Selective dissociation in dication-molecule reactions

The single electron transfer reactions between (13)CO(2)(2+) and (12)CO(2) and between (18)O(2)(2+) and (16)O(2) have been studied, using a position-sensitive coincidence technique, to test recently proposed explanations for the preferential dissociation of the (13)CO(2)(+) ion (the capture monocation) formed following electron transfer to (13)CO(2)(2+). In our studies of the carbon dioxide collision system, in agreement with previous work, the capture monocation shows a greater propensity to dissociate than the monocation formed from the neutral, (12)CO(2)(+) (the ejection monocation). The coincidence data clearly show that the dissociation pathways of the (13)CO(2)(+) and (12)CO(2)(+) ions are different and are consistent with the ejection monocation dissociating via population of the C(2)Sigma state, whilst the capture ion is predominantly directly formed in dissociative quartet states. This state assignment is in accord with an expected preference for one-electron transitions in the electron transfer process. A propensity for one-electron transitions also rationalizes our observation that following dissociative single electron transfer between (18)O(2)(2+) and (16)O(2) the ejection monocation ((16)O(2)(+)) preferentially dissociates; the opposite situation to that observed for carbon dioxide. The coincidence results for this reaction indicate the (16)O(2)(+) dissociation results from population of the B((2)Sigma) state. The less favoured dissociation of the capture monocation clearly involves population of a different electronic state(s) to those populated in the ejection ion. Indeed, the experimental data are consistent with the dissociation of the capture monocation via predissociated levels of the b((4)Sigma) state. Since the population of the B((2)Sigma) state from the neutral O(2) molecule involves a one-electron transition, and the population of the valence dissociative states of O(2)(+) from the dication are multi-electron processes, the preferential dissociation of the ejection monocation in this collision system can be rationalized by the same principles used to explain the electron transfer reactivity of CO(2)(2+) with CO(2).