxide radical

The study reports the successful modeling of electron density with invarioms for an open-shell compound, an organic nitroxide radical.

Probing Ionic Crystals by the Invariom Approach: An Electron Density Study of Guanidinium Chloride and Carbonate

A comparative study of two guanidinium salts, chloride and carbonate, is carried out to test the performance of the invariom approach for ionic crystals. Although treating them as formed by isolated ions with no charge transfer between them, the invariom approach provides features of interionic contacts that are amazingly similar to those obtained from conventional charge density analysis of high-resolution X-ray diffraction data, thus emerging as an easy way towards reliable description of chemical bonding peculiarities in ionic crystals.

Invariom approach as a new tool in electron density studies of ionic liquids: a model case of 1-butyl-2,3-dimethylimidazolium chloride BDMIM[Cl]

In this comparative study of an ionic liquid (IL), BDMIM[Cl], the invariom approximation emerges as a useful tool in search for ‘structure–property’ relations in ILs, as entangling complex interplay of interionic interactions that operate in them.

Invariom based electron density studies on the C/Si analogues haloperidol/sila-haloperidol and venlafaxine/sila-venlafaxine

Hirshfeld surfaces of haloperidol hydropicrate (left) and sila-haloperidol hydrochloride (right) show comparable sites of ED concentrations due to comparable intermolecular environments.

Zinc mediated allylations of chlorosilanes promoted by ultrasound: Synthesis of novel constrained sila amino acids

A simple, fast and efficient method for allylation and propargylation of chlorosilanes through zinc mediation and ultrasound promotion is reported. As a direct application of the resulting bis-allylsilanes, three novel, constrained sila amino acids are prepared for the first time.