Intramolecular coupling of BrO stretching vibrations in solid bromates, infrared and Raman spectroscopic studies on M(BrO3)2.6H2O (M = Mg, Co, Ni, Zn) and Ni(ClO3)2.6H2O

Infrared and Raman spectra of the isostructural cubic halate hexahydrates M(BrO3)2.6H2O (M = Mg, Co, Ni, Zn) and Ni(ClO3)2.6H2O (space group, Pa3; Z = 4) are presented. They are discussed with respect to the strength of the O-H...OXO2 hydrogen bonds (hydrogen bond acceptor capability, synergetic effect) and the order of the BrO stretching modes. In the case of undistorted bromate ions, e.g. at C3 lattice sites, the order of the symmetric (v1) and asymmetric (v3) XO stretching modes is v1 < v3 as for ClO3- but in contrast to IO3- with v1 > v3. The relative order of v1 and v3 of halate ions is mainly governed by the specific masses of the halogen atoms and the angles of the XO3- ions. The latter decrease in the sequence ClO3- (107degrees) > BrO3-> IO3- (< 100 degrees). The Raman scattering intensities of the asymmetric XO stretching modes v3 of the title compounds are unusually low (< 5% those of v1).