Synthon transferability probed with IR spectroscopy: cytosine salts as models for salts of lamivudine

Co-crystal screening of the anti-HIV drug lamivudine was carried out with dicarboxylic acids as co-formers, and three of the resulting crystalline solids, two salts and a co-crystal, were studied with SCXRD, PXRD and FTIR spectroscopy. Salts of cytosine, a molecule that incorporates critical structural features of lamivudine, with the same co-formers, were taken as model systems for IR spectroscopic studies of the synthons in the salts of lamivudine. It is shown that different systems with the same synthon show very similar spectral signatures in the regions corresponding to the synthon absorptions. This reveals again the modular nature of the supramolecular synthon.

C. N. R. Rao and the Growth of Solid State and Materials Chemistry as a Central Domain of Research

In celebrating Professor C. N. R. Rao's 80th birthday, this article recalls his singular contributions to solid state and materials chemistry for about sixty years. In so doing, the article also traces the growth of the field as a central domain of research in chemical sciences from its early origins in Europe. Although Rao's major work lies in solid state and materials chemistry - a field which he started and nurtured in India while its importance was being recognized internationally - his contributions to other areas of chemistry (and physics), viz., molecular spectroscopy, phase transitions, fullerenes, graphene, nanomaterials and multiferroics are equally significant. Illustrative examples of his work devoted to rare earth and transition metal oxides, defects and nonstoichiometry, metal-insulator transitions, investigation of crystal and electronic structures of a variety of solids by means of electron microscopies and photoelectron spectroscopy, superconducting cuprates, magnetoresistive manganites, multiferroic metal oxides of various structures and, last but not the least, development of new strategies for chemical synthesis of a wide variety of solids including nanomaterials and framework solids in different dimensionalities, are highlighted. The article also captures his exemplary role as a science teacher, science educationist and institution builder in post-Independence India.

Construction of Bis-pyrazole Based Co(II) Metal-Organic Frameworks and Exploration of Their Chirality and Magnetic Properties

In continuation of our interest in pyrazole based multifunctional metal-organic frameworks (MOFs), we report herein the construction of a series of Co(II) MOFs using a bis-pyrazole ligand and various benzene polycarboxylic acids. Employment of different acids has resulted in different architectures ranging from a two-dimensional grid network, porous nanochannels with interesting double helical features such as supramolecular chicken wire, to three-dimensional diamondoid networks. One of the distinguishing features of the network is their larger dimensions which can be directly linked to a relatively larger size of the ligand molecule. Conformational flexibility of the ligand also plays a decisive role in determining both the dimensionality and topology of the final structure. Furthermore, chirality associated with helical networks and magnetic properties of two MOFs have also been investigated.

Hydrogen-Bond-Driven Controlled Molecular Marriage in Covalent Cages

A supramolecular approach that uses hydrogen-bonding interaction as a driving force to accomplish exceptional self-sorting in the formation of imine-based covalent organic cages is discussed. Utilizing the dynamic covalent chemistry approach from three geometrically similar dialdehydes (A, B, and D) and the flexible triamine tris(2-aminoethyl)amine (X), three new 3+2] self-assembled nanoscopic organic cages have been synthesized and fully characterized by various techniques. When a complex mixture of the dialdehydes and triamine X was subjected to reaction, it was found that only dialdehyde B (which has OH groups for H-bonding) reacted to form the corresponding cage B3X2 selectively. Surprisingly, the same reaction in the absence of aldehyde B yielded a mixture of products. Theoretical and experimental investigations are in complete agreement that the presence of the hydroxyl moiety adjacent to the aldehyde functionality in B is responsible for the selective formation of cage B3X2 from a complex reaction mixture. This spectacular selection was further analyzed by transforming a nonpreferred (non-hydroxy) cage into a preferred (hydroxy) cage B3X2 by treating the former with aldehyde B. The role of the H-bond in partner selection in a mixture of two dialdehydes and two amines has also been established. Moreover, an example of unconventional imine bond metathesis in organic cage-to-cage transformation is reported.

