POLYHALIDE ANIONS IN CRYSTALS .3. THE BR-8(2-) ANION IN DIQUINUCLIDINIUM OCTABROMIDE, THE CRYSTAL-STRUCTURES OF ME(4)PBR(3) AND QUINUCLIDINIUM TRIBROMIDE, AND AB-INITIO CALCULATIONS ON POLYBROMIDE ANIONS

The crystal structure of diquinuclidinium octabromide contains the hit herto unreported centrosymmetric planar, roughly Z-shaped Br-8(2-) ani on. The Br-3(-) anion in Me(4)PBr(3) is only slightly distorted, where as the Br-3(-) distortion in quinuclidinium tribromide is considerable . Detailed examination of the geometry of asymmetric Br-3(-) anions in crystals shows that the two Br-Br bond lengths are correlated and tha t the correlation is representable by a simple power function, leading to an estimate of the Br-Br bond length in the symmetric Br-3(-) anio n of 2.541{24} Angstrom. A relationship is also shown to exist between the N ... Br and H(N)... Br distances in N-H(N)... Br hydrogen bonds in organoammonium and related bromides. The high linear correlation of the X-X bond lengths in the X(8)(2-) (X = Br, I) anions provides a us eful means of estimating Br-Br bond lengths from the corresponding I-I bond lengths and vice versa. Ab initio RHF/6-311G optimizations of t he equilibrium geometries of several Br-n(m-) anions reveal that linea r Br-n(-) are to be formulated as Br-(Br-2)(2k) when n = 1, 5, 9 = 4k + 1, and as Br-3(-) (Br-2)(2k) when n = 3, 7 = 4k + 3. The V-shaped Br -5(-) anion has an only slightly lower total electronic 6-311G energy than the linear Br-5(-). For Br-7(-) the energy is lowest for the C-3 v (trihedral) configuration and increases in the order C-3v < D-3h (pl anar) < D-infinity h (linear) for the other configurations examined.