Single‐crystal Raman studies on nickel iodate dihydrate, Ni(IO 3 ) 2 ·2H 2 O

Single‐crystal Raman spectra in the range 4000–35 cm −1 and the results of group theoretical treatment including Cartesian symmetry coordinates of nickel iodate dihydrate, Ni(IO 3 ) 2 · 2H 2 O, (space group Pbca , Z = 4) are presented and discussed with respect to the strength of the HO—HOIO 3 hydrogen bonds and the extent of intramolecular (intraionic) coupling of the stretching vibrations of both the hydrate H 2 O molecules and the iodate IO 3 − ions. The energies and relative intensities of the IO stretching modes in the various species of the unit‐cell group D 2 h reveal that the iodate ions of the title compound behave as if they possess pseudo C 2 v site symmetry with a at least partly uncoupled IO stretch due to the significantly shortest (175.5 pm) IO arm and a symmetric (ν s ) and an antisymmetric stretching vibration (ν as ) due to the remaining IO 2 group ( r I–O = 184.2 and 186.0 pm). The respective site‐group modes decrease in the order ν 3 a ↔ ν IO(1) (mean value of the respective unit‐cell group modes: 804 cm −1 ) > ν 1 ↔ ν s (772 cm −1 ) > ν 3 b ↔ ν as (744 cm −1 ) (ν 1 and ν 3 as for distorted C 3 v IO 3 − ions). For the IO 3 − bending modes (430–300 cm −1 ) and the lattice modes below 300 cm −1 the true vibrational modes are not known. The normally used assignment to a symmetric (ν 2 ) and an asymmetric bending vibration (ν 4 ) as for C 3 v IO 3 − ions is at least questionable. The hydrogen‐bond distances calculated from the wavenumbers of the OD stretching modes of matrix‐isolated HDO molecules do not fully agree with the data of a neutron diffraction study performed in 1972. Copyright © 2001 John Wiley & Sons, Ltd.