1973 Volume 46 Issue 10 Pages 2998-3004
The crystal structure of hemimellitic acid dihydrate (benzene-1,2,3-tricarboxylic acid dihydrate) has been determined by the X-ray diffraction method. The crystals are triclinic, with a space group of P\bar1 and with cell dimensions of a=11.02, b=9.12, c=8.72 Å, α=106.2°, β=140.2°, and γ=84.2°. The structure was determined by an inspection of a sharpened Patterson map. The final R value was 9.25% for 1532 observed reflections. The hemimellitic acid molecule has the approximate Cs symmetry. Three carboxyl groups twist by 4.5°, 86.8°, and 10.3° out of the plane of a benzene ring. The hemimellitic acid and water molecules are joined by seven independent O–H···O hydrogen bonds to form a three-dimensional network. The water molecules are hydrogen-bonded to each other around a center of symmetry to form a unique four-membered ring. The hydrogen bonds observed are classified by means of the combination of donor and acceptor groups in five types. These types of hydrogen bonds are discussed in the comparison with several carboxylic acid hydrates.
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