Crystal Structure of a Highly Efficient Clarifying Agent for Isotactic Polypropylene

Using a combined approach based on X-ray powder diffraction and solid-state NMR spectroscopy, we were able to determine the crystal structure of 1,3,5-tris(2,2-dimethylpropionylamino)benzene, an efficient clarifying agent for isotactic polypropylene. The XPRD data and ID solid-state NMR experiments allowed to exclude most of the possible primitive orthorhombic space groups with the exception of 11. The structure solution was carried out using real space methods including a close-contact penalty. Four space groups lead to reasonable wRp values below 10%. (CC)-C-13-C-13 double quantum (DQ) experiments of the labeled carbonyl group were measured with a supercycled symmetry based dipolar recoupling sequence. Taking into account the wRp values after Rietveld refinement as well as simulations of DQ build-up curves based on nine spin systems, the space group P2(1)2(1)2(1) is clearly favored. The largest dipolar coupling within these spin systems was about 60 Hz corresponding to a distance of 5 angstrom. 1,3,5-Tris(2,2-dimethylpropionylamino)benzene crystallizes in an orthorhombic metric (a = 14.91(5) angstrom, b = 24.05(8) angstrom, c = 6.80(2) angstrom). The molecules are arranged in a pseudohexagonal rod packing with medium-strong hydrogen bonds and pi-stacking. The antiferroelectric arrangement of neighbored rods leads to an averaging of the net dipolar moment in the whole crystal.