Role of Crystal Field in Mixed Alkali Metal Effect: Electron Paramagnetic Resonance Study of Mixed Alkali Metal Oxyfluoro Vanadate Glasses

The mixed alkali metal effect is a long-standing problem in glasses. Electron paramagnetic resonance (EPR) is used by several researchers to study the mixed alkali metal effect, but a detailed analysis of the nearest neighbor environment of the glass former using spin-Hamiltonian parameters was elusive. In this study we have prepared a series of vanadate glasses having general formula (mol %) 40 V2O5-30BaF(2)-(30 - x)LiF-xRbF with x = 5, 10, 15, 20, 25, and 30. Spin-Hamiltonian parameters of V4+ ions were extracted by simulating and fitting to the experimental spectra using EasySpin. From the analysis of these parameters it is observed that the replacement of lithium ions by rubidium ions follows a ``preferential substitution model''. Using this proposed model, we were able to account for the observed variation in the ratio of the g parameter, which goes through a maximum. This reflects an asymmetric to symmetric changeover of. the alkali metal ion environment around the vanadium site. Further, this model also accounts for the variation in oxidation state of vanadium ion, which was confirmed from the variation in signal intensity of EPR spectra.

Kinematic and Mechanical Profile of the Self-Actuation of Thermosalient Crystal Twins of 1,2,4,5-Tetrabromobenzene: A Molecular Crystalline Analogue of a Bimetallic Strip

A paradigm shift from hard to flexible, organic-based optoelectronics requires fast and reversible mechanical response from actuating materials that are used for conversion of heat or light into mechanical motion. As the limits in the response times of polymer-based actuating materials are reached, which are inherent to the less-than-optimal coupling between the light/heat and mechanical energy in them, 1 a conceptually new approach to mechanical actuation is required to leapfrog the performance of organic actuators. Herein, we explore single crystals of 1,2,4,5-tetrabromobenzene (TBB) as actuating elements and establish relations between their kinematic profile and mechanical properties. Centimeter-size acicular crystals of TBB are the only naturally twinned crystals out of about a dozen known materials that exhibit the thermosalient effect-an extremely rare and visually impressive crystal locomotion. When taken over a phase transition, crystals of this material store mechanical strain and are rapidly self-actuated to sudden jumps to release the internal strain, leaping up to several centimeters. To establish the structural basis for this colossal crystal motility, we investigated the mechanical profile of the crystals from macroscale, in response to externally induced deformation under microscope, to nanoscale, by using nanoindentation. Kinematic analysis based on high-speed recordings of over 200 twinned TBB crystals exposed to directional or nondirectional heating unraveled that the crystal locomotion is a kinematically complex phenomenon that includes at least six kinematic effects. The nanoscale tests confirm the highly elastic nature, with an elastic deformation recovery (60%) that is far superior to those of molecular crystals reported earlier. This property appears to be critical for accumulation of stress required for crystal jumping. Twinned crystals of TBB exposed to moderate directional heating behave as all-organic analogue of a bimetallic `strip, where the lattice misfit between the two crystal components drives reveriible deformation of the crystal.

mu-Oxamido binuclear copper (II) complexes: Synthesis, crystal structure, DNA interaction and antibacterial studies

Four binuclear copper (II) complexes Cu(oxpn)Cu(B)](2+) (2-5) bridged by N, N'-bis3-(methylamino) propyl] oxamide (oxpn), where, B is N, N-donor heterocyclic bases (viz. 2,2'-bipyridine (bpy, 2), 1,10-phenathroline (phen, 3), dipyrido3,2-d:2',3'-f]quinoxaline (dpq, 4) and dipyrido3,2-a:2',3'-c]phenazine (dppz, 5) are synthesized, characterized by different spectroscopic and single crystal X-ray data technique. The phen (3) and dpq (4) complexes were structurally characterized by X-ray data analysis. Their DNA binding, oxidative cleavage and antibactirial activities were studied. The dpq (4) and dppz (5) complexes are avid binders to the Calf thymus DNA (CT-DNA). The phen (3), dpq (4) and dppz (5) complexes show efficient oxidative cleavage of supercoiled DNA (SC DNA) through hydroxyl radical ((OH)-O-center dot) pathway in the presence of Mercaptopropionic acid (MPA). (C) 2013 Elsevier Ltd. All rights reserved.

