Thermal vibrations and electrostatic properties of parabanic acid at 123 and 298 K

The crystal structure of parabanic acid (1H,3H-imidazoletrione, C3H2N2O3, Mr = 114.1) at 123 K [monoclinic, P2(1)/n, Z = 4, a = 10.704 (2), b = 8.187 (2), c = 4.969 (1) A, beta = 92.32 (1)o] has been determined by neutron and X-ray diffraction. Neutron reflections [1658, (sin theta)/lambda less than or equal to 0.78 A-1, lambda = 1.0470 (1) A, mu = 0.75 cm-1] in full-matrix least-squares refinement (wR = 0.055) gave nuclear parameters with bond lengths having estimated standard deviations (e.s.d.'s) of 0.001 A. X-ray reflections [3150 with magnitude of F magnitude of greater than or equal to 3 sigma and (sin theta)/lambda less than or equal to 1.3 A-1, Mo Ka, lambda = 0.7093 (1) A, mu = 1.708 cm-1] were used in full-matrix least-squares refinement (R = 0.022) assuming Stewart's rigid pseudoatom model, so as to determine the charge-density distribution. Sharp features near the O nuclei in the residual electron density map noted by Craven & McMullan (CM) [Acta Cryst. (1979), B35, 934-945] in their 298 K study were absent at 123 K. New refinements using CM's 298 K data give significant third-order thermal parameters for O atoms due to anharmonic molecular stretching, with good agreement between probability density functions, (p.d.f.'s) from X-ray and neutron diffraction. When deconvoluted from the thermal vibrations in the crystal, deformation charge densities derived at 298 and 123 K are in satisfactory agreement and conform closely to the 2 mm symmetry of the isolated molecule. The molecular dipole moment is 2.3 (3) D [7.7 (10) x 10(-30) C m]. A map of molecular electrostatic potential indicates that the carbonyl O atoms in parabanic acid are weakly electronegative. This is confirmed by calculation of the electrostatic energy of interaction for the two distinct H-bonded pairs of molecules isolated from the crystal. Energy values are weakly attractive [-7 (3) and 5 (2) kJ mol-1]. Similar calculations are less reliable for pairs of molecules forming intermolecular C....O distances as short as 2.75 A. However, depending on the model, energy values are repulsive or negligibly small. Thus the high stability of parabanic acid crystals (m.p. 516 K with decomposition) cannot be explained.