Carmen Alvarez-Rúa

On the fitting of model electron densities into EM reconstructions: a reciprocal-space formulation

A fast method for fitting model electron densities into EM reconstructions is presented. The methodology was inspired by the molecular-replacement technique, adapted to take into account phase information and the symmetry imposed during the EM reconstruction. Calculations are performed in reciprocal space, which enables the selection of large volumes of the EM maps, thus avoiding the bias introduced when defining the boundaries of the target density.

Intermolecular Reaction of Internal Alkynes and Imines: Propargyl Tosylates as Key Partners in a Gold-Catalyzed [4 + 1] Unusual Cyclization Leading to Cyclopent-2-enimines

Propargyl tosylates react with N-tosylaldimines to afford cyclopent-2-enimines in a gold-catalyzed process that involves a deep reorganization of both substrates. The formal [4 + 1] cyclization is initiated by a 1,2-migration of the tosylate that eventually generates a substituted 1,3-diene. Subsequent interaction with the imine launches a series of reaction steps prior to a Nazarov-like cyclization to yield the final product.

Multiple hydrogen bonds and tautomerism in naphthyridine derivatives

The behaviour of three 2,7-disubstituted 1,8-naphthyridines able to exhibit tautomerism has been studied by NMR in solution and in two cases in the solid state. The three derivatives studied are 2,7-dihydroxy- (1), 2-acetamido-7-amino- (3) and 2,7-diacetamido-1,8-naphthyridine (4). To explore the problem of secondary interactions, a series of complexes, with up to four simultaneous hydrogen bonds, where the monomers are generated using pyridine and 4-pyridone as building blocks, have been theoretically studied. The calculated interaction energies have been correlated with the number of hydrogen bonds and with attractive and repulsive secondary interactions. Further analysis of the electron density and orbital interactions shows that the secondary interactions, both attractive and repulsive, have a purely electrostatic origin. The X-ray structure of compounds 3 and 4 have been determined. In the solid state these compounds exist in the “diamino” tautomers with the N–H proton of the amido groups pointing towards the naphthyridine nitrogen. DFT and GIAO calculations have been essential to disentangle the problem of the structure of these compounds.

Controlled Generation of Three Contiguous Stereocentres in the Michael Addition of 1-Pyrrolidinocyclohexene to (E)-(1-Methyl-2-oxoindolin-3-ylidene)acetophenone

Addition of 1-pyrrolidinocyclohexene to (E)-(1-methyl-2-oxoindolin-3-ylidene)acetophenone followed by acid hydrolysis was diastereoselectively controlled to give (±)-7a (3R*,1′S*,2′S*) or (±)-7b (3S*,1′R*,2′S*), the structures of which were supported by 1H NMR spectroscopic data and corroborated by X-ray diffraction analysis. The change in configuration at the C-2′ centre greatly affected the geometries of the two diastereomers, both in solid and in solution. An explanation of the observed diastereoselectivity is provided. An approach to the N-methylwelwitindolinone C skeleton from 7b as a starting material, by insertion of a rhodium carbenoid into the C(4)−H indole position, was abandoned because of the initial experimental results, which are also described. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Structure and microstructure of gypsum and its relevance to Rietveld quantitative phase analyses

~Received 2 December 2003; accepted 8 March 2004!Single crystals of gypsum were studied in a diffractometer equipped with a CCD two-dimensionaldetector. The microstructure of the crystal gave wide poorly shaped spots showing sometimescurved streaks around the spots, which made the integration process very difficult, yielding a lowquality structure. The crystal structure and microstructure of gypsum has been studied byhigh-resolution synchrotron powder diffraction of a ground single crystal. The intensities in thesynchrotron powder pattern can be reliably fitted although the peak shape displays anisotropic peakbroadening. The Rietveld results for gypsum were a56.52291(3) A, b515.19763(9) A, c56.52291(3) A, b5118.479(1)°,V5494.536(5) A

Imidazolium-Based Dicationic Cyclophanes. Solid-State Aggregates with Unconventional (C–H)+···Cl− Hydrogen Bonding Revealed by X-ray Diffraction

The first single-crystal X-ray crystallographic diffraction analysis of a dicationic heterophane showed a non-classic (C–H)+···Cl− hydrogen bond between the imidazolium rings and halide anions and the formation of unconventional charged assisted hydrogen bonds, which were the non-covalent forces driving the anion interactions shown by the dications 4·2X. Here is reported the halide-templated controlled synthesis and chemical response in basic media of 4·2X. Their structural properties were examined at the gas phase by electrospray ionization mass spectrometry in the negative-ion mode and in the solid-state by X-ray crystallography. Thus, the negative-ion ESI-MS response showed that the formation of non-covalent self-aggregates of macrocyclic dications is a consequence of hydrogen-bonded complexes with halide anions. Notably, X-ray diffraction of dication 4a·2Cl·2H2O provides evidence for the H-bonding network, which has a crucial role in crystal packing. The solid-state aggregates showed that chloride anions and water molecules formed channels among dications 4a+.

Hydrogen bonded driven anion binding by dicationic [14]imidazoliophanes

Imidazolium units represent the main structural motifs for the formation of unconventional C–H···Cl– hydrogen bonds which become the noncovalent forces driving the anion interactions exibited by dicationic [14]imidazoliophanes in the solid state, while C–H···O hydrogen bonds with water play a crucial role in governing the conformation of the solid–state aggregates, as well as in solution, wherein NMR studies also reveal the importance of hydrogen bonds in controlling the tendency to anion binding.

Quadrupolar [14](meta—ortho)2azolophanes. Intramolecular C—H…N and to Water C—H…O hydrogen bonding revealed by X-ray diffraction

Abstract The first X-ray crystallographic diffraction of an example of the title heterophanes built up from heterocyclic betaine subunits is reported and its quadrupolar character is confirmed. The crystal packing of 2·4H2O is mainly governed by hydrogen-bonding networks, strong intermolecular interactions with water together with weak interactions, either intramolecular or with water.

Bis­(pyridine)­iodo­nium(I) tetra­fluoro­borate

The title compound, C10H10IN2+·BF4−, or IPy2BF4, is found to crystallize in the monoclinic crystal system. The asymmetric unit contains two tetrafluoroborate anions, one independent IPy2 cation and two half-cations; for the latter the I atoms lie on centres of symmetry. A slight deviation from planarity is observed in the crystallographically independent IPy2 cation.

1,2-Diaryl(3-pyridyl)ethanone Oximes. Intermolecular Hydrogen Bonding Networks Revealed by X-ray Diffraction

The synthesis of a set of 1-aryl-2-aryl(3-pyridyl)ethanones 1-5 and the corresponding ketoximes 6-9 is reported. Structural studies of oximes 6, 7 and 9 were performed in solution using 1H-NMR and in the solid state by X-ray crystallography, providing evidence of H-bonding networks. The crystal packing was controlled by homomeric intermolecular oxime···oxime H-bond interactions for 6 and cooperative oxime···N(pyridyl) and CH/π interactions for 7 and 9.