ChemInform Abstract: CONFORMATION ANALYSIS. 6. THE MOLECULAR STRUCTURE OF GASEOUS TETRABROMOFORMALDAZINE AS DETERMINED BY ELECTRON DIFFRACTION

Abstract

The structure of gaseous tetrabromoformaldazine (BrZCNNCBrz) has been investigated by electron diffraction at a nozzle-tip temperature of 112 'C. In contrast to formaldazine itself which exists in two conformations, the tetrabromo com- pound was found only in a single, nonplanar form. The data were fit well by a model of C2 symmetry with planar NCBr2 groups, a torsion angle CNNC equal to 72.1 ' (5.2) relative to 0' for the planar anti form, and a torsion around the double bond NNCBr equal to 16.6' (3.9) taken in such a sense as to increase the separation between an unshared nitrogen electron pair and the bromine atom syn to it in the adjacent NCBrz group. Other geometrical parameter values with uncertainties estimated at 20 are r(C-Br) = 1.877 8, (0.005), r(N-N) = 1.381 8, (0.018), r(C=N) = 1.266 8, (0.008), LCNN = 114.5' (l.6), and LNCBr = 126.2' (1 .O) and 1 18.2' (I. 1) where the last two angles correspond to a rocking displacement of the CBrz group away from the nitrogen atom of the adjacent NCBrz group. The conformation is discussed in comparison with similar mole- cules. Our recent investigation of the structure of gaseous for- maldazine' (2,3-diaza-1,3-butadiene) revealed that the mol- ecule existed as a mixture of two rotational conformers, a more stable anti (4 = 0) form and a less stable gauche (4 = 120O) form. Replacement of the hydrogen atoms in formaldazine with much larger atoms such as bromine would be expected to create larger steric interactions between the terminal (bromine) atoms and the lone-pair electrons on the nitrogen atoms in a possible anti conformer, and between bromine atoms on opposite ends of the molecule in a gauche or syn conformer. Such interactions would clearly play an important role in de- termining both the nature and the relative amounts of con- formers of the molecule. No structure information has so far been reported for tetrabromoformaldazine and we decided to study it in connection with our program of work on the struc- tures and compositions of conjugated systems.