The crystal structure of anhydrous cytosine has been determined and refined by analysis of complete three-dimensional copper X-radiation data. The crystals are orthorhombic with space group P212121; the unit-cell dimensions are a = 13.041 Å, b = 9.494 Å, c = 3.815 Å. Refinement of the positional parameters for all 13 atoms and of the individual anisotropic temperature factors of the eight heavier atoms was by the method of least squares. For the 608 observed reflections of non-zero weight, the final R factor is 0.061. The molecule is found to exist in the amino form with a hydrogen atom bonded to N1 and none to N3 The amino nitrogen participates in two hydrogen bonds to carbonyl oxygens of other molecules. These bonds have relatively long lengths of 2.98 and 3.03 Å. N1 hydrogen bonds to N3 of a neighboring molecule at a distance of 2.84 Å.
David Lowell Barker (Thu,) studied this question.