Electrostatic properties of cytosine monohydrate from diffraction data.

The charge density distribution in deuterated cytosine monohydrate [4-(2H2)-amino-2(1H)-(1-2H)-pyrimidinone], C4H2D3N3O.D2O, Mr = 134.15, monoclinic, P2(1)/c, a = 7.714 (1), b = 9.825 (1), c = 7.506 (2) A, beta = 100.52 (1) degrees, Z = 4, has been determined from 1432 X-ray reflections [sin theta/lambda less than 1.15 A-1, Mo K alpha, lambda = 0.7093 (1) A] collected at 82 (2) K. Least-squares structure refinement assuming Stewart's rigid pseudoatom model gave R(F) = 0.041, with all nuclear positions and H/D anisotropic thermal parameters having fixed values previously determined by neutron diffraction. The cytosine and water molecules are both electrically neutral within experimental error (0.09 e). Molecular dipole moments are mu = 8.0 (1.4) debye for cytosine and 2.3 (3) debye for water. For the doubly hydrogen-bonded cytosine dimer (N1-D1...N3', N4-D4...O2') isolated from the crystal, the electrostatic energy of interaction is estimated to be -96 (27) kJ mol-1. Maps of the electrostatic potential for molecules isolated from the crystal indicate that the electronegativity of the hydrogen-bonding acceptor sites can be ranked with the water O atom, cytosine N(3) and carbonyl O(2) in decreasing order.