Single-Crystal Neutron Diffraction Study on Guanidine, CN₃H₅

Pure guanidine crystallizes in the
orthorhombic space group Pbca (no. 61) and a = 8.5022(2) Å, b = 9.0863(2) Å, c = 15.6786(4) Å
at 100 K, Z = 16, with two Y-shaped molecules in
the asymmetric unit. The compound features a three-dimensional network
of classical N–H···N hydrogen bonds. Here, we
present the results of a single-crystal neutron diffraction study,
performed at two different temperatures (100 and 273 K). The data
quality obtained at the HEiDi instrument (FRM II, Munich) allowed
to derive accurate positional and anisotropic displacement parameters
(ADP) for all the atoms, including H. The experimental hydrogen positions
confirm a model derived from theory. On the basis of the displacement
parameters, a TLS analysis of thermal motion proves that the guanidine
molecules behave in good approximation as rigid bodies and essentially
undergo libration. The unusual temperature behavior of one C–N
bond found in a preceding single-crystal X-ray study is an artifact
going back to this rigid-body movement. The existence of various hydrogen
bonds also manifests from a well-resolved IR spectrum, which was analyzed
in terms of individual vibrations on the basis of quasi-harmonic ab
initio phonon calculations.