Neutron‐Diffraction Study of the Structure of Potassium Oxalate Monohydrate: Lone‐Pair Coordination of the Hydrogen‐Bonded Water Molecule in Crystals

Local pseudosymmetry of the water molecule in BaBr2 · 2H2O, a neutron diffraction study

Barium bromide dihydrate, BaBr2 · 2H2O, Mr = 333.188, monoclinic C2/c, a = 1042.9(1), b = 719.5(1), c = 837.5(1) pm, β = 113.60(1)°, V = 0.5759(1) nm3, Z = 4, Dx = 3.843 Mg · m−3, λ(neutron) = 123.75 pm, μ(calc.) = 0.146 mm−1, F(000) = 61.746 fm, T = 297 K, R = 0.0494, wR = 0.0469 for 370 observed unique reflections. The present neutron diffraction study confirms the results of X-ray structure determination with respect to the heavy atoms. Additionally, previous spectroscopic findings can now be related to structural features. The energetic equivalence of the two crystallographically independent OH bonds can be explained by (i) the very similar hydrogen bond lengths [H … Br: 243.7(6) and 244.3(7) pm] and (ii) the symmetric coordination sphere of the tetrahedrally coordinated water molecules. The occurence of almost tetrahedral [Br2(H2O)2]2− units enables significant intermolecular vibrational interactions of the water molecules, which explains the unusually large Davydov splittings of the water bands.

Neutron diffraction study of Mn(NO3)2· 4H2O

Positions of the hydrogen atoms in the structure of Mn(NO3)2· 4H2O, were determined from 692 independent intensities, measured on a neutron diffractometer at the RA reactor in Vinča. The positional and isotropic thermal parameters for all atoms were refined by least squares to anRvalue of 0.10. All hydrogen interactions in the structure are normal, not bifurcated, and weak, ranging from 2.72(2) to 2.88(2) Å. The H–O–H and O–H…O angles are between 103(2) and 109(2)°, and 152(2) and 177(2)°, respectively.

Metal-water interactions and hydrogen bonding in dittmarite-type compounds M'M''PO4.H2O (M'=K+, NH4+; M''=Mn2+, Co2+, Ni2+). Correlations of IR spectroscopic and structural data

The vibrational behavior of the uncoupled nu(OD) modes and of the water librations in dittmarite-type compounds M'M''PO4.H2O (M'=K+, NH4+; M''=Mn2+, Co2+, Ni2+) is analyzed in terms of the influence of two types of metal-water interactions (M+-H2O and M2+-H2O), the hydrogen bonding and the repulsion potential of the lattices. The M+-H2O interaction is found to be the main factor, which influences nuOD. The strong K+-H2O interaction weakens in a higher degree the intramolecular O-H bonds than the corresponding M2+-H2O interactions (M2+=Mn, Co, Ni). As a result nuOD is shifted to lower wavenumbers in the potassium series than in ammonium one, irrespective of the synergetic effect of M2+, the hydrogen bond lengths and the repulsion potential of the lattices. The analysis of the spectroscopic data evidences for the dominating influence of the M2+-H2O interaction on the wagging mode. The blue shift of nuwag strictly follows the increasing synergetic effect of M2+, i.e. nuM'/Mn<nuM'/Co<nuM'/Ni, in all cases, irrespective of the strength of the M+-H2O interactions, the hydrogen bond lengths and the higher repulsion potential of the K-lattices. The rocking mode is insensitive to the replacement of the M2+ ions. Some relations between the spectroscopic and structural data are given.

A logical concept of structure prediction derived from supramolecular polymers of alkaline Earth metal halides formed by hydrogen bonding and complexation of the metal ion

Several different dimensional polymers derived from alkaline earth metal iodides are obtained as a result of supramolecular noncovalent bonding modes of the metal ion, namely complexation and hydrogen bonding. These polymers consist of complex cations linked to the halide ions by hydrogen bonds of the water ligands coordinated to the metal. They are built up in a logical way, depending on the ratio of complexing ligands to complexing and hydrogen-bonding ligands so that their dimensionality and, to a certain extent, their structure can be predicted.

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)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.