The Raman and infra-red spectra of some solid hydroxides

4. Summary

In each of the cases of LiOH, LiOH·H₂O, NaOH, Ca(OH)₂ and Mg (OH)₂ the stretching vibrations of the hydroxyls antisymmetrically with respect to the centre of inversion give rise in infra-red absorption to a single sharp intense maximum at about 3650 cm.⁻¹ as is to be theoretically expected. In all cases, the infra-red active hydroxyl frequency is found to be higher than the corresponding Raman active mode and the origin of this feature is explained.

The spectrum of Ca (OH)₂ recorded with single crystals exhibits on either side of the principal maximum due to the hydroxyl vibration, numerous other maxima which have hitherto remained unexplained. Some of them arise due to combinations between the observed Raman active lattice frequencies and the infra-red active hydroxyl frequency. The rest are explicable as combinations between the Raman active hydroxyl frequency and infra-red active lattice frequencies. Features similar to those observed in the spectrum of Ca (OH)₂ are exhibited by Mg (OH)₂ also, and have been explained in an analogous fashion. No experimental data regarding the infra-red active lattice frequencies of Ca (OH)₂ and the Raman active and infra-red active lattice frequencies of Mg (OH)₂ are available. The values of these are also revealed by the above analysis.