Size-dependent reactivity of aluminum cluster cations toward water molecules

We present gas-phase reaction of size-selected aluminum cluster cations (AlN+ , N = 3–19) with a water molecule. As major reaction products, AlNO+ and AlN(H2O)+ were observed for N ≥ 6 under single-collision conditions. The production of AlNO+ implies that AlN+ reacts with a H2O molecule to form AlNO+ and H2. In contrast to the previous report on H2 generation by AlN-, a wider size range of AlN+ exhibited high reactivity via a different mechanism. Selective formation of either AlNO+ or AlN(H2O)+ was observed in the size range of 6 ≤ N ≤ 13. Our results suggest that the internal energy of a precursor complex [AlN(H2O)+]∗ is relevant to the selectivity of the reaction products.