| Abstract: | The interaction of superoxide ion O2? with up to four water molecules O2?: (H2O)n, n = 1, 2, 4] has been investigated using ab initio molecular orbital theory. The binding energy of O2?: H2O is calculated to be ?20.6 kcal/mol in good agreement with gas phase experimental data. At the MP3/6-31G* level the O2?:H2O complex has a C2v structure with a double (cyclic) hydrogen bond between O2? and H2O. A Cs structure with a single hydrogen bond is only 0.7 kcal/mol less stable. Interaction of H2O with the doubly occupied π* orbital of O2? is preferred slightly over interaction with the singly occupied π* orbital. Natural bond orbital analysis suggests that both electrostatic and charge transfer interactions are important in anionic complexes. The charge transfer occurs predominantly in the O2? → H2O direction and is important in determining the relative stabilities of the different structures and states. Singly and doubly hydrogen-bonded structures for the O2?: (H2O)2 and O2?: (H2O)4 clusters were found to be similar in stability and the increase in binding of the cluster becomes smaller as each additional water molecule is added to the cluster. |
