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Co-crystals of hexamethylenetetramine and resorcinol were investigated by X-ray diffraction. The structure was refined in the superspace group Xmcm(0beta0)s0s, X=(1/2 1/2 0 1/2). In the average structure the resorcinol molecules are disordered between two orientations. The main effect of the modulation in the structure is a harmonic modulation of the occupation probabilities of the two orientations of the resorcinol molecule. However, the modulated order is not perfect and the resorcinol molecules remain partially disordered. Below 270 K the crystal undergoes a phase transition to a commensurately modulated structure with modulation vector q=(0 1/2 1/4) and superspace group X2/m(0betagamma)0s. The structure of the low-temperature phase could not be determined owing to the poor quality of the crystals affected by the reconstructive phase transition.

The 1,10-decanedioic acid-1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane (1/1) system, C(10)H(18)O(4).C(6)H(12)N(4), was studied at 215 (2) K. Its analysis provides important information with regard to the long-standing acid-carboxylate controversy in the urotropine-alkanedioic acid system. In the present structure, all the chain end-groups display a clear acid character. The asymmetric unit of this commensurate modulated phase contains two molecules of diacid as well as two molecules of urotropine. Furthermore, the chain packing suggests a possible order parameter for the lock-in transition.

The charge-density distribution has been determined from 333 independent X-ray reflections with sin theta/lambda < 1.47 A-1, which were collected with Ag K alpha radiation. As found in a recent refinement using room-temperature data, full-matrix least squares refinement with a multipole model gives rise to an almost complete correlation between certain octapole deformation terms. This is the worst case example of a problem which will always arise to some degree when applying the multipole model in a noncentrosymmetric space group. In this example, the correlation arises between terms which are predominant for describing the bonding density between atoms of the molecule, thereby causing the deformation charge density to have little meaning. However, the total molecular electrostatic potential and the molecular octapole moment are reliably obtained.