On the supramolecular symmetries of the piedfort units formed by 2,4,6-triaryloxy-1,3,5-triazines

The basic forms of supramolecular symmetries in the molecular diads (Piedfort units, PUs) observed in the crystal structures of 2,4,6-triaryloxy-1,3,5-triazines (n-RPOT, n = 2, 3, 4, R = halogen atoms or alkyl groups) are revisited, their extended canonical classification is given, and descriptions including symbols and graphical presentation are improved. The semirigid molecules in their column are stacked around a C-3 axis which may associate (i) with three parallel glide planes (C3(g)) having the angle of 120 degrees, (ii) centers of inversion (C-3i), or (iii) three perpendicular 2-fold axes (D-3) with an angle of 120 degrees. In addition, the,present work demonstrates that the columns formed by the C-3(g)-PUs (space group R3c) and C-3i-PUs (space group R3) are related by noncrystallographic rotations (ncr). Similarly, the columns containing D-3-PUs are formed from C3i-PUs by ncr's. It is proved that the occurrence of the D-3-PUS is inseparable from the presence of the C3i-PUs in the crystal structures of several 3-RPOT derivatives in space group P3c1.