Static deformation densities for cytosine and adenine

X-ray diffraction intensities for cytosine monohydrate have been measured at 97 K, to 2sin theta/lambda = 2.74 A-1, and used in a deformation refinement. Crystal data for cytosine monohydrate at 97 K: a = 7.728 (1), b = 9.817 (3), c = 7.520 (1), beta = 100.50 degrees, V = 560.94 A3, R = 0.0341 for 6456 unique reflections. The experimental static deformation density of cytosine compares very well with the corresponding theoretical 4-31G deformation density. Differences between theory and experiment occur at hydrogen-bonding sites. They are very similar to the corresponding differences between the theoretical deformation densities of formic acid monomer and cyclic dimer. The experimental static deformation density of 9-methyladenine was obtained in a deformation refinement against the low-temperature X-ray data of Craven & Benci [Acta Cryst. (1981), B37, 1584-1591]. It is similar to the maps presented by these authors. The comparison of the experimental deformation density of 9-methyladenine with the theoretical 4-31G deformation density of adenine shows qualitative agreement, which is improved when the 4-31G results are corrected for the absence of polarization functions in the basis set. Some of the remaining differences reflect the effects of substituting CH3 for H. The weak N...H hydrogen bonds have little influence on the deformation density in this structure.