Crystal structure and molecular dynamics of guanidinium perchlorate

Abstract

A single-crystal X-ray analysis has shown that guanidinium perchlorate crystallizes in the rhombohedral system [space group R3; hexagonal unit-cell dimensions: a= 7.606 (2)Å, c= 9.121 (2)Å and Z= 3]. Both the guanidinium cation and the perchlorate anion have a threefold symmetry axis and form hydrogen-bonded layers in which the N–H ⋯ O hydrogen bond is 3.06 Å in length.

Nuclear magnetic resonance studies carried out on the polycrystalline compound revealed a considerable reduction in the second moment from 19.5 to 3.2 G² between 170 and 250 K, approaching zero above 454 K, as well as an asymmetrical T₁ plot with a single minimum of 14 ms at 310 K. A model involving correlated motional processes (180 ° flips about the symmetry axes of the amino groups and a C₃ reorientation of the guanidinium ion) is consistent with experimental findings. The nature of the solid–solid phase transition at 454 K appears to be related to the onset of cationic self-diffusion.