Electronic origin of the dependence of hydrogen bond strengths on nearest-neighbor and next-nearest-neighbor hydrogen bonds in polyhedral water clusters (H2O)n, n = 8, 20 and 24

Abstract

The influence of the nearest neighbor and next-nearest neighbor water molecules on the strength of the hydrogen (H) bonds was examined for the polyhedral clusters of cubic (H₂O)₈, dodecahedral (H₂O)₂₀ and tetrakaidecahedral (H₂O)₂₄ cages. The relative stability and the characteristics of the H bond networks are also studied. The charge-transfer (CT) and dispersion interaction terms of every pair of H bonds are evaluated using perturbation theory based on the locally-projected molecular orbitals (LPMO PT). Every water molecule and every H-bonded pair in these polyhedral clusters are classified by the types of the neighbor molecules and H bonds. The relative binding energies among the polyhedral clusters are grouped by these classifications. The optimized O⋯O distances, which are strongly correlated with the calculated pairwise CT terms, are dependent on the 49 sub-groups of the H bonds determined by the type of the neighbor molecules. The electronic origin of this dependence is analyzed using Mulliken's charge-transfer theory, and employing a few assumptions, the analytical formulas for the contribution of the CT terms to the H bond energy are derived.