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 (similar to 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.