Halogen bonds, the formally noncovalent interactions where the halogen acts as a Lewis acid, have brought several controversies to the theoretical world regarding its nature and components, e.g., charge transfer (CT), electrostatics, dispersion, and polarization. The debate on whether all characteristics are accounted for by electrostatics is examined, highlighting the importance of the CT and repulsive interactions. A number of strongly halogen‐bonded complexes are as covalent as metal–ligand coordination bonds. Different levels of computational methods are reviewed with the objective of finding the best accuracy/cost ratios. The unusual electronic anisotropy of the halogen donor and its interaction with a Lewis base demand specific calculation schemes. From the wave‐function theory methods, only the ones with empirical corrections (spin‐component‐scaled MP2 or CCSD, and MP2.5) are suitable when CCSD(T) is unattainable. Density functional theory functionals with a high amount of exact exchange are fast and reliable methods for halogen bonds, but double hybrids are more robust if other types of interactions are involved. Molecular mechanics methods can be useful, but only when specific corrections are added to compensate for the inability of such methods to describe CT. The most common method introduces a virtual site with a partial positive charge to account for the quantum chemical effect of the halogen bond. This methodology has been successfully applied to study protein–ligand interactions for drug design. WIREs Comput Mol Sci 2014, 4:523–540. doi: 10.1002/wcms.1189 This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Computational Biochemistry and Biophysics Theoretical and Physical Chemistry > Thermochemistry

Highly reflective crystals of the nucleotide base guanine are widely distributed in animal coloration and visual systems. Organisms precisely control the morphology and organization of the crystals to optimize different optical effects, but little is known about how this is achieved. Here we examine a fundamental question that has remained unanswered after over 100 years of research on guanine: what are the crystals made of ? Using solution-state and solid-state chemical techniques coupled with structural analysis by powder XRD and solid-state NMR, we compare the purine compositions and the structures of seven biogenic guanine crystals with different crystal morphologies, testing the hypothesis that intracrystalline dopants influence the crystal shape. We find that biogenic “guanine” crystals are not pure crystals but molecular alloys (aka solid solutions and mixed crystals) of guanine, hypoxanthine, and sometimes xanthine. Guanine host crystals occlude homogeneous mixtures of other purines, sometimes in remarkably large amounts (up to 20% of hypoxanthine), without significantly altering the crystal structure of the guanine host. We find no correlation between the biogenic crystal morphology and dopant content and conclude that dopants do not dictate the crystal morphology of the guanine host. The ability of guanine crystals to host other molecules enables animals to build physiologically “cheaper” crystals from mixtures of metabolically available purines, without impeding optical functionality. The exceptional levels of doping in biogenic guanine offer inspiration for the design of mixed molecular crystals that incorporate multiple functionalities in a single material.

No abstract

External electric fields readily align birefringent block-copolymer mesophases. In this study the effect of gold nanoparticles on the electric-field-induced alignment of a lamellae-forming polystyrene-block-poly(2-vinylpyridine) copolymer is assessed. Nanoparticles are homogeneously dispersed in the styrenic phase and promote the quantitative alignment of lamellar domains by substantially lowering the critical field strength above which alignment proceeds. The results suggest that the electric-field-assisted alignment of nanostructured block copolymer/nanoparticle composites may offer a simple way to greatly mitigate structural and orientational defects of such films under benign experimental conditions.

In conventional studies of heterogeneous photocatalytic oxidation, it is crucial to evaluate by means of control experiments the relevance of the “non‐catalyzed” autoxidation reaction under irradiation. If the autoxidation is found to be negligible, it is usually considered that it can be safely disregarded. However, in the case of aromatic aldehydes’ synthesis such as benzaldehyde, irradiating with UV light may lead to a more complex mechanism than previously thought. Herein, we prove that neglecting the autoxidative reaction can lead to a misinterpretation of the mechanism as well as an overestimation of the catalyst's efficiency, even if the control experiment exhibits zero yield. As an example, we studied the synthesis of benzaldehyde from benzyl alcohol by UV‐A irradiation with and without TiO2 and CdS nanoparticles; without catalyst, after a long induction time, the aldehyde enhances its own production through an autocatalytic reaction activated by the same light wavelengths utilized for the nanocatalysts. The long induction time leads to considering the autocatalysis to be deceptively negligible. However, the nanoparticles act as a fast initiator of the benzaldehyde autocatalysis, in such a way that the oxidation rate reached a similar performance with nanoparticles and after removing them. This suggests that the commonly ignored autocatalysis of benzaldehyde indeed is a relevant parallel pathway to the heterogeneous catalysis mechanism.