Nuclear magnetic resonance study of molecular dynamics in guanidinium salts containing interionic hydrogen bonds

Abstract

Temperature dependences of the ¹H second moment and spin–lattice relaxation times obtained for polycrystalline guanidinium chloride, bromide, iodide and tetrafluoroborate are interpreted in terms of the same model of two strongly correlated motions: 180° flips around the C—N axis of the amino groups followed at once by reorientation of the whole guanidinium cation around its C₃ axis.

From measurements of the ¹⁹F second moment for the tetrafluoroborate the anion dynamics is described, and the contribution of the hydrogen-bond energy to the potential barrier hindering cation reorientation is estimated at 1.2 kJ mol^–1 per bond, forming ca. 25% of the whole activation energy.

A solid–solid structural hexagonal-cubic phase transition at 353 K is found for guanidinium iodide.