Very small crystals of anhydrous guanine (systematic name: 2-amino-1,7-dihydro-6H-purin-6-one), C 5 H 5 N 5 O, were obtained from an attempted solvothermal synthesis of a potassium complex. Data were collected at 120 K using a synchrotron radiation source. There is one essentially planar molecule in the asymmetric unit. Molecules are linked to each other by NÐHÁ Á ÁN and NÐHÁ Á ÁO hydrogen bonds to form sheets, between which there are %±% stacking interactions. This crystal structure determination demonstrates conclusively that, in the absence of any solvent or other molecules, guanine exists as the amino±keto tautomer in the solid state, with H atoms attached to N1 and N7 (purine numbering), unlike its monohydrate, which has H atoms on N1 and N9.
Crystals of a second polymorph of violuric acid monohydrate [systematic name: pyrimidine-2,4,5,6(1H,3H)-tetrone monohydrate], C(4)H(3)N(3)O(4).H(2)O, have higher density and a more extensive hydrogen-bonding arrangement than the previously reported polymorph. Violuric acid and water molecules form essentially planar hydrogen-bonded sheets, which are stacked in an offset ...ABCABC... repeat pattern involving no ring-stacking interactions.
bonds in the crystal structure but crystal packing is stabilized by eight C-H··· and four ··· interactions. Some of the C-H··· interactions are with the H···Cg distance shorter than 2.70Å (Cg is the centroid of five-or six-membered ring with delocalized bonds). Two of the C-H··· interactions are intramolecular and play a very significant role in the orientation of phenyl rings and -OCH 2 CH 3 fragments. Keywords: C-H··· interaction, Ni(II) complex, thiosemicarbazide (2), Hcit = C(OH)(COO-)(CH 2 COO-) 2 . They are monoclinic, P2 1 /n, and crystallize as linear polymers. The multidentate citrate anion coordinates through six out of its seven oxygen atoms, five of them acting as simple donors and the remaining one in a bridging mode. Two aqua molecules complete a LnO 9 coordination environment. Each citrate links to 3 lanthanide metal centers leading to the formation of "ladder like" chains parallel to [100] where the steps are defined by the Ln-O-Ln bridges (Ln…Ln: 4.321(4)/4.389(2)Å, for Gd/Nd respectively), connected to each other by longer citrate-based "linkers" (Ln…Ln : 6.145(4)/6.230(2)Å).The inter-linkage of chains is achieved through H-bonding to which the three water molecules provide five donor hydrogens.Magnetic susceptibility measurements in (1) In recent years, multimetallic Nb-based oxides have generated considerable interest in many fields : these oxides are mainly described as ferroelectrics, ion conductors and oxidation catalysts. In the frame of research aimed at developing new synthetic procedures of these Nb-containing oxides, water-soluble peroxo complexes of niobium(V) and tantalum(V) have been prepared and investigated spectroscopically and structurally.The compounds studied are of two types : peroxo-carboxylato and peroxo-polyaminocarboxylato (PAC) complexes, the latter ones will be described. The peroxo-PAC complexes were synthetized in the presence excess hydrogen peroxide. These conditions led to the oxidation in situ of the nitrogen atoms of the ligands into N-oxides. Two PAC ligands were selected : ethylenediaminetetraacetic acid (H 4 edta) and propylenediaminetetraacetic acid (H 4 pdta).The crystal structures of the guanidinium derivatives of the (edta)-and (pdta) Nb and also of the (edta)-Ta complexes have been determined at 100K. In all the three compounds, the metal exhibits an eightfold coordination by two bidentate peroxo ligands and a tetradentate bis(N-oxido) PAC ligand resulting in a distorted dodecahedral geometry. In the past years, it has been found that interaction of biomolecules with metal ions are of interest in studies of biological functions of metal ions, especially alkali and alkaline earth metals, because they have abundant distribution in the human body. In addition it is known that s-block metal ions interact with the negatively charged phosphate group of nucleic acids and thus stabilise the DNA and RNA structures.The study of the structural chemistry of nucleobases with transition metals is well known and many complexes have been synthesized and characterised...
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