Stacking interactions of resonance-assisted hydrogen-bridged rings and C 6 -aromatic rings.

Stacking interactions between six-membered resonance-assisted hydrogen-bridged (RAHB) rings and C-6-aromatic rings were systematically studied by analyzing crystal structures in the Cambridge Structural Database (CSD). The interaction energies were calculated by quantum-chemical methods. Although the interactions are stronger than benzene/benzene stacking interactions (-2.7 kcal mol(-1)), the strongest calculated RAHB/benzene stacking interaction (-3.7 kcal mol(-1)) is significantly weaker than the strongest calculated RAHB/RAHB stacking interaction (-4.7 kcal mol(-1)), but for a particular composition of RAHB rings, RAHB/benzene stacking interactions can be weaker or stronger than the corresponding RAHB/RAHB stacking interactions. They are also weaker than the strongest calculated stacking interaction between five-membered saturated hydrogen-bridged rings and benzene (-4.4 kcal mol(-1)) and between two five-membered saturated hydrogen-bridged rings (-4.9 kcal mol(-1)). SAPT energy decomposition analyses show that the strongest attractive term in RAHB/benzene stacking interactions is dispersion, however, it is mostly canceled by a repulsive exchange term; hence the geometries of the most stable structures are determined by an electrostatic term.