The energy of interaction between molecules is commonly expressed in terms of four key components: electrostatic, polarization, dispersion, and exchange-repulsion. Using monomer wave functions to obtain accurate estimates of electrostatic, polarization, and repulsion energies along with Grimme's dispersion corrections, a series of energy models are derived by fitting to dispersion-corrected DFT energies for a large number of molecular pairs extracted from organic and inorganic molecular crystals. The best performing model reproduces B3LYP-D2/6-31G(d,p) counterpoise-corrected energies with a mean absolute deviation (MAD) of just over 1 kJ mol(-1) but in considerably less computation time. It also performs surprisingly well against benchmark CCSD(T)/CBS energies, with a MAD of 2.5 kJ mol(-1) for a combined data set including Hobza's X40, S22, A24, and S66 dimers. Two of these energy models, the most accurate and the fastest, are expected to find widespread application in investigations of molecular crystals.

The new automated iterative Hirshfeld atom refinement method is explained and validated through comparison of structural models of Gly–l-Ala obtained from synchrotron X-ray and neutron diffraction data at 12, 50, 150 and 295 K. Structural parameters involving hydrogen atoms are determined with comparable precision from both experiments and agree mostly to within two combined standard uncertainties.

The relationship between the structure and the properties of a drug or material is a key concept of chemistry. Knowledge of the three-dimensional structure is considered to be of such...

Hydrogen atoms cannot hide from x-rays anymore but can instead be detected very reliably in routine measurements.

Quantum crystallography combines quantum chemistry and experimental diffraction or scattering to provide both enhanced wavefunctions and charge densities.

Chemically binding to argon (Ar) at room temperature has remained the privilege of the most reactive electrophiles, all of which are cationic (or even dicationic) in nature. Herein, we report a concept for the rational design of anionic superelectrophiles that are composed of a strong electrophilic center firmly embedded in a negatively charged framework of exceptional stability. To validate our concept, we synthesized the percyano-dodecoborate [B12(CN)12]2−, the electronically most stable dianion ever investigated experimentally. It serves as a precursor for the generation of the monoanion [B12(CN)11]−, which indeed spontaneously binds Ar at 298 K. Our mass spectrometric and spectroscopic studies are accompanied by high-level computational investigations including a bonding analysis of the exceptional B-Ar bond. The detection and characterization of this highly reactive, structurally stable anionic superelectrophile starts another chapter in the metal-free activation of particularly inert compounds and elements.

Crystallography and quantum mechanics have always been tightly connected because reliable quantum mechanical models are needed to determine crystal structures. Due to this natural synergy, nowadays accurate distributions of electrons in space can be obtained from diffraction and scattering experiments. In the original definition of quantum crystallography (QCr) given by Massa, Karle and Huang, direct extraction of wavefunctions or density matrices from measured intensities of reflections or, conversely, ad hoc quantum mechanical calculations to enhance the accuracy of the crystallographic refinement are implicated. Nevertheless, many other active and emerging research areas involving quantum mechanics and scattering experiments are not covered by the original definition although they enable to observe and explain quantum phenomena as accurately and successfully as the original strategies. Therefore, we give an overview over current research that is related to a broader notion of QCr, and discuss options how QCr can evolve to become a complete and independent domain of natural sciences. The goal of this paper is to initiate discussions around QCr, but not to find a final definition of the field.

Lewis diagrams and the octet rule [1] are central concepts in chemistry. Hypervalent molecules break the octet rule because they contain atoms with more than four electron pairs in their valence shell. [2] To describe them with the Lewis model requires hybridization schemes involving d orbitals (sp 3 d or sp 3 d 2 hybrids). [3, 4] The problem is that the formation of these hybrid orbitals requires large promotion energies. [5] Therefore, the significance of ionic resonance diagrams, which obviate the need for hypervalency, has long been discussed. [4] In this context, the electronic structure of SO 2 has been controversial. SO 2 can be described as a hypervalent molecule (Figure 1, left). Apart from d-orbital hybridization, multiple covalent bonding in SO 2 may be explained by three-center p pp p interactions of the sulfur 3p p orbital with non-bonding oxygen p p electrons (this interpretation goes back to Ref. [6]).However, other non-hypervalent ionic resonance structures can be formulated that preserve the octet rule ( Figure 1).The neutral Lewis structure is thought to be dominant, as the SÀO bond length in SO 2 is shorter than that in sulfur monoxide, SO (1.4299(3) in SO 2 from this study, compared to 1.481 for SO [7] ). The bond dissociation energy is also higher in SO 2 than in SO (547.3(8) kJ mol À1 vs. 517.1(8) kJ mol À1 [8] ). Furthermore, O 3 and O 2 , which are valence-isoelectronic with SO 2 and SO, have no available d orbitals, so they cannot be hypervalent, implying bond orders no higher than 1.5 and 2.0, respectively. This is consistent with the fact that the O À O distance in O 3 (1.2717(2) [9] ) is longer than in O 2 (1.15(8) , [10] 1.207 [7] ), in direct contrast with SO 2 relative to SO. This could support the notion that multiple covalent bonding is significant in SO 2 . Indeed, a large number of textbooks [11,12] adhere to this conclusion; for example, in ref. [12] it is stated that the S À O bond order is "at least 2".The simple empirical analysis above is in sharp contrast to the fact that computational studies have found significant ionic contributions to the SÀO bond, and very little sulfur dorbital participation. [13] Today, there is agreement among theoreticians that the role of d orbitals in the formation of bonds involving second and higher row elements is predominantly one of polarization functions, not of hybridization involving d orbitals. [5,14] In fact, the shorter and stronger bonds in SO 2 compared to SO (which is formally a double bond) support the conclusion that there are significant noncovalent contributions to the bonding. Indeed, calculations employing the electron localization function have shown that the polarity of a bond only depends on the electronegativity differences of the bonded atoms, so that molecules formerly classified as being hypervalent can be readily described with various ionic resonance structures. [15] So from a computational viewpoint, hypervalency is avoided by introducing ionic bonds.Experimentally, it has hitherto been difficult to obtain i...