Unusually strong hydrogen bond cooperativity in particular (H₂O)₂₀ clusters

Drawing upon an intuitive charge-transfer-based picture of hydrogen bonding, we demonstrate that cooperativity effects acting in concert can lead to unusually strong hydrogen bonds in neutral water clusters. The structure, vibrational, and NMR properties of a (H2O)20 pentagonal dodecahedron cluster containing such a strong hydrogen bond were studied using second-order perturbation theory and density functional theory. The hydrogen bond length was found to be shorter than 2.50 Å. A large redshift of over 2000 cm-1 with respect to the isolated water molecule was predicted for the OH stretching frequency of the donor water molecule. A large downfield shift to 13.5 ppm of the isotropic part of the 1H magnetic shielding tensor together with an unusually large shielding anisotropy of 49.9 ppm was obtained. The hydrogen bond energy was calculated using symmetry-adapted perturbation theory and was found to be more than three times stronger than a typical hydrogen bond in liquid water.