Aromatic rings act as hydrogen bond acceptors

Simple energy calculations show that there is a significant interaction between a hydrogen bond donor (like the > NH group) and the centre of a benzene ring, which acts as a hydrogen bond acceptor. This interaction, which is about half as strong as a normal hydrogen bond, contributes approximately 3kcal/mol (1 cal = 4.184 J) of stabilizing enthalpy and is expected to play a significant role in molecular associations. It is of interest that the aromatic hydrogen bond arises from small partial charges centred on the ring carbon and hydrogen atoms: there is no need to consider delocalized electrons. Although some energy calculations have included such partial charges, their role in forming such a strong interaction was not appreciated until after aromatic hydrogen bonds had been observed in protein-drug complexes.