DETERMINATION OF THE CHANGING DIPOLE MOMENT OF $OH^{-}$ IN CRYSTALS OF Li(OH). $Ca(OH)_{2}$ AND $Mg(OH)_{2}$
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Date
1962
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Ohio State University
Abstract
Lithium hydroxide crystallizes tetragonally and belongs to the space group $D^{7}_{th}-p4/nm$. Calcium hydroxide and magnesium hydroxide are isomorphic and belong to the space group $D^{3}_{3d}-P 3m$. These crystals contain two hydroxyl ions per unit cell with their dipoles oriented parallel to the crystalline c-axis and are related to each other through a center of inversion. Consequently from the crystal symmetry two hydroxyl internal vibrations, belonging respectively to the species $A_{1g}$ and $A_{2u}$ are predicted Experiments indicate these hydroxyl vibrations to be split by $14 cm^{-1}, 26 cm^{-4}$ and $45 cm^{-1}$ respectively in LiOH, $Ca(OH)_{2}$ and $Mg(OH)_{2}$. This frequency separation between the infrared and Raman hydroxyl internal modes is indicative of a small intermolecular coupling between the hydroxyl ions. By assuming an oscillating dipole-dipole interaction it is possible to calculate the magnitude of the hydroxyl changing dipole moment. The value for $\frac{\partial \mu}{\partial q}$ corresponding to the $OH^{-}$ internal vibration in Li(OH), $Ca(OH)_{2}$ and $Mg(OH)_{2}$ is determined to be 1.35, 1.54 and 1.81 Debye/{\AA}.
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Author Institution: U. S. Naval Ordnance Laboratory