Selective dissociation in dication-molecule reactions

The single electron transfer reactions between (CO22+)-C-13 and (CO2)-C-12 and between O-18(2)2+ and O-16(2) have been studied, using a position-sensitive coincidence technique, to test recently proposed explanations for the preferential dissociation of the (CO2+)-C-13 ion (the capture monocation) formed following electron transfer to (CO22+)-C-13. 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, (CO2+)-C-12 (the ejection monocation). The coincidence data clearly show that the dissociation pathways of the (CO2+)-C-13 and (CO2+)-C-12 ions are different and are consistent with the ejection monocation dissociating via population of the C-2 Sigma(+)(g) 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 O-18(2)2+ and O-16(2) the ejection monocation (O-16(2)+) preferentially dissociates; the opposite situation to that observed for carbon dioxide. The coincidence results for this reaction indicate the O-16(2)+ dissociation results from population of the B((2)Sigma(-)(g)) 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(-)(g)) state. Since the population of the B((2)Sigma(-)(g)) 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 CO22+ with CO2.