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 1D
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%. 13C13C 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 P212121 is clearly favored. The largest dipolar coupling within these
spin systems was about 60 Hz corresponding to a distance of 5 Å.
1,3,5-Tris­(2,2-dimethylpropionylamino)­benzene crystallizes in an orthorhombic
metric (a = 14.91(5) Å, b =
24.05(8) Å, c = 6.80(2) Å). The molecules
are arranged in a pseudohexagonal rod packing with medium-strong hydrogen
bonds and π-stacking. The antiferroelectric arrangement of neighbored
rods leads to an averaging of the net dipolar moment in the whole
crystal.