Absolute chirality and crystal structure of barium nitrite monohydrate, Ba(NO2)2.H2O

The absolute chirality of barium nitrite monohydrate, Ba(NO2)2.H2O [Mr=247.4, hexagonal, space group P61, a=7.070(1), c=17.886(2) A, V=774.3 A3, R=0.017 for 1792 observed reflections] has been determined at room temperature through a combination of X-ray diffraction and optical study. For a crystal that is optically dextrorotatory in the visible region of the spectrum, the space group is found to be P61. Classical polarizability theory is successfully used to calculate the optical rotation and refractive indices in this crystal and results in good agreement with experiment are obtained. Using the results of these calculations and the principles of anisotropic polarizability theory, a correlation between the sign of the observed optical rotation and the arrangement of the atoms in the crystal structure is established. The interactions of most importance to the optical rotation are shown to be those between the highly polarizable Ba ions (Ba2+) and the O atoms of the water molecules [O(W)]. The interdependence of the Ba2+ and O(W) species in their response to the propagating electromagnetic field results in the consideration of polarizable Ba-O(W) 'units' giving a quasimolecular contribution to the optical rotation. A structural helix of these Ba-O(W) units is identified as the principal structural origin of the optical rotation. (orig.)