The reorientational dynamics of ammonium dicyanamide ND4[N(C N)2] and the kinetics as well as the mechanism of the solid-state isomerization reaction from ammonium dicyanamide into dicyandiamide (N C-N= C(NH2)(2)) was studied by means of H-2 and N-14 solid-state NMR spectroscopy in a temperature range between 38 and 390 K. Whereas in previous investigations the mechanism of the solid-state transformation was investigated by means of vibrational and magic angle spinning solid-state NMR spectroscopy as well as neutron diffraction, we here present a comprehensive H-2 study of the ammonium ion dynamics prior to and during the course of the reaction, thereby highlighting possible cross correlations between dynamics and reactivity involving the ammonium ion. The ND4+ group was found to undergo thermally activated random jumps in a tetrahedral potential, which is increasingly distorted with increasing temperature, giving rise to an asymmetrically compressed or elongated tetrahedron with deviations from the tetrahedral angle of up to 6 degrees. The correlation time follows an Arrhenius law with an activation energy of E-a = 25.8(2) kJ mol(-1) and an attempt frequency of tau(-1)(0) = 440(80) THz. The spin- lattice relaxation times were fitted according to a simple Bloembergen-Purcell-Pound type model with a T-1 minimum of 4 ms at 230 K. Temperature-dependent librational amplitudes were extracted by line-shape simulations between 38 and 390 K and contrasted with those obtained by neutron diffraction, their values ranging between 5 and 28 degrees. The onset and progress of the solid-phase transformation were followed in situ at temperatures above 372 K and could be classified as a strongly temperature-dependent, heterogeneous two-step reaction proceeding with rapid evolution of ammonia and comparatively slow subsequent reintegration into the solid. On the microscopic level, this correlates with a rapid proton transfer-possibly triggered by a coupling between the ammonium ion dynamics and phonon modes on the terahertz time scale-and an essentially decoupled nucleophilic attack of ammonia at the nitrile carbon, giving rise to significantly differing time constants for the two processes.