Quinoxaline: Z′ = 1 form

A new Z′ = 1 crystal structure of quinoxaline (or 1,4-diaza­naphthalene), C8H6N2, with one-fifth the volume of the earlier known Z′ = 5 structure was obtained by means of an in situ cryocrystallization technique

1,2,3-Trifluoro­benzene

In the title compound, C6H3F3, weak electrostatic and dispersive forces between C(δ+)—F(δ−) and H(δ+)—C(δ−) groups are at the borderline of the hydrogen-bond phenomenon and are poorly directional and further deformed in the presence of π–π stacking inter­actions. The mol­ecule lies on a twofold rotation axis. In the crystal structure, one-dimensional tapes are formed via two anti­dromic C—H⋯F hydrogen bonds. These tapes are, in turn, connected into corrugated two-dimensional sheets by bifurcated C—H⋯F hydrogen bonds. Packing in the third dimension is furnished by π–π stacking inter­actions with a centroid–centroid distance of 3.6362 (14) Å

1,3-Difluoro­benzene

The weak electrostatic and dispersive forces between C(δ+)—F(δ−) and H(δ+)—C(δ−) are at the borderline of the hydrogen-bond phenomenon and are poorly directional and further deformed in the presence of other dominant inter­actions, e.g. C—H⋯π. The title compound, C6H4F2, Z′ = 2, forms one-dimensional tapes along two homodromic C—H⋯F hydrogen bonds. The one-dimensional tapes are connected into corrugated two-dimensional sheets by further bi- or trifrucated C—H⋯F hydrogen bonds. Packing in the third dimension is controlled by C—H⋯π inter­actions

Amorphous 1-propanol interstellar ice beyond its melting point

The recent discovery of 1-propanol (CH3CH2CH2OH) in the interstellar medium (ISM) is of tremendous interest since fatty alcohols have been proposed as constituents of proto-cell membranes. Motivated by this discovery, we present the laboratory midinfrared (MIR) and vacuum ultra-violet (VUV) absorption spectra of 1-propanol ice under astrochemical conditions, mimicking an icy mantle on cold dust in the ISM. Both MIR and VUV spectra were recorded at ultra-high vacuum (UHV) of ∼ 10-9 mbar and at temperatures ranging from 10 K to sublimation. The morphology of the 1-propanol ice deposited at 10 K was amorphous. By warming the ice to temperatures of 140 K and above, with subsequent recording of IR spectra, we observe complete sublimation of 1-propanol molecules from the substrate around 170 K. No amorphous-to-crystalline phase change was observed upon warming to higher temperatures. Additionally, We observe the IR and VUV signatures of 1-propanol ice on the substrate well beyond its melting point (147 K). To the best of our knowledge, this is the first reported observation of a molecular ice staying well beyond its melting point under such conditions. This result shows that the morphology of icy mantles on ISM cold dust grains is more complex than previously thought. Our atomistic molecular dynamics (MD) simulations capture the experimental trends and shed light on the microscopic origin of this unusual phase behaviour of 1-propanol

Towards crystal structure prediction of complex organic compounds - a report on the fifth blind test

Following on from the success of the previous crystal structure prediction blind tests (CSP1999, CSP2001, CSP2004 and CSP2007), a fifth such collaborative project (CSP2010) was organized at the Cambridge Crystallographic Data Centre. A range of methodologies was used by the participating groups in order to evaluate the ability of the current computational methods to predict the crystal structures of the six organic molecules chosen as targets for this blind test. The first four targets, two rigid molecules, one semi-flexible molecule and a 1: 1 salt, matched the criteria for the targets from CSP2007, while the last two targets belonged to two new challenging categories - a larger, much more flexible molecule and a hydrate with more than one polymorph. Each group submitted three predictions for each target it attempted. There was at least one successful prediction for each target, and two groups were able to successfully predict the structure of the large flexible molecule as their first place submission. The results show that while not as many groups successfully predicted the structures of the three smallest molecules as in CSP2007, there is now evidence that methodologies such as dispersion-corrected density functional theory (DFT-D) are able to reliably do so. The results also highlight the many challenges posed by more complex systems and show that there are still issues to be overcome

Misassigned C-H...Cu agostic interaction in a copper(II) ephedrine derivative is actually a weak, multicentred hydrogen bond

Agostic interactions and weak hydrogen bonds to metal acceptors, M, are chemically and geometrically distinct, and this distinction must be maintained in the classification of new C-H...M interactions

Crystal structure prediction of a co-crystal using a supramolecular synthon approach: 2-methylbenzoic acid-2-amino-4-methylpyrimidine

The crystal structure prediction (CSP) of the 1:1 binary crystal of 2-methylbenzoic acid and 2-amino-4-methylpyrimidine was performed in the recently held fourth blind test (organized by the Cambridge Crystallographic Data Centre) using a supramolecular synthon based approach. Further work was done on this system after the blind test was concluded. A combined quantum mechanical and Cambridge Structural Database (CSD) study of the individual components was carried out to assess the viability of possible synthons both energetically and statistically. CSP of a few selected co-crystals from the CSD and from our own in-house experimental database was carried out in order to standardize the methodology for the prediction of the crystal structure of the target compound. The CSP of the co-crystal was accomplished successfully showing the applicability of a synthon based approach to CSP for this category of substance, especially with a combination of computational and experimental techniques

Studying the Role of CO···CO, CO···N–O, and N–O···N–O Dipole–Dipole Interactions in the Crystal Packing of 4‑Nitrobenzoic Acid and 3,3′-Dinitrobenzophenone Polymorphs: An Experimental Charge Density Study

An experimental charge density study of 4-nitrobenzoic acid and 3,3′-dinitrobenzophenone polymorphs has been performed to study the role played by various intermolecular interactions in the crystal packing. The study highlights the importance of dipole–dipole interactions analyzed with the help of Bader’s quantum theory of atoms in molecules (QTAIM), low-reduced density gradient (RDG) based topological descriptors of electron density and computations. The molecular pairs held by antiparallel CO···CO, CO···N–O, and N–O···N–O type dipole–dipole interactions were found to contribute significantly toward stability especially in the case of 3,3′-dinitrobenzophenone, Form <b>II</b>. The low-RDG based descriptors were used for understanding the nature of these dipole–dipole interactions, which indicates the involvement of multiple atoms in stabilizing many of these interactions. The significant contributions from the O···C, O···O, and O···N contacts arising from these dipole–dipole interactions in crystal packing are also revealed from the Hirshfeld surface analysis of these crystals

Crystal Structure and Prediction

The notion of structure is central to the subject of chemistry. This review traces the development of the idea of crystal structure since the time when a crystal structure could be determined from a three-dimensional diffraction pattern and assesses the feasibility of computationally predicting an unknown crystal structure of a given molecule. Crystal structure prediction is of considerable fundamental and applied importance, and its successful execution is by no means a solved problem. The ease of crystal structure determination today has resulted in the availability of large numbers of crystal structures of higher-energy polymorphs and pseudopolymorphs. These structural libraries lead to the concept of a crystal structure landscape. A crystal structure of a compound may accordingly be taken as a data point in such a landscape