Olefinic vs aromatic π-H interaction: A theoretical investigation of the nature of interaction of first-row hydrides with ethene and benzene

The nature and origin of the π-H interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides (BH₃, CH₄, NH₃, H₂O, and HF) has been investigated by carrying out high level ab initio calculations. The results indicate that the strength of the π-H interaction is enhanced as one progresses from CH₄ to HF. Unlike conventional H-bonds, this enhancement cannot be simply explained by the increase in electrostatic interactions or the electronegativity of the atom bound to the π H-bonded proton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy are important. Thus, the inductive energy is highly correlated to the olefinic π-H interaction as we progress from CH₃ to HF. On the other hand, both electrostatic and inductive energies are important in the description of the aromatic π-H interaction. In either case, the contribution of dispersion energies is vital to obtain an accurate estimate of the binding energy. We also elaborate on the correlation of various interaction energy components with changes in geometries and vibrational frequencies. The red-shift of the v_Y-H mode is highly correlated to the inductive interaction. The dramatic increase in the exchange repulsion energies of these π complexes as we progress from CH₄ to HF can be correlated to the blue-shift of the highly IR active out-of-plane bending mode of the π system.