Spin conservation accounts for aluminum cluster anion reactivity pattern with O2

The reactivity pattern of small (similar to 10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long- standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even- numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd- numbered clusters increased appreciably when singlet oxygen was used in place of ground- state ( triplet) oxygen. Conversely, monohydride clusters AlnH-, in which addition of the hydrogen atom shifts the spin state by converting formerly open- shell structures to closed- shell ones ( and vice versa), exhibited an opposing trend: The odd- n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.