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Using a density functional theory approach with the B3LYP and PBE0 functionals and the 6-G31(d,p) basic set, we simulate the partial vibrational spectra of isolated H2O and C60 molecules, of a periodic H2O structure (ice XI) and of a single H2O molecule encaged in a buckyball (H2O@C60). A very significant result of the H2O@C60 calculation is that the C-atoms of the hosting fullerene lattice (which for pure C60 are subjected to a vibrational frequency spectrum that terminates at ∼ 1600 cm−1) were found to participate in the high frequency modes at ∼ 3800 cm−1 of the H2O guest, indicating a coupling between the modes of the two sub systems. The effect of temperature between 5 and 300 K on the atomic kinetic energies, Ke(X) (X = C,O,H) were also deduced. The calculated Ke(C) in C60 using B3LYP was found to be in better agreement with experiment than that of PBE0. Surprisingly, the zero point Ke(H) in the hydrogen bonds (HBs) free H2O@C60, as well as its T-dependence, were found to be nearly the same as those of the HBs containing condensed bulks of H2O, namely ice and liquid water. This result is at variance with that of other H2O confining systems measured using deep inelastic neutron scattering, where anomalous Ke(H) values were generically observed. An attempt to understand this behavior is made in view of the observed coupling between the caged H2O modes and those of the C60 host. |
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