Role of surface chemistry in modulating drug release kinetics in titania nanotubes

Surface chemistry and the intrinsic porous architectures of porous substrates play a major role in the design of drug delivery systems. An interesting example is the drug elution characteristic from hydrothermally synthesised titania nanotubes with tunable surface chemistry. The variation in release rates of Ibuprofen (IBU) is largely influenced by the nature of the functional groups on titania nanotubes and pH of suspending medium. To elucidate the extent of interaction between the encapsulated IBU and the functional groups on titania nanotubes, the release profiles have been modelled with an empirical Hill equation. The analysis aided in establishing a probable mechanism for the release of IBU from the titania nanotubes. The study of controlled drug release from TiO2 has wider implication in the context of biomedical engineering. (C) 2014 Elsevier B.V. All rights reserved.

Self-assembly of Chloro-Bridged Arene-Ruthenium Based Rectangle: Synthesis, Structural Characterization and Sensing Study

Self-assembly of a chloro-bridged half-sandwich p-cymene ruthenium(II) complex Ru-2(mu-Cl-2)(eta(6)-p-cymene)(2)Cl-2] 1 with linear ditopic donor L; trans-1,2-bis(4-pyridyl) ethylene] in presence of 2 eq. AgNO3 in CH3CN yielded a chloro-bridged molecular rectangle 2. The rectangle 2 was isolated as nitrate salt in high yield (90 %) and characterized by infra-red, H-1 NMR spectroscopy including ESI-MS analyses. Molecular structure of 2 was determined by single crystal X-ray diffraction study The diffraction analysis shows that 2 adopts a tetranuclear rectangular geometry with the dimensions of 5.51 angstrom x 13.29 angstrom and forming an infinite supramolecular chain with large internal porosity arising through multiple pi-pi and CH-pi interactions between the adjacent rectangles. Furthermore, rectangle 2 is used as selective receptor for phenolic-nitroaromatic compounds such as picric acid, dinitrophenol and nitrophenol.

Comprehending the Formation of Eutectics and Cocrystals in Terms of Design and Their Structural Interrelationships

The phenomenon of cocrystallization, which encompasses the art of making multicomponent organic solids such as cocrystals, solid solutions, eutectics, etc. for novel applications, has been less studied in terms of reliably and specifically obtaining a desired cocrystallization product and the issues that govern their formation. Further, the design, structural, and functional aspects of organic eutectics have been relatively unexplored as compared to solid solutions and cocrystals well-established by crystal engineering principles. Recently, eutectics were proposed to be designable materials on par with cocrystals, and herein we have devised a systematic approach, based on the same crystal engineering principles, to specifically and desirably make both eutectics and cocrystals for a given system. The propensity for strong homomolecular synthons over weak heteromolecular synthons and vice versa during supramolecular growth was successfully utilized to selectively obtain eutectics and cocrystals, respectively, in two model systems and in two drug systems. A molecular level understanding of the formation of eutectics and cocrystals and their structural interrelationships which is significant from both fundamental and application viewpoints is discussed. On the other hand, the obscurity in establishing a low melting combination as a eutectic or a cocrystal is resolved through phase diagrams.

Polymer-grafted multiwall carbon nanotubes functionalized by nitrene chemistry: effect on cooperativity and phase miscibility

The demixing of polystyrene (PS) and poly(vinyl methylether) (PVME) was systematically investigated in the presence of surface functionalized multiwall carbon nanotubes (MWNTs) by melt rheology. As PS-PVME blends are weakly interacting blends, the contribution of conformational entropy increases, resulting in thermo-rheological complexity wherein the concentration fluctuation persists even beyond the critical demixing temperature. These phenomenal changes were followed here in the presence of MWNTs with different surface functional groups. Polystyrene was synthesised by atom transfer radical polymerization and was immobilized onto carboxyl acid functionalized multiwall carbon nanotubes (COOH-MWNTs) via nitrene chemistry in order to improve the phase miscibility in PS-PVME blends. Interestingly, blends with 0.25 wt% polystyrene grafted multiwall carbon nanotubes (PS-g-MWNTs) delayed the spinodal decomposition temperature in the blends by similar to 33 degrees C with respect to both control blends and those with COOH-MWNTs. While the localization of COOH-MWNTs in PVME was explained from a thermodynamic point of view, the localization of PS-g-MWNTs was understood to result from favorable PS-PVME contact and the degree of surface coverage of PS on the surface of MWNTs. The length of the cooperative rearranging region (xi) decreased in presence of PS-g-MWNTs, suggesting confinement effects on large scale motions and enhanced interchain concentration fluctuation.

