PSEUDOINVERSION CENTERS IN SPACE GROUP P1 AND A REDETERMINATION OF THE CRYSTAL-STRUCTURE OF 3,4-DIMETHOXYCINNAMIC ACID - A STUDY OF NON-CRYSTALLOGRAPHIC SYMMETRY

A survey of the 11691 P1BAR crystal structures in the Cambridge Structural Database shows that 1166 have Z = 4. Of these, a mere 20 have local pseudocenters of symmetry relating the atoms in the two halves of the asymmetric unit. The coordinates of these local pseudocenters often include the special values of 0, 1/4 and 1/2, but can also be perfectly general. As an example of the latter, the crystal structure of 3,4-dimethoxycinnamic acid (C11H12O4, M(r) = 208.21) was re-examined at both room temperature and at 173 K. The poor refinement reported in the original study of this compound was ascribed to the low data/parameter ratio and the presence of the pseudocenter. The present study, however, clearly demonstrates that the refinement is unaffected by the presence of the pseudocenter when the data/parameter ratio is reasonable (> 5). This could be true in this specific case because the center is located at a general position: 0.217, 0.433, 0.319. At room temperature, a = 8.449 (1), b = 15.034 (2), c = 8.449 (1) angstrom, alpha = 99.47 (1), beta = 94.57 (1), gamma = 101.53 (1)-degrees, V = 1029 angstrom3, 1893 reflections, 367 variables, R = 0.041, wR = 0.038. At 173 K, a = 8.412 (2), b = 14.959 (2), c = 8.341 (2) angstrom, alpha = 100.28 (2), beta = 94.80 (2), gamma = 101.83 (2)-degrees, V = 1003 angstrom3, 2543 reflections, R = 0.041, wR = 0.040.