Single-crystal neutron refinement of creatine monohydrate at 20 K and 123 K

The molecular structure and bonding of guanidine and related compounds have been a source of considerable interest for theoretical chemists over the past 30 years. Here we present a study of one such compound, creatine monohydrate, using low-temperature pulsed neutron diffraction data to refine the known X-ray crystal structure. The molecule crystallises in space group P2(1)/c with a = 12.492(4), b = 4.967(1), c = 12.051(4) Angstrom, beta = 109.10(2)degrees at 20 K and a = 12.489(7), b = 4.987(1), c = 12.075(6) Angstrom, beta = 108.86(3)degrees at 123 K, Z = 4. The final refinements included anisotropic temperature factors on all atoms and converged to wR(F) = 0.055 at 20 K and 0.045 at 123 K (190 parameters). The extensive network of hydrogen bonds is now well defined, and a thermal parameter analysis reveals significant torsional motion of the methyl group at both temperatures.