The crystal structure of terephthalic acid

Abstract

387 C(5) and C(6) relatively large ones. The anisotropic parameters of the ruthenium atom correspond to values of B ranging from 2.8 (in a direction approximately parallel to e) to 2.0 (approximately along a). The disordered modification of disorder was proposed by Trotter (1958) for bis-indenyliron. In view of the near equality of the lengths of the c axes in the two modifications, there appears to be a simple relationship between the ordered and the disordered structures. This relationship is shown in Fig. 5. Visual intensity data for the hkO reflections of the disordered modification of bisindenylruthenium (see Experimental) were obtained from a set of Weissenberg photographs prepared with Cu Ka radiation. The Pat-terson projection onto (001) could be interpreted on the basis of four ruthenium atoms, each of half weight, in the unit cell; phases calculated from these ruthe-nium positions led to the electron-density projection shown in Fig.4. Since this projection is down a relatively short (6.2 A) axis, separate molecules must be resolved. Accordingly, it seems quite clear that each molecule can assume either of two orientations with equal probability, one orientation being related to the other by a twofold rotation about an axis perpendicular to the rings and passing approximately through the centers of the two C(8)-C(9) bonds. A similar sort We are grateful to Dr D. Hall for providing us with a sample of the compound. One of us (N.C.W.) is grateful to the U.S. Rubber Company and to the National Science Foundation for providing financial support. X-ray diffraction photographs of terephthalic acid have shown the existence of two polymorphic forms which have closely related triclinic unit-cell dimensions. The crystal structures of both these forms have been determined from two-dimensional data, and a full three-dimensional analysis, including the calculation of anisotropic temperature factors, has been carried out for one of these forms. The R index over 470 F(hkl) was 7.5 %. The bond lengths indicated a small amount of quinonoid character in the benzene ring, and a slight departure from planarity between the ring and the carboxyl groups. The molecules pack together in the crystal in systems of infinite chains linked together by double hydrogen bonds of length 2.608/~.