COMPUTER-SIMULATION OF WATER CLUSTERS CONTAINING AN H2O-O-2 CHARGE-TRANSFER COMPLEX

A proposed mechanism for the photonucleation of water vapour in the pr esence of oxygen involves the initial formation of a charge-transfer ( CT) complex of molecular oxygen and water, H2O+O2-. The large dipole o f this complex then attracts the surrounding polar water molecules, th ereby forming a cluster. To investigate the properties of such a clust er, we have carried out computer simulations at 300 K using two differ ent water models. For both models, we found that the complex resided p referentially at the surface of the cluster despite its large dipole m oment. This preference was in accord with the results of ab initio cal culations carried out on a small cluster. The internal energy differen ce between a cluster of n water molecules and a cluster of (n - 1) wat er molecules and one complex was of the order of 10-20 kJ mol(-1), the precise value depending on the cluster size. This energetic stabilisa tion reflects the fact that the CT complex can make three hydrogen bon ds in contrast to the two made by a water molecule.