Aza analogues of nucleic acid bases: experimental determination and computational prediction of the crystal structure of anhydrous 5-azauracil

Abstract

The crystal and molecular structure of 5-azauracil, C3H3O2N3, Mr=113.07Da, was determined from X-ray diffraction data. The material crystallizes in the orthorhombic space group Pbca with eight molecules of 5-azauracil in a cell of dimensions a=6.5135(3), b=13.5217(4), c=6.5824(4)Å, crystal density Dc=1.779gcm−3. The structure was determined using direct methods and refined by full-matrix least-squares to a conventional R index of 0.0337 for 763 observed reflections and 86 parameters. Two strong hydrogen bonds, N1H1…O4 and N3H3…N5, and several weaker intermolecular interactions produce a crinkled sheet structure. This crystal structure was independently predicted by a search for minima in the lattice energy, as calculated using an ab initio optimised molecular structure and a distributed multipole model for the electrostatic interactions. Indeed, the global minimum in the search corresponded to the same Pbca space group, with rms errors in the cell lengths of 3.7%. There is a larger energy gap separating the observed hydrogen bonding motif structure from alternative structures, with different hydrogen bonds and connectivity, for 5-azauracil than for 6-azauracil and uracil.