Rotational barriers in ammonium hexachlorometallates as studied by NMR, tunnelling spectroscopy and ab initio calculations

• Published in: Chemical physics 2004-03, Vol.299 (1), p.113-122
• Main Authors: Birczyński, A., Lalowicz, Z.T., Łodziana, Z.
• Format: Article
• Language: English
• Subjects: Ammonium compounds; DFT; Tunnelling spectroscopy
• ISSN: 0301-0104
• DOI: 10.1016/j.chemphys.2003.12.014

Ammonium hexachlorometallates, (NH 4) 2MCl 6 with M=Pd, Pt, Ir, Os, Re, Se, Sn, Te and Pb, comprise a set of compounds with systematically changing properties. The compounds may be ordered according to decreasing tunnelling frequency (TF) of ammonium ions, which is related to the increasing potential barrier. It was also observed that TF correlates directly with the lattice constant of a particular compound. This correlation is explained by density-functional theory (DFT). The size of the unit cell is governed by the nature of metal–chlorine interaction, with respect to which the studied compounds can be divided into two subgroups. The group containing d-metals (Pd, Pt, Ir, Os, Re) has stronger covalent M–Cl bond and smaller unit cell. The second category contains p-elements (Se, Sn, Te, Pb), which exhibit larger ionic charges and the larger unit cell. The differences in the electronic structure explain observed variation of the tunnelling frequencies for NH 4 +. The theory provides also M–Cl distances and barriers for C 2 and C 3 rotations of ammonium ions in respective compounds, which show good agreement with experimental values.