Selective dissociation in dication–molecule reactions

Abstract

The single electron transfer reactions between ¹³CO₂²⁺ and ¹²CO₂ and between ¹⁸O₂²⁺ and ¹⁶O₂ have been studied, using a position-sensitive coincidence technique, to test recently proposed explanations for the preferential dissociation of the ¹³CO₂⁺ ion (the capture monocation) formed following electron transfer to ¹³CO₂²⁺. 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, ¹²CO₂⁺ (the ejection monocation). The coincidence data clearly show that the dissociation pathways of the ¹³CO₂⁺ and ¹²CO₂⁺ ions are different and are consistent with the ejection monocation dissociating via population of the C²Σ⁺_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 ¹⁸O₂²⁺ and ¹⁶O₂ the ejection monocation (¹⁶O₂⁺) preferentially dissociates; the opposite situation to that observed for carbon dioxide. The coincidence results for this reaction indicate the ¹⁶O₂⁺ dissociation results from population of the B(²Σ⁻_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(⁴Σ⁻_g) state. Since the population of the B(²Σ⁻_g) state from the neutral O₂ molecule involves a one-electron transition, and the population of the valence dissociative states of O₂⁺ 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.