Reactions of Aluminum Oxide Cluster Cations with Ethane: A Mass-Spectrometric and Vibrational Spectroscopy Study

The pathways for the reactions of aluminum oxide cluster ions with ethane have been measured. For selected ions (Al₂O⁺, Al₃O₂ ⁺, Al₃O₄ ⁺, Al₄O₇ ⁺) the structure of the collisionally-stabilized reaction intermediates were explored by measuring the photodissociation vibrational spectra from 2600 cm^-1-3100 cm^-1. Density functional theory was used to calculate features of the potential energy surfaces for the reactions and the vibrational spectra of intermediates. Generally, more than one isomer contributes to the observed spectrum. The oxygen-deficient clusters Al₂O⁺ and Al₃O₂ ⁺ have large C-H activation barriers, so only the entrance channel complexes in which intact C₂H₆ binds to aluminum are observed. This interaction leads to a substantial (~200 cm^-1) red shift of the C-H symmetric stretch in ethane, indicating significant weakening of the proximal C-H bonds. In Al₃O₄ ⁺, the complex formed by interactions with three C₂H₆ is investigated and, in addition to entrance channel complexes, the C-H activation intermediate Al₃O₄H⁺(C₂H₅)(C₂H₆)₂ is observed. For oxygen-rich Al₄O₇ ⁺, the C₂H₆ is favored to bind at an aluminum site far from the reactive superoxide group, reducing the reactivity. As expected, oxygen-rich species and open-shell cluster ions have smaller barriers for C-H bond activation, except for Al₃O₄ ⁺ which is predicted and observed to be reactive.