The nature and origin of the
![](/images/gifchars/pi.gif)
-H interaction in both the ethene (olefinic) and benzene (aromatic)complexes of the first-row hydrides (BH
3, CH
4, NH
3, H
2O, and HF) has been investigated by carrying outhigh level ab initio calculations. The results indicate that the strength of the
![](/images/gifchars/pi.gif)
-H interaction is enhanced asone progresses from CH
4 to HF. Unlike conventional H-bonds, this enhancement cannot be simply explainedby the increase in electrostatic interactions or the electronegativity of the atom bound to the
![](/images/gifchars/pi.gif)
H-bondedproton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchangecomponents of the total binding energy are important. Thus, the inductive energy is highly correlated to theolefinic
![](/images/gifchars/pi.gif)
-H interaction as we progress from CH
3 to HF. On the other hand, both electrostatic and inductiveenergies are important in the description of the aromatic
![](/images/gifchars/pi.gif)
-H interaction. In either case, the contribution ofdispersion energies is vital to obtain an accurate estimate of the binding energy. We also elaborate on thecorrelation of various interaction energy components with changes in geometries and vibrational frequencies.The red-shift of the
Y-H mode is highly correlated to the inductive interaction. The dramatic increase in theexchange repulsion energies of these
![](/images/gifchars/pi.gif)
complexes as we progress from CH
4 to HF can be correlated to theblue-shift of the highly IR active out-of-plane bending mode of the
![](/images/gifchars/pi.gif)
system.