click on figure for more details (fig.1)High-resolution neutron powder diffraction using D2B could not only show that this cation exists in the solid state but also reveal the "freezing" of various internal motions when cooling, and its subsequent distortion [2, 3]. Besides that, by comparing the deuterated and hydrogenated compound an interesting difference in the phase behaviour was found.
Both compounds N2H7I and N2D7I 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 a 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).
click on figure for more details (fig.1)
Group-subgroup relations of the phases N2H7I/N2D7I. (1)N--axis disordered in three dimensions. H atoms disordered. (2)a´= b´~acubic, c´~acubic, N--N axis disordered in two dimensions. H atoms disordered. (3)a´´= b´´= root 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.
The molecule now undergoes different transitions when cooling down depending on whether the hydrogenated or deuterated cation is present. In the hydrogenated case a tetragonal phase is observed at 220K followed by an orthorhombic phase at 202K, wheras the deuterated compound already undergoes a transition to an orthorhombic phase when cooling to 260K. Both compounds transform into a monoclinic low-temperature form at around 155K. The structure of the orthorhombic phase was determined from high resolution powder diffraction 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 molecule is no longer disordered. The reduction of orientation disorder leads to larger N-N distance (2.81(1)Å) than observed in the disordered 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 structure 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).
click on figure for more details (fig.2)
Orientation of the N2D7I+ - cations in the iodine "cubed" of the orthorhombic phase. One D atom of each terminal ND3 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 cell distortion when going from the cubic to the orthorhombic cell shows that the cell constant b along which the strong linear hydrogen bonds are orientated is shorter than a. The formation of one stronger and two weaker hydrogen bonds explains this distortion. In the tetragonal hydrogenated phase the terminal hydrogen atoms are still spinning around the N-N axis as in the cubic phase, hence no hydrogen bond network is formed which can induce an orthorhombic distortion. Fig.1 shows the group-subgroup relations of the N2D7I/N2H7I phases. The tetragonal structure is only found in N2H7I, the Pm3m and Pcan structures are found for both N2D7I and N2H7I.