LATTICE VIBRATION-SPECTRA .72. OH STRETCHING FREQUENCIES OF SOLID HYDROXIDES - CORRELATION WITH STRUCTURAL AND BONDING DATA

The OH stretching frequencies of OH- ions in solid hydroxides can be either higher or lower than the free ion value. The high-energy shift found for OH- ions which are not involved in H-bonds like OH-...X (OH-, H-bond donor) has been analysed with respect to metal-oxygen interactions, repulsion of the lattice potential and H-bonds of the type XH...OH-. The upshift of nu(OH) increases with a decrease in (i) the mean M...O distances, (ii) the relative volume increment of OH- and (iii) the mean H...O distances (OH-, H-bond acceptor). In the case of alkali and alkaline earth metal hydroxides, nu(OH) obeys the function nu(OH) = A + C/(r(M-O))2BAR + D/(r(M-O))3BAR. In the case of transition metal hydroxides, covalent parts of the metal-oxygen interaction counteract the high-energy shift. H-bonding of the type OH-...X gives rise to a downshift of nu(OH). These H-bonds are strengthened in the case of metal-oxygen interactions (synergetic effect, H.D. Lutz, Struct. Bonding (Berlin), 69 (1988) 97). The criteria of ascertainment whether H-bonds OH-...X are present or not are discussed and extended to weak bonds.