Raman and infrared spectra of barium and strontium hydroxides and hydroxide hydrates

Abstract

The Raman and i.r. spectra of the barium and strontium hydroxides and hydroxide hydrates Ba(OH) 2·8H 2O, Ba(OH) 2·3H 2O, β- and γ-Ba(OH) 2·1H 2O, α- and β-Ba(OH) 2, Sr(OH) 2·8H 2O, Sr(OH) 2·1 H 2O and Sr(OH) 2 are reported and structural and bonding features are discussed. The Raman spectra of the lower hydrates of barium and strontium hydroxide, viz. Ba(OH) 2·3H 2O, Sr(OH) 2·1H 2O, and β- and γ-Ba(OH) 2·1H 2O, do not show any water band, either as stretching or bending modes. This behaviour is probably due to the strong hydrogen bonding of the water of crystallization in the barium and strontium hydroxide hydrates. The hydroxide ions of most compounds under investigation, however, are not hydrogen bonded. This is shown from the small negative temperature shift of the OH (or OD) stretching modes in isotopically dilute samples. A positive temperature shift of the uncoupled stretching modes, as found for some of the hydroxide ions in Sr(OH) 2 and α- and β-Ba(OH) 2, is an unfailing indicator that hydrogen bridges are present. The stretching vibrations of non-hydrogen-bonded hydroxide ions have been found to vary from 3690 to 3500 cm −1. The possible reason for these large frequency shifts, i.e., weakening or hardening of the stretching frequency of free OH ions by the lattice potential, is discussed. Some rotatory modes of hydroxide ions and water molecules have been observed at unusually high frequencies above 1000 cm −1.