DB D DB Highresolution twoaxis diffractometer DB Applications NEUTRONS FOR SCIENCE Selected examples • The structural chemistry of non-rigid molecules • Ab-initio structure solution from powders The diamminehydrogen ion N 2 D 7 + Proton transfer along hydrogen bonds is an important process in biological and chemical systems.There has been considerable theoretical interest in model systems of the type: H 3 O+-H 2 O, NH 4 +-H 2 O, NH 4 +-NH 3 and others. The diaquahydrogen ion O 2 H 5 + is known to exist in the crystalline state and has been characterized structurally. The isoelectronic N 2 H 7 + ion was known to exist in the gas phase revealing a [NH 3 __H.....NH 3 ]+ topology. High-resolution neutron powder diffraction using D2B was able to show that this cation exists not only in the solid state but also reveals the "freezing" of various internal motions when cooling, and its subsequent distortion. Besides that, by comparing the deuterated and hydrogenated compound an interesting difference in the phase behaviour was found. Both compounds N 2 H 7 I and N 2 D 7 I crystallize at room temperature in a cubic phase.This cubic phase is the consequence of an orientational disorder of the cation: its centre of gravity is located in the middle of the cube of iodine atoms.The N-N axis is statistically equally orientated along the three cubic axis and the terminal hydrogen (deuterium) atoms are disordered around each N-N axis (Fig.1). The molecule now undergoes different transitions when cooling down depen- ding on whether the hydrogenated or deuterated cation is present. In the hydro- genated case a tetra- gonal phase is obser- ved at 220K followed by an orthorhombic phase at 202K, whe- ras the deuterated compound already undergoes a transi- tion to an orthor- hombic phase when cooling to 260K. Both compounds trans- form into a monocli- nic low-temperature form at around 155K. 1 - Group-subgroup relations of the phases N 2 H 7 I/N 2 D 7 I. (1) N--axis disordered in three dimensions. H atoms disordered. (2) a´= b´~acubic, c´~acubic, N--N axis disorde- red in two dimensions. H atoms disordered. (3) a´´= b´´= 2a´, c´´= 2c´. N-N axis ordered. H atoms still disordered, origin shifted to 01/20. (4) a´´´~ a´´, b´´´~ b´´ (a´´´ not= b´´´), c´´´ ~ c´´.Terminal H atoms ordered, bridging H atoms distorted, cation on 2/m. (5) a IV ~ a´´´, b IV ~ b´´´, c IV ~ c´´´. Terminal H atoms ordered, bridging H atom disordered, cation on 1. 2 - Orientation of the N 2 D 7 I+ - cations in the iodine "cube" of the orthorhombic phase. One D atom of each terminal ND 3 group forms a strong linear hydrogen bond to an iodine atom, the other two D atoms point approximately to the middle of the edges of the cubes (bifurcated hydrogen bond). The structure of the orthorhombic phase was determined from high resolution powder diffrac- tion data. On cooling-down, the three-dimensional disorder of the N-N axis along the three cubic axes is reduced to a two-dimensional disorder leading to the contraction of the axis along which the mole- cule is no longer disordered. The reduction of orientational disorder leads to a larger N-N distance (2.81(1)Å) than observed in the disorde- red cubic phase (2.52(4)Å). The temperature dependent powder diffraction patterns show that on transforming into the orthorhombic phase the diffuse scattering due to the rotational disorder of the terminal deuterium atoms spinning around the N-N axis vanishes.The "freezing-out" of this rotational disorder leads to the appearance of hydrogen bonds between these terminal hydrogens and the surrounding iodine atoms. A close examination of the structu- re revealed that one D-I distance (2.78(2)Å) was significantly shorter than the other two (3.03(3)Å) and (3.17(4)Å).This is the result of an almost linear hydrogen bond of the type N-D......I. The other N-D bonds point between two iodines qualifying them as bifurcated hydrogen bonds (Fig.2). 1 2 The cell distortion when going from the cubic to the orthorhombic cell shows that the cell cons- tant b along which the strong linear hydrogen bonds are orientated is shorter than a. The for- mation of one stronger and two weaker hydro- gen bonds explains this distortion. In the tetrago- nal hydrogenated phase the terminal hydrogen atoms are still spinning around the N-N axis as in the cubic phase, hence no hydrogen bond net- work is formed which can induce an orthorhom- bic distortion. Figure 1 shows the group-subgroup relations of the N 2 D 7 I/N 2 H 7 I phases.The tetrago- nal structure is only found in N 2 H 7 I, whereas the Pm3m and Pcan structures are found for both N 2 D 7 I and N 2 H 7 I.