Alloxan – electrostatic properties of an unusual structure from X-ray and neutron diffraction

Abstract

The crystal structure of alloxan [2,4,5,6(1H,3H)pyrimidinetetrone, C4H2N204, Mr= 142.07, tetragonal, space group P4t2~2, Z = 4] has been determined from neutron diffraction data ( s i n 0 / A 2o-.] Alloxan has an unusually high crystal density (1.93 g cm -3 at 295 K), apparently due to short intermolecular O = C . . O distances (2.73 A, at 123 K). There is no H-bonding except for a weak bifurcated interaction with long H-..O distances (2-32, 2.35 A,) at 123 K. Bond lengths and angles from neutron diffraction (corrected for thermal motion) have e.s.d.'s less than 0.002 A and 0.1 °. They indicate that the effects of conjugation in alloxan are small. With fixed values for all nuclear positional and thermal parameters as determined by neutron diffraction, the X-ray data were used to determine the charge-density distribution assuming Stewarrs rigid pseudoatom model. The resulting electron population parameters were also used to map the electrostatic potential for an isolated molecule and for a selected group of molecules removed from the lattice. Electrostatic properties show the bifurcated H-bond to be weakly attractive. In the carbonyl groups, electron density at the C atom is depleted from both sides of the molecular plane, thus deshielding the atomic nucleus. Although the shortest intermolecular C.. .O distances are formed with O atoms carrying a small negative charge, -0.11 (3)e, these interactions are weak. It appears that the van der Waals radius for carbonyl C atoms should be smaller than for aromatic C atoms by at least 0.2 A,.