Tejender S. Thakur

Intermolecular atom–atom bonds in crystals – a chemical perspective

Short atom–atom distances between molecules are almost always indicative of specific intermolecular bonding. These distances may be used to assess the significance of all hydrogen bonds, including the C–H⋯O and even weaker C–H⋯F varieties.

Extending the Supramolecular Synthon Based Fragment Approach (SBFA) for Transferability of Multipole Charge Density Parameters to Monofluorobenzoic Acids and their Cocrystals with Isonicotinamide: Importance of C–H···O, C–H···F, and F···F Intermolecular Regions

An extension of the supramolecular synthon-based fragment approach (SBFA) method for transferability of multipole charge density parameters to include weak supramolecular synthons is proposed. In particular, the SBFA method is applied to C-H···O, C-H···F, and F···F containing synthons. A high resolution charge density study has been performed on 4-fluorobenzoic acid to build a synthon library for C-H···F infinite chain interactions. Libraries for C-H···O and F···F synthons were taken from earlier work. The SBFA methodology was applied successfully to 2- and 3-fluorobenzoic acids, data sets for which were collected in a routine manner at 100 K, and the modularity of the synthons was demonstrated. Cocrystals of isonicotinamide with all three fluorobenzoic acids were also studied with the SBFA method. The topological analysis of inter- and intramolecular interaction regions was performed using Baders AIM approach. This study shows that the SBFA method is generally applicable to generate charge density maps using information from multiple intermolecular regions.

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.

Synthesis of Octabromoperylene Dianhydride and Diimides: Evidence of Halogen Bonding and Semiconducting Properties

A facile synthesis of octabromoperylene-3,4,9,10-tetracarboxylic dianhydride (Br8-PDA) (1), its diimides (Br8-PDIs) (2a-e), and bis-, tris-, and tetra-amino substituted diimides (5a-c) with six, five, and four remaining substitutable Br atoms, respectively, is reported. Octabromination results in facile chemical/electrochemical reduction, radical anion formation, and red-shifted optical properties. For the first time, diverse halogen-bonding interactions were identified in the PDA/PDI, which along with the attractive electronic features enhance the electron-transport characteristics compared to the di-/tetra-brominated PDIs (3/4).

Crystalline Supramolecular Nanofibers Based on Dehydrobenzoannulene Derivatives

Supramolecular nanofibers (SNFs) composed of low-molecular-weight π-conjugated molecules exhibit attractive optical and electrical properties and are expected to be the next optoelectronic materials. In this work, five crystalline SNFs have been constructed from three dehydrobenzoannulene (DBA) derivatives. The DBAs were designed to assemble in one dimension in a strategy based on anisotropic crystal growth. The crystallinity of the SNFs allowed the molecular arrangements in the SNFs to be determined. Therefore the mechanism of construction and correlations between the molecular arrangements and optical and electrical properties could be considered. The results clearly indicate that the properties of the SNFs are affected by the chemical structures and molecular arrangements. Moreover, one of the SNFs exhibits a high charge-carrier mobility (Σμ=0.61 cm(2) V(-1) s(-1)) because of its crystallinity and appropriate molecular arrangement. This systematic experimental study based on a proposed strategy has provided information for improving the electrical properties of SNFs. This strategy will lead to highly functional SNFs.

New crystalline salt forms of levofloxacin: conformational analysis and attempts towards the crystal structure prediction of the anhydrous form

Structural information on the anhydrous forms of levofloxacin, a broad spectrum fluoroquinolone antibiotic drug, is limited as it readily converts into the more stable hemihydrate form. Obtaining single crystals of the anhydrous form will be a challenge in this situation. Herein, we report six novel carboxylic acid salt forms of levofloxacin and our attempts towards the crystal structure prediction of its anhydrous form. In-depth analysis of various possible conformations of levofloxacin was performed with the help of high level density functional theory calculations. Useful structural insights obtained from crystal structure analysis of two known hydrates and six new salt forms of levofloxacin were utilized to identify the most probable supramolecular synthons. Synthon information obtained from these crystalline forms was used in the re-ranking of computationally generated crystal structures to arrive at a set of most probable crystal structures for the anhydrous form. A comparison of the crystal structures of the two hydrate forms with the predicted anhydrous form of levofloxacin was performed to understand the hydration–dehydration behaviour.

Structural Transformation between Supramolecular Nanofibers with Drastic Change of Conductivity by Heat and Ultrasound

Transformation with every fiber of its being: Dehydrobenzoannulene derivatives with a boomerang shape, dipole moment, and substituents that make diverse interactions enable the construction of stimuli-responsive nanofibers despite the lack of stimuli-responsive groups in these compounds. Interestingly, the supramolecular nanofibers obtained after ultrasonic treatment displayed an 80% decrease in conductivity as compared to untreated nanofibers. This is the first example of an electronic wire for which the conductivity can be controlled by ultrasound.

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.

Correction to “S-Enantiomer of the Antitubercular Compound S006-830 Complements Activity of Frontline TB Drugs and Targets Biogenesis of Mycobacterium tuberculosis Cell Envelope”

[This corrects the article DOI: 10.1021/acsomega.7b01281.].

Experimental and computational crystal structure landscape study of nigerloxin: a fungal metabolite from Aspergillus niger

The crystal structure landscape for the bioactive fungal metabolite, nigerloxin, has been explored in the present study using a combined experimental and computational approach. Two new solvate forms of nigerloxin, have been identified from crystallization screening experiments. Additionally, a crystal structure prediction (CSP) study of nigerloxin anhydrate and monohydrate forms has been performed in the study. A comparison of the radical scavenging properties of nigerloxin and related β-hydroxy-γ-pyrone analogues have been performed with the help of bond dissociation enthalpy (BDE) and HOMO–LUMO energy gap computations, which indicate the lowest BDE and HOMO–LUMO gaps for nigerloxin. Various growth units present in the experimental and predicted crystal structures were identified using the supramolecular synthon approach and were utilized in the identification of the most probable crystal growth pathways available for nigerloxin. A total of twelve distinct crystal growth pathways were identified for nigerloxin anhydrate from the CSP based crystal structural landscape study. The 1-D nigerloxin water tape synthon (1HA) was identified as the most frequently observed growth unit in the predicted structures and was also found present in the two experimental solvate forms. The higher order supramolecular constructs derived from the synthon 1HA were identified as the most probable crystal growth pathways for the nigerloxin monohydrate.