Origin of the attraction and directionality of the NH/π interaction:: Comparison with OH/π and CH/π interactions

High-level ab initio calculations were carried out to evaluate the interaction between the pi face of benzene and ammonia as a model of NH/pi interaction. The intermolecular interaction energy was calculated from the extrapolated MP2 interaction energy at the basis set Limit and a CCSD(T) correction term. The calculated interaction energy (-2.22 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The monodentate complex is slightly more stable than the bidentate and tridentate complexes. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the monodentate complex (0.13 kcal/mol) is repulsive. The large gain in the attraction by electron correlation correction (-2.36 kcal/mol) indicates that the dispersion interaction is significantly important for the attraction. The electrostatic energy (-1.01 kcal/mol) is also important for the attraction. The benzene-water (OH/pi) interaction energy (-3.17 kcal/mol) is larger than the benzene-ammonia (NH/pi) interaction. The dispersion interaction is again important for the attraction in the benzene-water complex. The attraction in the benzene-ammonia complex is stronger than that in the benzene-methane (CH/pi) complex (-1.45 kcal/mol). The amount of electrostatic energy is mainly responsible for the magnitude of the attractions in these three complexes. The directionality for the NH/pi and OH/pi interactions is mainly controlled by the electrostatic interaction.