Computer simulation of water clusters containing an H2O–O2 charge-transfer complex

Abstract

A proposed mechanism for the photonucleation of water vapour in the presence of oxygen involves the initial formation of a charge-transfer (CT) complex of molecular oxygen and water, H ₂ O ⁺ O ₂ ^- . The large dipole of this complex then attracts the surrounding polar water molecules, thereby forming a cluster. To investigate the properties of such a cluster, we have carried out computer simulations at 300 K using two different water models. For both models, we found that the complex resided preferentially at the surface of the cluster despite its large dipole moment. This preference was in accord with the results of ab initio calculations carried out on a small cluster. The internal energy difference between a cluster of n water molecules and a cluster of (n-1) water molecules and one complex was of the order of 10–20 kJ mol ^-1 , the precise value depending on the cluster size. This energetic stabilisation reflects the fact that the CT complex can make three hydrogen bonds in contrast to the two made by a water molecule.