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inorganic compounds
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The crystal structure of (H3O)2Cr4O13 is isotypic with K2Cr4O13. The finite tetrachromate anion in the title structure consists of four vertex-sharing CrO4 tetrahedra and exhibits a typical zigzag arrangement. The crystal packing is stabilized by hydrogen bonds between these anions and hydronium cations. The two different hydronium cations are surrounded by nine O atoms of tetrachromate anions, with O
O distances ranging between 2.866 (8) and 3.282 (7) Å.
O distances ranging between 2.866 (8) and 3.282 (7) Å.organic compoundsOpen access
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The co-crystal, C7H9N4O2+·ClO4−·C20H24O6·3.25H2O, consists of theobrominium (3,7-dimethyl-2,6-dioxo-1H-purin-9-ium) cations, perchlorate anions and dibenzo-18-crown-6 and water molecules. The crown ether is in a bent conformation, in which the planes of the aromatic rings subtend an angle of 63.7 (1)°. Intermolecular O—HO hydrogen bonding between the water molecules and the O atoms of the cyclic ether delimit an empty space reminiscent of a hollow cage. The water molecules are additionally linked to the cations by N—HO hydrogen bonding. One of the positions of the water molecules is occupied only fractionally (25%) and is located outside this framework.
O hydrogen bonding between the water molecules and the O atoms of the cyclic ether delimit an empty space reminiscent of a hollow cage. The water molecules are additionally linked to the cations by N—HO hydrogen bonding. One of the positions of the water molecules is occupied only fractionally (25%) and is located outside this framework. 
