X-ray and neutron diffraction investigation at 298 K of NH4I · NH3 and ND4I · ND3 containing the cation H3N – H … NH3 + and D3N – D … ND3 +

Abstract

Abstract NH4I · NH3 = N2H7I and ND4I · ND3 = N2D7I both crystallize at room temperature in the cubic space group Ptriim (Z = 1) with a = 512.9(3) pm and a = 513.4(1) pm. The primitive unit cells contain the cation N2H7 + = H3N – … NH3 and N2D7 + = D3N – D … ND3, respectively. The cubic symmetry is the consequence of an orientational disorder of the cation, whose N – H … N or N – D … N axis in the various unit cells points statistically in the direction [100], [010] or [001]. The time-averaged center of gravity of the cation lies in 1/2 1/2 1/2. So far the existence of the cation N2H7 + has been detected only in the gas phase. The cations are isoelectronic with the cations O2H5 + and O2D5 +, which have long been known to exist in the solid state. A single crystal structure analysis of N2H7I with X-rays at room temperature yielded an N – N distance of 269(5) pm. Since the terminal hydrogen/deuterium atoms are positionally disordered around the N – N axis these atoms could not be located. The deuterated compound N2D7I was investigated by neutron diffraction methods. The Rietveld refinement of a data set obtained on the high resolution powder diffractometer D2B at the ILL, Grenoble, confirmed the results of our X-ray investigation. Due to the orientational disorder of the cation and the positional disorder of the terminal D atoms, the motion of the terminal D atoms is confined to the surface of a sphere with its center in 1/2 1/21 /2. In order to consider their contribution to the structure factor, SASH functions have been successfully used to describe their scatteringlength density. The N – N distance was calculated to be 252(4) pm. The structures of the ordered low temperature phases will be reported separately.