Synthesis, crystal structure, DNA interaction and cleavage activities of mononuclear and trinuclear copper(II) complexes

Mono- and trinuclear copper(II) complexes with 2-1-(2-dimethylamino-ethylamino)-ethyl]-phenol (HL) have been synthesized and structurally characterized. The mononuclear complex Cu(L)(H2O)(ONO2)] (1) crystallizes in monoclinic space group P2(1) /n with a square pyramidal Cu(II) center coordinated by the tridentate Schiff base (L) and a water ligand in the equatorial plane and an oxygen atom from nitrate in the axial position. The trinuclear complex (CuL)(3)(mu(3)-OH)](ClO4)(2)center dot H2O (2) crystallizes in hexagonal space group P6(3); all three copper atoms are five-coordinate with square pyramidal geometries. The interactions of these complexes with calf-thymus DNA have been investigated using absorption spectrophotometry. The mononuclear complex binds more strongly than the trinuclear complex. The DNA cleavage activity of these complexes has been studied on double-stranded pBR 322 plasmid DNA by gel electrophoresis experiments in the absence and in the presence of added oxidant (H2O2). The trinuclear complex cleaves DNA more efficiently than the mononuclear complex in the presence of H2O2.

New luminescent 2-methoxy-6-(4-methoxy-phenyl)-4-p-tolyl-nicotinonitrile: Synthesis, crystal structure, DFT and photophysical studies

In the current communication, we report the synthesis, spectroscopic, crystal structure, DFT and photophysical studies of a new nicotinonitrile derivative, viz. 2-methoxy-6-(4-methoxy-phenyl)-4-p-tolyl-nicotinonitrile (2) as a potential blue light emitting material. The compound 2 was synthesized in good yield via a simple route. The acquired spectral and elemental analysis data were in consistent with the chemical structure of 2. The single crystal study further confirms its three dimensional structure, molecular shape, and nature of short contacts. Its DFT calculations reveal that compound 2 possesses a non-planar structure and its theoretical IR spectral data are found to be in accordance with experimental values. In addition, its UV visible and fluorescence spectral measurements prove that the compound exhibits good absorption and fluorescence properties. Also, it shows positive solvatochromic effect when the solvent polarity was varied from non-polar to polar. (c) 2014 Elsevier B.V. All rights reserved.

A Dinuclear Cadmium(II) Schiff Base Thiocyanato Complex: Crystal Structure and Fluorescence

A new dinuclear cadmium(II) complex, Cd(L)(NCS)](2) (1) has been synthesized using a potentially tetradentate Schiff base ligand HL, 2-((E)-(2-(diethylamino) ethylimino) methyl)-6-methoxyphenol, obtained by the condensation of 2-diethylaminoethylamine and o-vanillin, and characterized by different physicochemical techniques. Crystal structure of the title complex was unambiguously established by single crystal X-ray diffraction which reveals that metal centers are connected by bridging phenolato and chelating methoxy oxygen atoms of the coordinating Schiff bases and embedded in severely distorted octahedral geometries. Fluorescence properties of the ligand and its complex, studied at room temperature indicate that later may serve as strong fluorescent

Ligational behaviour of (E)-2-amino-N ` -1-(2-hydroxyphenyl) ethylidene]benzohydrazide towards later 3d metal ions: X-ray crystal structure of nickel(IV) complex

Ligational behaviour of (E)-2-amino-N'-1-(2-hydroxyphenyl)ethylidene]benzohydrazide (Aheb) towards later 3d metal ionscopper(II), cobalt(II), manganese(II), zinc(II), cadmium(II) and nickel(IV)] has been studied. Their structures have been elucidated on the basis of spectral (IR, H-1 NMR, UV-Vis, EPR and FAB-mass), elemental analyses, conductance measurements, magnetic moments, and thermal studies. During complexation Ni(II) ion has got oxidized to Ni(IV). The changes in the bond parameters of the ligand on complexation has been discussed by comparing the crystal structure of the ligand with that of its Ni(IV) complex. The X-ray single crystal analysis of Ni(aheb)(2)]Cl-2 center dot 4H(2)O has confirmed an octahedral geometry around the metal ion. EPR spectra of the Cu(II) complex in polycrystalline state at room (300 K) and liquid nitrogen temperature (77 K) were recorded and their salient features are reported. (C) 2014 Elsevier B.V. All rights reserved.

