NON-TOPOLOGICAL PHASE TRANSITION OF N-2,4-DINITROPHENYL-o-ANISIDINE. K. Takada, K. Taguchi, M. Yasui and F. Iwasaki, Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182, Japan

N-(2,4-dinitrophenyl)-o-anisidine (1) has two crystalline modifications, orange and red forms. The orange form transforms to the red form at the wide temperature range, 383~431K, and these transition points are depending on the specimens. During the crystals of the orange form were kept at 393 K for several hours, some orange crystals partially turned to the red form. The red region of these partially changed crystals could be kept in room temperature for long time. X-ray oscillation and Weissenberg patterns of the orange, the red and the boundary parts of these crystals observed by a Weissenberg-type diffractometer with the imaging plates revealed that the b axis of the transformed red form was always coincident with the b axis of the orange form, although powder diffraction rings with preferred orientation were also observed in the red part. Crystal data: Orange form, P2/a, a=21.561(5), b=4.219(1), c=16.749(5)Å, [[beta]]=122.30(1)deg., Z=4, R=0.060 for 1842 reflections. Red form, P2l, a=15.693(2), b=10.981(3), c=3.842(1)Å, [[beta]]=98.39(2)deg., Z=2, R=0.079 for 886 reflections. The molecules of these forms are rotational isomers. In the orange form an intramolecular hydrogen bond is formed between the NH and o-nitro groups, while in the red form the NH group is hydrogen-bonded to the o-nitro and methoxy groups bifurcately, so that the methoxyphenyl group rotates by about 60deg. during the transformation. In the crystals the molecules are arranged approximately perpendicular to the shortest axes, b for the orange form and c for the red form. However, the diffraction patterns showed that the solid-phase transformation occurred coinciding of the b axes of both forms. This fact suggests that a mechanism of the transformation cannot be understood from topological arrangements. The transformation from the orange to the red forms retaining the b axes should be performed by twisting of the molecules together with an intramolecular rotation of the o-methoxyphenyl group. Such motion needs a large space, so that this rearrangement may occur from defects in the crystals as shown from the wide transition temperatures and the fact of changing to a polycrystalline state.