H2O/Olefinic-π Interaction inside a Carbon Nanocage

The H2O/CH2=CH2-type hydrogen-bonding (H-bonding) model was experimentally constructed using a water complex of an open-cage C-60 derivative, in which an olefinic double bond and a single molecule of H2O are geometrically confined. To investigate OH/pi-type H-bonding, that is, H2O center dot center dot center dot(C=C) interaction, we performed H-1 NMR spectroscopic studies that demonstrated the monotonic down-field shift of the proton signal corresponding to H2O with remarkable rotational perturbation by lowering the temperature. From the temperature dependence of the angular momentum correlation time (tau(j)), the interaction energy was quantitatively estimated to be ca. 0.3 kcal/mol. The computational studies were thoroughly conducted to clarify its inherent nature. As a consequence, the orientation of H2O was found to play a prominent role in varying the bonding strength as well as contribution from the electrostatic attraction and orbital-orbital interaction significantly driven by the favorable orbital overlap identified as pi(C=C) -> sigma*(OH) interaction.