Ashley T. Hulme

Occurrence of ribbons of cyclic water pentamers in a metallo-organic framework formed by spontaneous fixation of CO2

In the reaction of equimolar amounts of copper(II) acetate with 2,2′-dipyridylamine (DPA) in aqueous tetrahydrofuran, in presence of KOH, aerial CO2 is spontaneously fixed to the carbonate anion yielding [Cu(DPA)(CO3)]·3H2O (1). X-ray crystallography shows the presence of zigzag ribbons of cyclic water pentamers in the channels of a chain-like metallo-organic framework. The water ribbons are stabilised by hydrogen bonds to the metallo-organic backbone. Each (H2O)5 pentamer is approximately planar.

Toward the Prediction of Organic Hydrate Crystal Structures.

Lattice energy minimization studies on four ordered crystal structures of ice and 22 hydrates of approximately rigid organic molecules (along with 11 corresponding anhydrate structures) were used to establish a model potential scheme, based on the use of a distributed multipole electrostatic model, that can reasonably reproduce the crystal structures. Transferring the empirical repulsion-dispersion potentials for organic oxygen and polar hydrogen atoms to water appears more successful for modeling ice phases than using common water potentials derived from liquid properties. Lattice energy differences are reasonable but quite sensitive to the exact conformation of water and the organic molecule used in the rigid molecule modeling. This potential scheme was used to test a new approach of predicting the crystal structure of 5-azauracil monohydrate (an isolated site hydrate) based on seeking dense crystal packings of 66 5-azauracil···water hydrogen-bonded clusters, derived from an analysis of hydrate hydrogen bond geometries involving the carbonyl- and aza-group acceptors in the Cambridge Structural Database. The known structure was found within 5 kJ mol(-1) of the global minimum in static lattice energy and as the third most stable structure, within 1 kJ mol(-1), when thermal effects at ambient temperature were considered. Thus, although the computational prediction of whether an organic molecule will crystallize in a hydrated form poses many challenges, the prediction of plausible structures for hydrogen-bonded monohydrates is now possible.

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

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.

Clarification of the crystalline forms of androsterone

The crystal structure of androsterone form 1 has been redetermined at low temperature and the unit cell of what was thought to be a second polymorph is now determined from the full crystal structure to be that of a hemihydrate.

5-Fluorouracil and thymine form a crystalline solid solution

The crystal structure of a 5-fluorouracil-thymine [5-fluoropyrimidine-2,4(1H,3H)-dione-5-methylpyrimidine-2,4(1H,3H)-dione, C(4)H(3)FN(2)O(2)xC(5)H(6)N(2)O(2)] solid solution has been determined. Both of the crystallographically independent sites can accommodate either 5-fluorouracil or thymine molecules, leading to occupational disorder [C(5-x)H(6-3x)F(x)N(2)O(2)xC(5-y)H(6-3x)F(y)N(2)O(2), with x = 0.52 and y = 0.7 for determination (I), x = 0.55 and y = 0.69 for (II), and x = 0.67 and y = 0.76 for (III)]. The 5-fluorouracil-thymine ratio in the crystal structure is influenced by the 5-fluorouracil-thymine ratio in the crystallization solution, though it does not exactly mirror it. The crystal structure comprises interpenetrating hydrogen-bonded nets, containing four independent hydrogen bonds.