Dynamics of Encapsulated Water inside Mo132 Cavities

The structure and dynamics of water confined inside a polyoxomolybdate molecular cluster [{(Mo)Mo(5)O(21)(H(2)O)(6)}(12){Mo(2)O(4)(SO(4))}(30)](72-) metal oxide nanocapsule have been studied by means of molecular dynamics simulations under ambient conditions. Our results are compared to experimental data and theoretical analyses done in reverse micelles, for several properties. We observe that the characteristic three-dimensional hydrogen bond network present in bulk water is distorted inside the cavity where water organizes instead in concentric layered structures. Hydrogen bonding, tetrahedral order, and orientational distribution analyses indicate that these layers are formed by water molecules hydrogen bonded with three other molecules of the same structure. The remaining hydrogen bond donor/acceptor site bridges different layers as well as the whole structure with the hydrophilic inner side of the cavity. The most stable configuration of the layers is thus that of a buckyball with 12 pentagons and a variable number of hexagons. The geometrical constraints make it so that the bridges between the layers display a significant degree of frustration. The main modes of motion at short times are correlated fluctuations of the entire system with a characteristic frequency. Switches of water molecules between layers are rare events, due to the stability of the layers. At long times, the system shows a power law decay (pink noise) in properties like the fluctuations in the number of molecules in the structures and the total dipole moment. Such behavior has been attributed to the complex relaxation of the hydrogen bond network, and the exponents found are close to those encountered in bulk water for the relaxation of the potential energy. Our results reveal the importance of the competition between the confinement and the long-range structure induced in this system by the hydrogen bond network.