Role of Weak Hydrogen Bonds and Halogen Bonds in 5-Halo-1,3-dimethyluracils and Their Cocrystals-A Combined Experimental and Computational Study

Seven single crystals containing either N,N-dimethyluracil (DMHU) or one of its 5-halogenated derivatives (DMXU; X = F, Cl, Br, I) were prepared using N,N-dimethylformamide as the crystallization solvent. Single crystal X-ray diffraction and quantum chemical calculations carried out at the spin component scaled local MP2 level of theory were then used to study the intramolecular halogen and nonconventional hydrogen bonds present in the structures. The results were compared to and contrasted with the previously repotted data for uracil and its halogenated derivatives. In particular, the intermolecular interactions in DMIU were compared to the halogen and hydrogen bonds in 5-iodouracil that, in contrast to DMHU and its derivatives, displays conventional hydrogen bonds involving its strong N-H donor sites. The crystallographic and computational analyses showed that, while non conventional hydrogen bonds are present in both DMHU and DMXU; halogen bonds could only be identified in DMBrU and DMIU, in which case they play an important role in directing the resulting crystal structures.