Proton dynamics in letovicite: Part I. Static 1H and 15N NMR MAS experiments and lineshape simulations

Abstract

Synthetic letovicite (NH 4 ) 3 H(SO 4 ) 2 has been investigated using 1 H static, low-speed MAS, and 15 N MAS NMR spectroscopy. Experiments were carried out in the temperature range of 215–425 K. The 1 H MAS NMR spectra show three different resonances. The resonance assigned to the ammonia protons is broad and spinning sidebands cannot be resolved in the low-speed MAS NMR spectra. On the other hand, the acidic protons in the ferro- and paraphase show narrow signals with sideband patterns that enable a chemical shift anisotropy analysis. The chemical shift parameters of the free protons in the paraphase (δ iso = 13.2 ppm, δ aniso = 4.5 ppm, η = 0.0) differ completely from those of the protons in the ferrophase (δ iso = 14.1 ppm, δ aniso = 8.5 ppm, η = 1.0). The lowering of the chemical shift anisotropy δ aniso by a factor of two and the change of the asymmetry parameter η imply a tetrahedral site jump mechanism of the protons. Three different ammonia tetrahedra can be distinguished by 15 N MAS NMR spectroscopy in the P 2 /n phase below 273 K. Two resonances are prominent for the ferrophase (space group C 2 /c ) corresponding to the two different crystallographic sites. Both resonances move together into a single resonance in the high-temperature phase that can be interpreted as fast dynamics of ammonia groups and its local environment so that the two crystallographic sites are locally nearly equal.