Computational studies of the stability of the (H2O)100 nanodrop

The stability of the (H2O)100 nanodrop, experimentally known from a polyoxomolybdate crystal structure (Müller et al., Inorg. Chem. Commun. 6 (2003) 52) and other structures inferred from clathrate structures and from the literature, are studied by quantum-chemical B3LYP and MP2 computations. Positions for the H atoms in the 100-molecule cluster are suggested, which will then be of C2h/Ci-symmetry when considering the oxygens/all atoms. The free energies are compared to the trends for smaller clusters with 15-30 molecules. For the small clusters both cage-based structures and denser structures with a larger number of H-bonds obtained from Bandow and Hartke who used an evolutionary algorithm (Bandow and Hartke, J. Phys. Chem. A 110 (2006) 5809) are investigated. The dense structures are most often found to be lower in electronic energy. The cage-based structures, to which the structure of the experimentally found (H2O)100 cluster can be categorized, gain in stability when their Gibbs free energies at 298 K are considered, although which of a dense or a cage-based structure that is predicted to be the most stable depends on the quantum-chemical method employed. Additional cage-based clusters in the 35-81 molecular range were constructed for comparison. The constructed (H2O)42 cluster has an extraordinary high symmetry (S6), even when the hydrogens are considered. The experimental cluster with 100 molecules and the constructed cluster with 42 molecules are found to be in fair agreement with a plausible overall global-minimum energy vs. cluster size trend inferred from the smaller clusters.