Long Range π-Type Hydrogen Bond in the Dimers (HF)2, (H2O)2, and H2O−HF

Based on ab initio calculations at MP2 and MP4 levels, a chemically intuitive π-type hydrogen-bond model is proposed to illustrate and interpret the small deviations from a strictly linear hydrogen bond X ··· H − Y in the dimers (HF) 2, (H 2 O) 2, and H 2 O−HF. The computational results show that π-type hydrogen-bond interaction is secondary and is an attraction between the H atom of the H−X bond and lone pair on Y. In particular, the orientations of lone pairs are detected, by the energy scan at the MP2/6-311+G (2df, 2p) level with a probing point charge of q = − 1, which is important to show the existence of the π-type hydrogen bond. Furthermore, the interaction energy of π-type hydrogen bond, D π, and stabilized energy of bent hydrogen bond, Δ E sb, are also calculated and discussed.