Tuning nuclearity of clusters by positional change of functional group: Synthesis of polynuclear clusters, crystal structures and magnetic properties

Four neutral polynuclear magnetic clusters, (Mn6Mn2Na2I)-Mn-III-Na-II(N-3)(8)(mu(1)-O)(2)(L-1)(6)(CH3OH)(2)] (1), (Mn6Na2I)-Na-III(N-3)(4)(mu(4)-O)(2)(L-2)(4)(CH3COO)(4)] (2), Ni-5(II)(N-3)(4)(HL1)(4)(HCOO)(2)(CH3OH)(2)(H2O)(2)]center dot 2CH(3)OH (3) and (Ni4Na2I)-Na-II(N-3)(4)(HL2)(6)]center dot 2CH(3)OH (4) have been synthesized using tetradentate ligands H2L1-2 along with azide as a co-ligand. H2L1-2 are the products formed in situ upon condensation of 2-hydroxy-3-methoxybenzaldehyde with 1-aminopropan-2-ol and 1-aminopropan-3-ol, respectively. Single crystal X-ray diffraction and bond valence sum calculation showed that complex 1 is composed of both Mn-III and Mn-II. Complex 3 contains coordinated formate, which was formed upon in situ oxidation of methanol. The magnetic study over a wide range of temperatures of all the complexes (1-4) showed that 1 and 2 are antiferromagnetic whereas other two (3-4) are predominantly ferromagnetic. The estimated ground states of the complexes are S approximate to 3(1), S = 4(2), S = 5(3) and S approximate to 4(4), respectively. (C) 2014 Elsevier B.V. All rights reserved.

New columnar liquid crystal materials based on luminescent 2-methoxy-3-cyanopyridines

A new series of donor-acceptor-donor (D-A-D) type luminescent mesogens carrying 2-methoxy-3-cyanopyridine as a central core linked with variable alkoxy chain lengths (m = 6 and 8) as terminal substituents was synthesized and characterized using spectral methods. The newly synthesized molecules were subjected to single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), differential scanning calorimetric (DSC), polarizing optical microscopy (POM), and fluorescence emission studies in order to ascertain their mesogenic and photophysical properties. The SCXRD data on 4a and 4b reveal that the presence of short intermolecular contacts, viz. C-H center dot center dot center dot N, C-H center dot center dot center dot O, C-H center dot center dot center dot pi, and pi center dot center dot center dot pi interactions, is responsible for their crystal packing. The measured torsion angle values indicate that molecules possess distorted non-planar structure. The DSC, POM, and PXRD studies confirm that all the molecules show thermotropic liquid crystalline behaviour and exhibit rectangular columnar phase. Further, their UV-visible and fluorescence spectral studies reveal that the target molecules are luminescent displaying a strong absorption band in the range of 335-340 nm and a blue fluorescence emission band in the range of 395-425 nm (both in solution and film state) with good fluorescence quantum yields (10-49 %).

Crystal structure of a putative aspartic proteinase domain of the Mycobacterium tuberculosis cell surface antigen PE_PGRS16

We report the crystal structure of the first prokaryotic aspartic proteinase-like domain identified in the genome of Mycobacterium tuberculosis. A search in the genomes of Mycobacterium species showed that the C-terminal domains of some of the PE family proteins contain two classic DT/SG motifs of aspartic proteinases with a low overall sequence similarity to HIV proteinase. The three-dimensional structure of one of them, Rv0977 (PE_PGRS16) of M. tuberculosis revealed the characteristic pepsinf-old and catalytic site architecture. However, the active site was completely blocked by the N-terminal His-tag. Surprisingly, the enzyme was found to be inactive even after the removal of the N-terminal His-tag. A comparison of the structure with pepsins showed significant differences in the critical substrate binding residues and in the flap tyrosine conformation that could contribute to the lack of proteolytic activity of Rv0977. (C) 2013 The Authors. Published by Elsevier B.V. on behalf of Federation of European Biochemical Societies. All rights reserved.

