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.