Sensitivity of meteorological input and soil properties in simulating aerosols (dust, PM10, and BC) using CHIMERE chemistry transport model

The objective of this study is to evaluate the ability of a European chemistry transport model, `CHIMERE' driven by the US meteorological model MM5, in simulating aerosol concentrations dust, PM10 and black carbon (BC)] over the Indian region. An evaluation of a meteorological event (dust storm); impact of change in soil-related parameters and meteorological input grid resolution on these aerosol concentrations has been performed. Dust storm simulation over Indo-Gangetic basin indicates ability of the model to capture dust storm events. Measured (AERONET data) and simulated parameters such as aerosol optical depth (AOD) and Angstrom exponent are used to evaluate the performance of the model to capture the dust storm event. A sensitivity study is performed to investigate the impact of change in soil characteristics (thickness of the soil layer in contact with air, volumetric water, and air content of the soil) and meteorological input grid resolution on the aerosol (dust, PM10, BC) distribution. Results show that soil parameters and meteorological input grid resolution have an important impact on spatial distribution of aerosol (dust, PM10, BC) concentrations.

Polymorphism in cocrystals of urea:4,4 `-bipyridine and salicylic acid:4,4 `-bipyridine

Polymorphic cocrystals of urea:4,4'-bipyridine and salicylic acid: 4,4'-bipyridine were obtained by crystallization from different solvents. The urea tape is a rare phenomenon in cocrystals but it is consistent in urea:4,4'-bipyridine polymorphic cocrystals. The polymorph obtained from MeCN has symmetrical N-H...N hydrogen bond distances on either side of the urea tape. However, the other form obtained from MeOH has unsymmetrical N-H...N hydrogen bond lengths. In the polymorphic cocrystals of salicylic acid:4,4'-bipyridine, the basic supramolecular synthon acid-pyridine is the same but the 3D packing is different. Both the polymorphic pairs of cocrystals come under the category of packing polymorphs. All polymorphs were characterized by single-crystal X-ray diffraction (SCXRD), PXRD, DSC, FT-IR and HSM. N-H...N and the acid-pyridine supramolecular synthons were insulated by FT-IR vibrational spectroscopy.

Component Selection in the Self-Assembly of Palladium(II) Nanocages and Cage-to-Cage Transformations

Dynamic supramolecular systems involving a tetratopic palladium(II) acceptor and three different pyridine-and imidazole-based donors have been used for self-selection by a synergistic effect of morphological information and coordination ability of ligands through specific coordination interactions. Three different cages were first synthesized by two-component self-assembly of individual donor and acceptor. When all four components were allowed to interact in a reaction mixture, only one out of three cages was isolated. The preferential binding affinity towards a particular partner was also established by transforming a non-preferred cage into a preferred cage by interaction with the appropriate ligand. Computational studies further supported the fact that coordination interaction of imidazole moiety to Pd-II is enthalpically more preferred compared to pyridine, which drives the selection process. Analysis of crystal packing of both complexes indicated the presence of strong hydrogen bonds between nitrate and water molecules and also H-bonded 3D networks of water. Both complexes exhibit promising proton conductivity (10(-5) to ca. 10(-3) Scm(-1)) at ambient temperature under a relative humidity of circa 98% with low activation energy.

Obtaining Synthon Modularity in Ternary Cocrystals with Hydrogen Bonds and Halogen Bonds

Design of ternary cocrystals based on synthon modularity is described. The strategy is based on the idea of extending synthon modularity in binary cocrystals of 4-hydroxybenzamide:dicarboxylic acids and 4-bromobenzamide:dicarboxylic acids. If a system contains an amide group along with other functional groups, one of which is a carboxylic acid group, the amide associates preferentially with the carboxylic acid group to form an acidamide heterosynthon. If the amide and the acid groups are in different molecules, a higher multicomponent molecular crystal is obtained. This is a stable pattern that can be used to increase the number of components from two to three in a multicomponent system. Accordingly, noncovalent interactions are controlled in the design of ternary cocrystals in a more predictable manner. If a single component crystal with the amideamide dimer is considered, modularity is retained even after formation of a binary cocrystal with acidamide dimers. Similarly, when third component halogen atom containing molecules are introduced into these binary cocrystals, modularity is still retained. Here, we use acidamide and Br/I center dot center dot center dot O2N supramolecular synthons to obtain modularity in nine ternary cocrystals. The acidamide heterosynthon is robust to all the nine cocrystals. Heterosynthons may assist ternary cocrystal formation when there is a high solubility difference between the coformers. For a successful crystal engineering strategy for ternary cocrystals, one must consider the synthon itself and factors like shape and size of the component molecules, as well as the solubilities of the compounds.

