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)degrees, beta = 106.87(1)degrees, and gamma = 90.97(1)degrees, final R1 = 0.0398 (I > 2 sigma(I) for 780 unique r eflections) was redetermined by single-crystal neutron diffraction studies. Each of the four crystallographically different H2O molecules serves as do nor 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 mol ecules are extremely asymmetrically bound. The difference of the two intern al O-H distances of the H2O molecules Delta r(O-H) reaches 8.1 pm at the mo st. As a consequence, the stretching modes of the H2O molecules are fully d ecoupled. The OH- ion donates a very weak O-H-... I hydrogen bond and accep ts three HO-H ... OH- bonds. The different strength of the hydrogen bonds i s discussed with respect to the acceptor capability of the hydrogen-bond ac ceptor groups and the various OH frequency versus r(O-H) and r(H ... Y) hyd rogen-bond distance correlation curves reported in the literature.