An ab initio and neutron diffraction study of ammonium chloride

Ab initio density functional calculations have been carried out on the orde red phase of ammonium chloride and the results compared with experimental d ata. Powder neutron diffraction measurements have been carried out on (ND4C l)-N-14 and (ND4Cl)-N-15 at a temperature of 5 K; the lattice parameter is 3.8224(4) A and the N-D bond length is 1.0318(8) A. Calculated structural a nd spectroscopic quantities agree well with the results of the neutron diff raction refinements, and with measured spectroscopic data. The calculated b arrier to reorientation of the ammonium ion about the fourfold axis is 29 k J mol(-1), and the calculated compressibility is 3.78 x 10(-11) Pa-1. The o ctopole moment of the ammonium ion in the crystal has been estimated from t he calculated electron density, and is about 10-20% greater than for the fr ee ion. The octopole moment does not Vary appreciably with the orientation of the ammonium ion in the lattice. There is no evidence of any covalent in teraction between the ammonium and chloride ions.