The use of restraints in Rietveld refinement of molecular compounds; a case study using the crystal structure determination of tryptamine free base.
The previously unknown crystal structure of the biogenic compound tryptamine, in the form of a free base (C(10)H(12)N(2)), has been solved from X-ray powder diffraction data using simulated annealing followed by restrained Rietveld refinement [space group P2(1)2(1)2(1), a = 12.28593 (6), b = 8.53351 (4), c = 8.49385 (4) A, Z = 4, final reduced-chi(2) = 5.255]. A restrained Rietveld refinement was carried out in which the global weight factor, f, of the stereochemical restraints was gradually lowered. The effect of the relaxation of restraints on the crystal structure and on chi(2) was studied and a criterion for the final choice of f is reported. The crystal structure reported here shows efficient packing involving weak intermolecular hydrogen bonding and a herringbone-type packing pattern.