Competition between hydrogen and halogen bonding in halogenated 1‐methyluracil: Water systems

The competition between hydrogen‐ and halogen‐bonding interactions in complexes of 5‐halogenated 1‐methyluracil (XmU; X = F, Cl, Br, I, or At) with one or two water molecules in the binding region between C5‐X and C4O4 is investigated with M06‐2X/6‐31+G(d). In the singly‐hydrated systems, the water molecule forms a hydrogen bond with C4O4 for all halogens, whereas structures with a halogen bond between the water oxygen and C5‐X exist only for X = Br, I, and At. Structures with two waters forming a bridge between C4O and C5‐X (through hydrogen‐ and halogen‐bonding interactions) exist for all halogens except F. The absence of a halogen‐bonded structure in singly‐hydrated ClmU is therefore attributed to the competing hydrogen‐bonding interaction with C4O4. The halogen‐bond angle in the doubly‐hydrated structures (150–160°) is far from the expected linearity of halogen bonds, indicating that significantly non‐linear halogen bonds may exist in complex environments with competing interactions. © 2016 Wiley Periodicals, Inc.