We present gas-phase reaction of size-selected aluminum cluster cations (Al N +, N = 3-19) with a water molecule. As major reaction products, Al N O+ and Al N (H 2O)+ were observed for N ≥ 6 under single-collision conditions. The production of Al N O+ implies that Al N + reacts with a H2O molecule to form Al N O+ and H2. In contrast to the previous report on H2 generation by Al N -, a wider size range of Al N + exhibited high reactivity via a different mechanism. Selective formation of either Al N O+ 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)+]â̂ - is relevant to the selectivity of the reaction products.
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