Halogen Bonds in Crystal Engineering: Like Hydrogen Bonds yet Different

CONSPECTUS: The halogen bond is an attractive interaction in which an electrophilic halogen atom approaches a negatively polarized species. Short halogen atom contacts in crystals have been known for around 50 years. Such contacts are found in two varieties: type I, which is symmetrical, and type II, which is bent. Both are influenced by geometric and chemical considerations. Our research group has been using halogen atom interactions as design elements in crystal engineering, for nearly 30 years. These interactions include halogen center dot center dot center dot halogen interactions (X center dot center dot center dot X) and halogen center dot center dot center dot heteroatom interactions (X center dot center dot center dot B). Many X center dot center dot center dot X and almost all X center dot center dot center dot B contacts can be classified as halogen bonds. In this Account, we illustrate examples of crystal engineering where one can build up from previous knowledge with a focus that is provided by the modern definition of the halogen bond. We also comment on the similarities and differences between halogen bonds and hydrogen bonds. These interactions are similar because the protagonist atoms halogen and hydrogen are both electrophilic in nature. The interactions are distinctive because the size of a halogen atom is of consequence when compared with the atomic sizes of, for example, C, N, and O, unlike that of a hydrogen atom. Conclusions may be drawn pertaining to the nature of X center dot center dot center dot X interactions from the Cambridge Structural Database (CSD). There is a clear geometric and chemical distinction between type I and type II, with only type II being halogen bonds. Cl/Br isostructurality is explained based on a geometric model. In parallel, experimental studies on 3,4-dichlorophenol and its congeners shed light on the nature of halogen center dot center dot center dot halogen interactions and reveal the chemical difference between Cl and Br. Variable temperature studies also show differences between type I and type II contacts. In terms of crystal design, halogen bonds offer a unique opportunity in the strength, atom size and interaction gradation; this may be used in the design of ternary cocrystals. Structural modularity in which an entire crystal structure is defined as a combination of modules is rationalized on the basis of the intermediate strength of a halogen bond. The specific directionality of the halogen bond makes it a good tool to achieve orthogonality in molecular crystals. Mechanical properties can be tuned systematically by varying these orthogonally oriented halogen center dot center dot center dot halogen interactions. In a further development, halogen bonds are shown to play a systematic role in organization of LSAMs (long range synthon aufbau module), which are bigger structural units containing multiple synthons. With a formal definition in place, this may be the right time to look at differences between halogen bonds and hydrogen bonds and exploit them in more subtle ways in crystal engineering.

Improved Erlenmeyer Synthesis with 5-Thiazolone and Catalytic Mn(II) Acetate. Crystal Structure Confirmation of the Reaction Stereochemistry

2-Phenylthiazolin-5-one (5, a thioazlactone) condenses with various aldehydes in the presence of the mild base Mn(II) acetate as catalyst in CH2Cl2 solution. This leads to the corresponding Erlenmeyer reaction products (6) in excellent yields in the case of aromatic aldehydes and moderate yields in others. The mildness of the reaction conditions is apparently enabled by the aromaticity of the (putative) intermediate thiazolone anion. The structure and stereochemistry (Z) of the product derived from i-BuCHO was confirmed by single crystal X-ray diffraction. This study overcomes key limitations of the classical Erlenmeyer synthesis and also introduces the relatively nontoxic Mn(II) acetate as a reagent in heterocyclic chemistry.

Raman Evidence for Coupling of Superconducting Quasi-Particles with a Phonon and Crystal Field Excitation in Superconductor Ce0.6Y0.4FeAsO0.8F0.2

We report inelastic light scattering experiments on superconductor Ce0.6Y0.4FeAsO0.8F0.2 from 4K to 300K covering the superconducting transition temperature T-c similar to 48.6K. A strong evidence of the superconductivity induced phonon renormalization for the A(1g) phonon mode near 150cm(-1) associated with the Ce/Y vibrations is observed as reflected in the anomalous red-shift and decrease in the linewidth below T-c. Invoking the coupling of this mode with the superconducting gap, the superconducting gap (2 Delta) at zero temperature is estimated to be similar to 20meV i.e the ratio 2 Delta(0)/k(B)T(c) is similar to 5, suggesting Ce0.6Y0.4FeAsO0.8F0.2 to belong to the class of strong coupling superconductors. In addition, the mode near 430cm(-1) associated with Ce3+ crystal field excitation also shows anomalous increase in its linewidth below T-c suggesting strong coupling between crystal field excitation and the superconducting quasi-particles.

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.