MolBridge: a program for identifying nonbonded interactions in small molecules and biomolecular structures

Identification and analysis of nonbonded interactions within a molecule and with the surrounding molecules are an essential part of structural studies, given the importance of these interactions in defining the structure and function of any supramolecular entity. MolBridge is an easy to use algorithm based purely on geometric criteria that can identify all possible nonbonded interactions, such as hydrogen bond, halogen bond, cation-pi, pi-pi and van der Waals, in small molecules as well as biomolecules. The user can either upload three-dimensional coordinate files or enter the molecular ID corresponding to the relevant database. The program is available in a standalone form and as an interactive web server with Jmol and JME incorporated into it. The program is freely downloadable and the web server version is also available at http://nucleix.mbu.iisc.ernet.in/molbridge/index.php.

Polymorphic phase transition among the titania crystal structures using a solution-based approach: from precursor chemistry to nucleation process

Nanocrystalline titania are a robust candidate for various functional applications owing to its non-toxicity, cheap availability, ease of preparation and exceptional photochemical as well as thermal stability. The uniqueness in each lattice structure of titania leads to multifaceted physico-chemical and opto-electronic properties, which yield different functionalities and thus influence their performances in various green energy applications. The high temperature treatment for crystallizing titania triggers inevitable particle growth and the destruction of delicate nanostructural features. Thus, the preparation of crystalline titania with tunable phase/particle size/morphology at low to moderate temperatures using a solution-based approach has paved the way for further exciting areas of research. In this focused review, titania synthesis from hydrothermal/solvothermal method, conventional sol-gel method and sol-gel-assisted method via ultrasonication, photoillumination and ILs, thermolysis and microemulsion routes are discussed. These wet chemical methods have broader visibility, since multiple reaction parameters, such as precursor chemistry, surfactants, chelating agents, solvents, mineralizer, pH of the solution, aging time, reaction temperature/time, inorganic electrolytes, can be easily manipulated to tune the final physical structure. This review sheds light on the stabilization/phase transformation pathways of titania polymorphs like anatase, rutile, brookite and TiO2(B) under a variety of reaction conditions. The driving force for crystallization arising from complex species in solution coupled with pH of the solution and ion species facilitating the orientation of octahedral resulting in a crystalline phase are reviewed in detail. In addition to titanium halide/alkoxide, the nucleation of titania from other precursors like peroxo and layered titanates are also discussed. The nonaqueous route and ball milling-induced titania transformation is briefly outlined; moreover, the lacunae in understanding the concepts and future prospects in this exciting field are suggested.

Fluorine prefers hydrogen bonds over halogen bonds! Insights from crystal structures of some halofluorobenzenes

Crystal structures of a series of isomers of chlorofluorobenzene, bromofluorobenzene and iodofluorobenzene, all of which are liquids under ambient conditions, are determined by a technique of in situ cryocrystallography. These simple dihalo substituted benzenes provide clear insights into subtle interplay of packing interactions preferred by fluorine and heavier halogens for example, C-H center dot center dot center dot X hydrogen bonds vs. X center dot center dot center dot X halogen bonds (X=F, Cl, Br, I). The interaction patterns noted here are purely characteristic of halogens, having not been influenced by other stronger interactions. Variability of principal supramolecular synthons among the isomers highlights the importance of molecular shape and relative position of interacting atoms while preserving the basic intermolecular bonds. Mutually exclusive occurrence of homo (I center dot center dot center dot I) and hetero (I center dot center dot center dot F) halogen bonds in polymorphs of 4-iodofluorobenzene questions the robustness and reliability of these interactions.