Dynamic properties of solid ammonium cyanate

Ammonium cyanate is well-known to undergo a solid-state reaction to form urea. Knowledge of fundamental physicochemical properties of solid ammonium cyanate is a prerequisite for understanding this solid-state chemical transformation, and this paper presents a comprehensive study of the dynamic properties of the ammonium cation in this material. The techniques usedincoherent quasielastic neutron scattering (QENS) and solid-state 2H NMR spectroscopyprovide insights into dynamic properties across a complementary range of time scales. The QENS investigations (carried out on a sample with natural isotopic abundances) employed two different spectrometers, allowing different experimental resolutions to be probed. The 2H NMR experiments (carried out on the deuterated material ND4+OCN−) involved both 2H NMR line shape analysis and 2H NMR spin−lattice relaxation time measurements. The results of both the QENS and 2H NMR studies demonstrate that the ammonium cation exhibits reorientational dynamics across a wide temperature range, and several dynamic models (based on knowledge of the crystal structure of ammonium cyanate) were considered in this work. It is found that a tetrahedral jump model for the dynamics of the ammonium cation provides the best description for both the QENS and 2H NMR data, and there is excellent agreement between the values of activation parameters established from these two experimental approaches, with estimated activation energies of 22.6 ± 2.1 kJ mol−1 from QENS and 21.9 ± 1.0 kJ mol−1 from 2H NMR spin−lattice relaxation time measurements. The wider implications of the results from this work are discussed