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

Abstract

Abstract The crystal structure of barium hydroxide iodide tetrahydrate Ba(OH)I · 4 H2O (space group P[unk], No. 2, Z = 2, a = 628.1(1) pm, b = 806.1(2) pm, c = 811.8(1) pm, α = 90.47(1)°, β = 106.87(1)°, and γ = 90.97(1)°, final R1 = 0.0398 (I > 2σ(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 Δr 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.