X-ray analysis of the dibenzyl series III—The structure of stilbene, tolane, and azobenzene

Abstract

The first two parts of this work described a detailed quantitative analysis of the structure of dibenzyl, which crystallizes in the monoclinic system, space group C 5 2 h (P2 1 / a ), with two centro-symmetrical molecules in the unit cell. From preliminary data already published it is known that stilbene, azobenzene, and tolane, also monoclinic, are in many ways similar to dibenzyl, but differ from it in the fact that their c axes are doubled in length, the unit cells containing four instead of two chemical molecules. The comparison is best seen from the figures given in Table I. The structural change encountered when we pass from dibenzyl to the other three compounds is not merely a simple doubling of the c axis with some small displacement of what were formerly (in dibenzyl) an identically oriented set of molecules. This is shown by rotation photographs taken about the c axis of stilbene, where the new reflexions of odd l index, forming the intermediate layer lines, are not weak but have an average intensity quite comparable to the other reflexions. Further, it has previously been shown (Prasad, loc. cit .) that stilbene, tolane, and azobenzene possess a pseudo-orthorhombic structure, with the (201 ¯ ) as an approximate plane of symmetry. Thus the (202 ¯ ) and the (200) reflexions are of nearly equal intensity and spacing, and so are the (203 ¯ ) and (201), (204 ¯ ) and (202), (403 ¯ ), and (401 ¯ ), etc. With stilbene, which has been examined in greater detail than the other two compounds, it is found that this approximate symmetry breaks down to some extent in the higher orders. For example, the (207), (208), and (209) reflexions are definitely present, although weak, but only avery faint trace of their symmetrical reflexions, the (209 ¯ ), (2010 ¯ ), and (2011 ¯ ) can be seen. Perhaps the most notable exception occurs in the (60 l ) reflexions, where the (601) is entirely absent, but the (607 ¯ ) has medium intensity. These observations, for stilbene, also serve to define which of the two almost identical crystal directions we have chosen as the c axis. If the c axis were taken in the (202 ¯ ) plane instead of the (200), then its length would be 15·7 instead of 15·9 A, and the angle β would be 112° instead of 114°.