The Crystal and Molecular Structure of Pyromellitic Acid Dihydrate (Benzene-1,2,4,5-tetracarboxylic Acid Dihydrate)

Abstract

Abstract The crystal structure of pyromellitic acid dihydrate has been determined by the method of X-ray diffraction. The crystal is triclinic in the P\bar1 space group, with Z=1 and cell dimensions of a=10.05, b=6.45, c=5.45 Å, α=74.5°, β=112.2°, and γ=77.3°. The structure was derived from the three-dimensional Patterson function and was later refined by the least-squares method, using the anisotropic thermal parameters, to give a final R index of 7.4%. All the hydrogen atoms were located on the difference-synthesis map. The average C(ring)–C(ring) distance is 1.386 Å, while the average C(ring)–C(carboxyl) distance is 1.496 Å. One carboxyl group is twisted by 17.9°, and the other is twisted by 74.5°, out of the plane of the benzene ring. The C–O(H) and C=O distances differ significantly from each other, the average values being 1.302 and 1.211 Å respectively. Four types of hydrogen bonds, with an average O–H···O distance of 2.732 Å, form a three-dimensional network. Thus, each molecule is joined, through hydrogen bonds around the center of symmetry, to two other molecules, forming an endless chain in the [1 \bar1 1] direction. Water molecules are arranged on the plane parallel to (1 1 0) so as to hold the chains together by hydrogen bonds.