Knowledge Bank

Room and low temperature infrared spectra of thin films of calcium sulfate dihydrate, containing small percentages of hydrated OH and OD in $CaSO4+2D2O$ and $CaSO4\cdot 2H2O$, respectively, have been obtained between 3700 and $2300cm-1$ Based on probability theory, the two absorptions observed in the OH stretching region of the nearly deuterated hydrate have been assigned to intra- and intermolecularly uncoupled vibrations of the hydrated isotopic species HOD. It will be shown that these uncoupled bands arise from two physically different OH vibrations of a water molecule, which is distorted by its local environment in the crystal, and not from the presence of proton tunnelling in $CaSO4\cdot 2H2O$ as has been previously suggested. The same conclusions apply to the two uncoupled OD(HOD) absorptions observed in the OD stretching region of partially deuterated $CaSO4\cdot 2H2O$. The large observed splitting of the two uncoupled OH (and OD) absorptions indicates that the water molecule is distorted to a greater degree from $C2v$ symmetry than is found in the neutron diffraction study of $CaSO4\cdot 2H2O$.