The observed and energetically feasible crystal structures of 5-substituted uracils

Abstract

A search of the Cambridge Structural Database for crystal structures of 5-substituted uracils shows that, although there is a recurrent motif with symmetric hydrogen bonding and interdigitation of the 5-substituent R, a range of other hydrogen bonded ribbons, sheets and three-dimensional motifs are possible. In order to try and rationalize this, we have performed a combination of experimental studies and computational searches for low energy structures for the 12 simple 5-substituted uracils with R = H, CH₃, CH₂CH₃, CHCH₂, CN, OH, NH₂, NO₂, F, Cl, Br and I. Crystallization experiments on these compounds yielded the first single crystal X-ray determinations of 5-ethyluracil and 5-cyanouracil, as well as low temperature redeterminations of the disordered structures of 5-chlorouracil and 5-bromouracil. The lattice energies were calculated for the known crystal structures and compared with the computed lattice energy landscape for each molecule (except R = Br and I). Although the symmetric ribbon motif often dominates the computed crystal energy landscape, all of the molecules show a variety of different hydrogen bonding structures within a small energy range (5 kJ mol⁻¹) of the global minimum and exhibit quite a diverse range of energetically competitive motifs. Thus, the range of crystallization outcomes, from polymorphism and other multiple forms, to the difficulty in growing single crystals (R = CHCH₂ and NH₂) probably reflects the sensitivity of the various hydrogen bonding motifs to the substituent and limited range of crystallization conditions that can be applied.