Analytical Chemistry and Spectroscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The Raman and infrared spectra of MgSO3·3H2O and of deuteriated and isotopically dilute samples of this hydrate were recorded from 100 to 4000 cm-1 at 90-300 K and analysed with regard to the bonding structure of the water of crystallization, the space group of this compound, i.e. P212121, Pnma or Pn21a, and assignment and coupling (with the H2O and D2O librations) of the internal modes of the sulphite ions. The orientational behaviour of single crystal Raman studies on isotopically dilute samples are firstly used for assigning the uncoupled OH (OD) stretching modes to the H positions in the lattice. From the three types of water of crystallization (in the case of space group P212121 or Pn21a), the orientationally disordered H2O1 and H2O11 are involved in very strong hydrogen bonds, uncoupled OH (OD) modes 3027 (2282) and 2930 (2240) cm-1, caused by the strong hydrogen bond acceptor strength of the sulphite ions. Both water molecules are extremely distorted with OH (OD) mode splittings up to 393 (218) cm-1, the largest known so far. H2O111 forms weak bifurcated hydrogen bonds. Both the stretching and bending vibrations of the sulphite ions coincide with rotational modes of the water molecules.
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