Water clusters confined in nonpolar cavities by ab initio calculations

Encapsulation of (H2O)(n) clusters (n = 1-22) in fullerene cages of different diameters (0.73-1.41 nm) has been investigated using gradient-corrected density functional theory. A linear relationship between cavity volume and maximum number of the encapsulated water molecules has been obtained. The interaction between water molecules and the fullerene wall was identified as physisorption with an adsorption energy of about 1.1 kcal/mol per molecule. The equilibrium configurations of small confined water clusters (n < 12) roughly resemble those of gas-phase clusters, whereas larger water clusters tend to adopt cage-like configurations when they are encapsulated in fullerene cages of sufficiently large diameter (i.e., > 1.4 nm). The dipole moments of water clusters in the confined phase are smaller than those in the gas phase due to the screening effect of the outer fullerene cage. These results might shed some light on the behavior of water clusters confined in the nonpolar cavities of biological interests.