The competition between halogen bonds (Br•••O) and C-H•••O hydrogen bonds: the structure of the acetone-bromine complex revisited

A combination of single-crystal X-ray (at 110 and 200 K) and high-resolution neutron powder diffraction (110 K only) using perdeuterated samples has been used to re-determine the crystal structure of the well-known acetone-Br-2 complex. The results indicate a significant interaction between the carbonyl oxygen and the nearest Br atom leading to a marked elongation of the molecular Br-Br bond. In the earlier widely cited crystal structure, no such elongation was reported. Furthermore, the new structure indicates the presence of additional weaker C-H center dot center dot center dot O and C-H center dot center dot center dot Br interactions. In this simple complex, in which the halogen bonding is strong compared to the hydrogen bonding (C-H center dot center dot center dot O), the former interaction is found to occur close (similar to 12 degrees) to the plane of the carbonyl group, with the C-H center dot center dot center dot Br complementing the stronger halogen bond.