Some apparent periodic errors in the crystal lattice of the molecular complexes of 4:4'-dinitrodiphenyl with 4-iodo- and 4-bromodiphenyl

Abstract

[Plate 23]Oscillation photographs from crystals of the complexes of 4:4/-dinitrodiphenyl with 4-bromoand 4-iododiphenyl show a mixture of sharp and diffuse spots. When the oscillation is about the c-axis the spots on every seventh layer-line are observed to be sharp, those on all the other lines being diffuse. The diffuseness varies from line to line, but in all cases corresponds to an elongation of the maxima of the interference function in the reciprocal-lattice space in the 6*-direction. The spacing of the diffuse layer-lines in the c-axis photographs from the complex with 4-bromodiphenyl is anomalous, and the positions of the lines correspond to those of optical ghosts accompanying the sharp layer-lines, such as would be expected if a fundamental spacing of 3-69 A were periodically distorted, the period of the distortion being rather less than 3£ times that of the fundamental spacing. The diffuseness of the spots suggests that the structure also contains faults in the 6-direction. A suggested explanation of the effects in terms of the structure is put forward.In the second part of the paper, a mathematical discussion of diffraction by a simplified model, consisting of a lattice with a periodic error in spacing in the c-direction, and faults consisting of sudden random changes in the phase of a periodic error in lattice planes per pendicular to the 6*-axis, is discussed, and it is shown that the diffraction pattern given by such a structure exhibits features similar to those observed.

Organic molecular compounds with modulated crystal structures

“…Generally, the mutual frustrations of conflicting intramolecular and intermolecular forces are believed to be one possible reason for the modulation of the crystal structures of molecular compounds [67,68]. As seen in the above mentioned "historical" examples of the 4 : 4 0 -dinitrodiphenyl complexes [35,36] and in the urea inclusion compounds [37], also a conflict in the packing of the molecules due to "non-compatible" sizes of the involved molecules can result in a modulation of the crystal structure. Depending on the rigidity of the molecules then also the molecular conformation might be modulated.…”

“…Some of the first examples questioning perfect threedimensional periodicity in organic compounds might be the molecular complexes of 4 : 4 0 -dinitrodiphenyl with 4-chloro-, 4-bromo-, 4-iodo-and 4 : 4 0 -diiododiphenyl, published by James and Saunder in the years 1947 and 1948 [35,36]. The structures are built by layers of dinitrodiphenyl molecules in a face-centred array creating tubular cavities, in which the halogenated diphenyl molecules are placed.…”