π-Stacked Quadruply Hydrogen-Bonded Dimers:  π-Stacking Influences H-Bonding

[reaction: see text] The effects of pi-stacking on the stability of multiply hydrogen-bonded systems are investigated using hybrid DFT calculations on pi-stacked quadruply H-bonded dimers of ureidopyrimidinone in its different tautomeric forms. Both the strengths of the hydrogen bonds and the relative occurrence of tautomers are influenced by pi-stacking; electrostatics and natural bond orbital analysis are used to explain these observations. Finally, these conclusions are independent of the precise nature of the multiply hydrogen-bonded systems, including the DNA base pairs.