PECULIARITIES OF TETRASYSTEM ORGANIC CRYSTALS. STRUCTURAL CLASS P , Z = 8(1,1,1,1). A.E. Obodovskaya, A.N. Kolyshev, P.M. Zorky. Chemistry Department, Moscow State University, Moscow 119899, Russia

The so-called polysystem organic crystals, in which chemically identical molecules occupy more than one orbit, specifically multisystem crystals with k3 (three or more molecules per asymmetrical unit), are of particular interest. The reasons for their occurence are yet unknown.

We have considered about ten representatives of the structural class P , Z = 8(1,1,1,1) with k = 4. In all of them the molecules are arranged in a specific way; their orientations appear to be not arbitrary.

A good example is the crystal structure of chlordiazepoxide, C₁₆H₁₄N₃OCl (CMABOX10). The substance contains two similar but crystallographically non-equivalent H-bonded dimers A - C and B - D; each of them exhibits the presence of local two-fold axis. These two axes skew at 88.1deg. , the distance between them is 2.40 Å. At the same time the A - C and B - D dimers can be transformed into one another by the third two-fold axis, which skew with each of the axes of the dimers at 44.1deg. . Thus, in this crystal one can see a tetramer with pseudosymmetry 2 composed of two dimers with the same pseudosymmetry.

In other representatives of this class we have found not only two-fold axes but also axes 2q of hypersymmetry; in such cases symmetrically non-equivalent molecules can be transformed into one another by a rotation through 180deg. and a shift (this shift is small in chlordiazepoxide).