Neutron single crystal study of barium hydroxide iodide tetrahydrate, a compound with most strongly distorted hydrate H2O molecules

The crystal structure of barium hydroxide iodide tetrahydrate Ba OH . I4H 2 O space group P 1 over bar, No.2, Z 2, a 628.1 1 pm, b 806.1 2 pm, c 811.8 1 pm, alpha 90.47 1 deg., beta 106.87 1 deg., and gamma 90.97 1 deg., final R1 0.0398 I gt; 2 sig. I for 780 unique reflections was redetermined by single crystal neutron diffraction studies. Each of the four crystallographically different H2O molecules serves as donor of a very weak hydrogen bond to adjacent I ions and quite a strong one to an OH ion or another H2O molecule H2O II . Hence, the hydrate H2O molecules are extremely asymmetrically bound. The difference of the two internal O H distances of the H2O molecules Delta r O H reaches 8.1 pm at the most. As a consequence, the stretching modes of the H2O molecules are fully decoupled. The OH ion donates a very weak O H ... I hydrogen bond and accepts three HO H ... OH bonds. The different strength of the hydrogen bonds is discussed with respect to the acceptor capability of the hydrogen bond acceptor groups and the various OH frequency versus r O H and r H ... Y hydrogen bond distance correlation curves reported in the literature