On the Nature of Bonding in Lone Pair•••π-Electron Complexes: CCSD(T)/Complete Basis Set Limit Calculations

The nature of the stabilization in lone pair center dot center dot center dot pi-electron complexes was investigated using the highly accurate CCSD(T) method based on the complete basis set limit, as well as the DFT-SAPT perturabative method. Specifically, we studied various structures of benzene center dot center dot center dot water, benzene center dot center dot center dot dimethylether, and 1,2,4,5-tetracyanobenzene center dot center dot center dot water complexes. The lone pair center dot center dot center dot pi-electron interactions between an unsubstituted aromatic ring and a water molecule are repulsive in the whole range of vertical distances. Partial stabilization results by rotating the water molecule by 900 (with the water and aromatic ring being localized in parallel planes) or by decreasing the negative charge at oxygen and simultaneously increasing the polarizability of the system, which provides stabilization even for genuine lone pair center dot center dot center dot pi-electron interactions. In these cases, a substantial part of the stabilization stems from dispersion energy. Substituting an aromatic ring by electron-withdrawing cyano groups represents the most powerful way to achieve a substantial stabilization of genuine lone pair center dot center dot center dot pi-electron interactions. This stabilization is comparable to quite strong H-bonding, originating in electrostatic and, to a slightly lesser degree, dispersion energies.