Lattice parameters for urea have been measured at seven temperatures in the range 12 to 173 K and the crystal structure has been determined at 12, 60 and 123 K from neutron diffraction data. With 342 reflections (sin 0/A <0.77 A -I) measured for an octant of reciprocal space, full-matrix least-squares refinement gave Rw(F2) = 0.030, 0.029 and 0.029 at 12, 60 and 123 K. The atomic anisotropic thermal parameters are consistent with overall rigid-body motion together with intramolecular librations of the NH2 groups.

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Cytosine is orthorhombic, space group P212t21, with a = 13.044 (2), b = 9.496 (1), c= 3.814 (1)/~, Z= 4. Cytosine monohydrate is monoclinic, space group P21/c, with a = 7.783 (2), b = 9.825 (2), c = 7.668 (2) ~, /~=99°34 (1)', Z=4. The bond lengths of the carbonyl group, previously reported to be different in cytosine and the monohydrate, are presently found to be 1.241 and 1.251 ~, respectively. This and other differences in molecular structure are of marginal significance.

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