ORGANIZATION OF WATER AROUND A BERYLLIUM CATION

Ab initio molecular orbital calculations are used to examine the possible structures of hydrated Be2+ ions with up to six water molecules partitioned between the first and second coordination shells. The potential energy surfaces of Be[H2O]42+, Be[H2O]52+, and Be[H2O]62+ are found to have many local minima with different arrangements of the surrounding water molecules. In each of these cases, however, the lowest energy configuration has four water molecules in the first shell with any remaining water molecules relegated to the second shell, suggesting that the hydration number of Be2+ is indeed 4. We have further shown some energetic and geometrical details of how the second shell of water molecules is organized around the tetraaquated beryllium ion.