Electrostatic properties of 1-methyluracil from diffraction data

1-Methyluracil (1-methyl-2,4-dioxopyrimidine), C5H6-N2O2, M(r) = 126.12, orthorhombic, Ibam, a = 13.188 (6), b = 13.175 (5), c = 6.214 (3) A, V = 1079.7 (8) A3, Z = 8, Dx = 1.552 g cm-3, lambda (Mo K alpha) = 0.7107 A, mu = 1.317 cm-1, F(000) = 528, T = 123 K, R(F2) = 0.068 for 2996 reflections with sin theta/lambda less than 1.08 A-1. The electronic charge-density distribution has been analyzed in terms of Stewart's rigid pseudoatom model, using restricted Slater radial functions and angular multipole terms extending to octapoles for C, N and O, and quadrupoles for H pseudoatoms. Three different structure refinements have been carried out based on two X-ray diffraction data sets from different crystals collected at temperatures differing by about 20 K. The molecular dipole moment is 6.4 (27) D. Maps of the electrostatic potential for a molecule isolated from the crystal show that atoms O2 and O4 confer overall electro-negativity on one side of the molecule while the CH groups and the C1 methyl group confer electropositivity on the other. For the centrosymmetric hydrogen-bonded dimer (N3-H3...O4'; H...O distances 1.77 A) the electrostatic potential shows electropositive bridges between the molecules. These features are lacking for the C-H...O interactions (distances H6...O2, 2.37; H11...O4, 2.34 A). The electron density and its Laplacian have been determined at the intramolecular bond-critical points and also for the intermolecular H...O interactions. Values for the former are characteristic of covalent bonds. Values of the electron density and Laplacian for the C-H...O interactions are very small and have little or no significance in terms of their e.s.d.'s. The electrostatic energy of interaction for the N-H...O hydrogen-bonded dimer is -10 (12) kJ mol-1. The attractive electrostatic energy increases to -67 (33) kJ mol-1 for a centrosymmetric planar tetramer in which the C-H...O interactions are also formed.