W0380

Form III of 2,2’,4,4',6,6'-Hexanitroazobenzene (HNAB-III). Bruno Morosin¹, Mark A. Rodriguez¹, Edward Graeber¹ and Charles F. Campana²,¹Sandia National Laboratories, P.O. Box 5800, MS-1405, Dept. 01822, Albuquerque, NM 87185, ²Bruker AXS, Inc., 5465 East Cheryl Parkway, Madison, WI 53711.

2,2’,4,4',6,6'-Hexanitroazobenzene (HNAB) is extremely complex in its crystallographic behavior, as evidenced by the relationships among the various phase transformations. There are five crystallographic polymorphs of HNAB. Two of these structures were solved by the authors over 30 years ago¹. The present structure (HNAB-III) was finally solved as a pseudo-merohedral twin (monoclinic space group P2₁ [a =15.4015(8) Å, b = 5.5240(3) Å, c = 22.1182(11) Å, β = 110.367(2)°], emulating orthorhombic space group C222₁ [a =15.4015(8) Å, b = 41.471(18) Å, c = 5.5240(3) Å]) using a dual space recycling method² as implemented in the Bruker XM program.

The structure was refined with the Bruker XL program using a two-fold twinning law. The structure has been refined to R1 = 3.62% with Z = 4 and a twinning ratio of 0.448. The absolute structures of the two independent molecules are inverted with respect to each other.

Details of the twin analysis will be presented. The HNAB-III structure will be compared to the other published polymorphs as well as with the related compound, hexanitrostilbene.