Size-dependent reactivity of aluminum cluster cations toward water molecules

We present gas-phase reaction of size-selected aluminum cluster cations (Al-N(+), N = 3-19) with a water molecule. As major reaction products, AlNO+ and Al-N(H2O)(+) were observed for N >= 6 under single-collision conditions. The production of AlNO+ implies that Al-N(+) reacts with a H2O molecule to form AlNO+ and H-2. In contrast to the previous report on H-2 generation by Al-N(-), a wider size range of Al-N(+) exhibited high reactivity via a different mechanism. Selective formation of either AlNO+ or Al-N(H2O)(+) was observed in the size range of 6 <= N <= 13. Our results suggest that the internal energy of a precursor complex [Al-N(H2O)(+)]* is relevant to the selectivity of the reaction products.