IR SPECTROSCOPIC CHARACTERIZATION OF NH4-ANALCIME

Abstract

The IR spectrum of ammonium-exchanged natural analcime (basalt, Nidym River, Siberian platform) exhibits several features that suggest a lowered symmetry for the NH4+ ion and that this is influenced by hydrogen bonding within the framework. These features are: the pronounced splitting into three components and high-frequency shift of the ν4-bending mode; appearance of the ν1-stretching mode which is predicted to become IR-active when the ideal T d symmetry of NH4+ ion is violated, and the low-frequency shift of the ν1- and ν3-stretching modes. The absence of absorption lines in the 1800–2400-cm−1 region indicates that hydrogen bonding between the framework and the NH+4 ion is very weak. The three-component splitting of the ν4-bending mode indicates that the symmetry of NH+4 ion is lower than C3v. This implies that at least two N–H bonds of the NH+4 ion are disturbed by hydrogen bonding. Computer analysis of the normal vibrations of the NH+4 molecule for different symmetry types (using harmonic approximation) indicates that the best fit to the observed ν4 triplet frequencies for C1 symmetry implies a deviation of the valent angle ∠H–N–H from ideal T d symmetry of around ±2.5°. The factors governing the behaviour of the NH+4 ion in the analcime structure are discussed. The geometry of the nearest environment of the NH+4 ion in the analcime structure is analyzed with respect to the present IR data.