T. N. Guru Row

Net atomic charges and molecular dipole moments from spherical-atom X-ray refinements, and the relation between atomic charge and shape

Refinement of the population and radial dependence of the spherical atomic valence shell is introduced in a general crystallographic least-squares program. The radial dependence is described by an expansion-contraction parameter κ, which, in the nine data sets tested, indicates contraction of positively and expansion of negatively charged atoms in agreement with theoretical concepts such as those incorporated in Slaters analytical rules for atomic orbitals. H atoms appear more contracted than concluded previously on the basis of a comparison of X-ray and neutron thermal parameters of sucrose. An average value of 1.40 for the radial contraction of H is used in structures for which no neutron thermal parameters are available. The resulting net charges are used to calculate X-ray molecular dipole moments whose magnitude and direction are in good agreement with theoretical and other experimental results, though some differences may be expected because of matrix effects. Net molecular charges in the one-dimensional conductor TTF-TCNQ agree with results obtained earlier by direct integration of the charge density over the molecular volume. A charge transfer from Si to V in the superconducting alloy V3Si is also in agreement with earlier results.

Hydrogen and fluorine in crystal engineering: systematics from crystallographic studies of hydrogen bonded tartrate–amine complexes and fluoro-substituted coumarins, styrylcoumarins and butadienes

Abstract Three aspects of crystal engineering in molecular crystals are presented to emphasize the role of intermolecular interactions and factors influencing crystal packing. Hydrogen bonded tartrate–amine complexes have been analyzed with the propensity for formation of multidirectional hydrogen bonding as a key design element in the generation of materials for second harmonic generation (SHG). The invariance of the framework in DBT and its possible implications on SHG is outlined. The role of Fluorine in orienting molecules of coumarins, styrylcoumarins and butadienes for photodimerization is described with particular emphasis on its steering capability. Usage of coumarin as an design element for the generation of polymorphs of substituted styrylcoumarins is examined with specific examples.

C–halogen···π interactions and their influence on molecular conformation and crystal packing: a database study

Abstract A Cambridge Structural Database study was undertaken to investigate the role of C–X···π interactions (X=F, Cl, Br, I) in conformational and crystal engineering control in organic crystal structures. This analysis reveals that organic fluorine has a greater propensity for the formation of such interactions rather than accepting hydrogen bonds. These results also compare well with the recently analyzed C–H···π interactions and depict the orientational dependency and the control of crystal packing in organic molecules.

Magnetoresistance in ordered and disordered double perovskite oxide, Sr2FeMoO6

We have prepared crystallographically ordered and disordered specimens of the double perovskite, Sr 2 FeMoO 6 , and investigated their magnetoresistance (MR) behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder X-ray diffraction patterns and reconfirmed by Mossbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the MR at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state.

Evaluation of the interchangeability of C–H and C–F groups: insights from crystal packing in a series of isomeric fluorinated benzanilides

A series of mono- and difluorinated benzanilides have been synthesized and characterized to unravel the importance of interactions involving organic fluorine. The effect of fluorine substitution on the molecular conformation and the solid-state organization of the molecules in the crystalline lattice have been discussed in terms of changes in molecular conformation and the supramolecular aggregation. It is noteworthy that the meta- and para-isomers (mono and difluorinated) are isostructural, with the ortho-isomers possessing minor conformational variations. Furthermore, the molecular conformation as obtained by theoretical DFT calculations is different from the solid-state conformation highlighting the importance of co-operative features brought by N–H⋯O and C–H⋯O hydrogen bonds, along with weak intermolecular interactions involving organic fluorine and aromatic C–H⋯π contacts in the solid state.

C–halogen…π interactions in proteins: a database study

A Protein Data Bank (PDB) study was undertaken to investigate the role of C–X…\pi interactions (X = F, Cl, Br, I) in protein structures. From this analysis, it is evident that the propensity for the formation of C– X…\pi interactions is higher in case of fluorine than in other halogens. These results compare well with the recently reported C–X…\pi interactions in small molecules and depict the orientational dependence of these interactions.

Weak interactions involving organic fluorine: a comparative study of the crystal packing in substituted isoquinolines

Abstract Weak interactions involving fluorine have been analyzed in the structure of 6-methoxy-1,2-diphenyl-1,2,3,4-tetrahydroisoquinoline with fluorine substitution at para -, meta - and ortho - positions, respectively, on the 1-phenyl ring. The packing modes in the crystalline lattice as determined by X-ray diffraction techniques generate motifs via F⋯F, C–H⋯F and C–F⋯π interactions. The three structures as compared to the parent compound depict conformational changes in the saturated tetrahydroisoquinoline moiety. The salient features of the four structures in terms of weak interactions involving fluorine suggest that organic fluorine does resemble the other halogens.

Structure and dielectric properties of recurrent intergrowth structures formed by the Aurivillius family of bismuth oxides of the formula Bi2An−1BnO3n+3

Crystal structures and dielectric properties of three recurrent intergrowth structures Bi9Ti6CrO27, Bi9Ti6FeO27, and BaBi8Ti7O27 formed by the Aurivillius family of bismuth oxides of the formula Bi2An−1BnO3n+3 are reported. The intergrowths exhibit ferroelectricity and accordingly belong to the noncentrosymmetric space group Cmm2. The ferroelectric curie temperatures of the intergrowths are in the 630–1070 K range.

Unusually short chalcogen bonds involving organoselenium: Insights into the Se-N bond cleavage mechanism of the antioxidant ebselen and analogues

Structural studies on the polymorphs of the organoselenium antioxidant ebselen and its derivative show the potential of organic selenium to form unusually short Se⋅⋅⋅O chalcogen bonds that lead to conserved supramolecular recognition units. Se⋅⋅⋅O interactions observed in these polymorphs are the shortest such chalcogen bonds known for organoselenium compounds. The FTIR spectral evolution characteristics of this interaction from solution state to solid crystalline state further validates the robustness of this class of supramolecular recognition units. The strength and electronic nature of the Se⋅⋅⋅O chalcogen bonds were explored using high-resolution X-ray charge density analysis and atons-in-molecules (AIM) theoretical analysis. A charge density study unravels the strong electrostatic nature of Se⋅⋅⋅O chalcogen bonding and soft-metal-like behavior of organoselenium. An analysis of the charge density around Se-N and Se-C covalent bonds in conjunction with the Se⋅⋅⋅O chalcogen bonding modes in ebselen and its analogues provides insights into the mechanism of drug action in this class of organoselenium antioxidants. The potential role of the intermolecular Se⋅⋅⋅O chalcogen bonding in forming the intermediate supramolecular assembly that leads to the bond cleavage mechanism has been proposed in terms of electron density topological parameters in a series of molecular complexes of ebselen with reactive oxygen species (ROS).

Redetermination of curcumin: (1E,4Z,6E)-5-hydr­oxy-1,7-bis­(4-hydr­oxy-3-methoxy­phen­yl)hepta-1,4,6-trien-3-one

The uncertainty associated with the position of one H atom in curcumin,