An X-ray and NMR study of structure and ion motions in C(NH2)3PF6

Abstract

A single-crystal X-ray analysis has shown that guanidinium hexafluorophosphate C(NH2)3PF6 crystallises in the trigonal space group R3m, with a=7.427(3) AA, alpha =67.89(2) degrees and Z=2. The guanidinium cation has the symmetry 3m (C3v), the two inequivalent hexafluorophosphate anions 3m (D3d). The ions are connected by the weak N-H . F hydrogen bonds. Proton and fluorine spin-lattice relaxation times as well as second moments have been measured over a wide range of temperatures. An analytical solution of a set of coupled differential equations describing the time variations of nuclear magnetisations was derived for four unlike spin systems. The analysis of all cross-relaxation effects enabled the authors to interpret the results in terms of C3 reorientation of the cation and isotropic reorientation of two dynamically inequivalent anions. The activation parameters of the motions were found. The phase transitions and a convergence of cation and anion rotational correlation times at one of them (333 K) were discovered.