Single-Sided Delocalized Polarization of the C₆₀ Cage and Reduced Infrared Intensities and Dipole Moment of H₂O@C₆₀

The C₆₀ fullerene cage can encapsulate a small molecule like water and provides room to leave the encapsulated component rather isolated, but the true nature of the intracomplex interactions should be further elucidated for better understanding and utility of this series of complexes. Here, an analysis toward this goal is conducted for H₂O@C₆₀ by infrared spectral measurements and theoretical calculations. It is shown that the response of the π electrons of the C₆₀ cage upon encapsulating a water molecule is single-sided and delocalized in that the electron density is partially transferred from the -z side to the +z side of the cage (when the z axis is taken along the water dipole) but almost only inside the cage, explaining the significant reduction of the dipole moment and the infrared intensities. Those infrared intensities have a large temperature dependence in a way that the bands gain intensities upon lowering the temperature down to 10 K, possibly due to coupling with lattice phonons.