Size Dependence of Blackbody Radiation Induced Hydrogen Formation in Al+(H2O) n Hydrated Aluminum Cations and Their Reactivity with Hydrogen Chloride

Trapped hydrated aluminum cluster ions are studied by FT-ICR mass spectrometry on a time scale of several seconds. Blackbody radiation, besides causing the fragmentation of the cluster by stepwise loss of individual water molecules, induces in hydrated aluminum clusters Al+(H2O) n an intracluster reaction yielding hydrated hydroxide and releasing molecular hydrogen. Local deviations of the individual rate constants for the water loss process from a linear size dependence are due to increased rigidity of certain sizes due to the formation of stabilizing hydrogen-bonded bridges. In comparison with previously studied hydrated ions, surface versus internal solvation is discussed. The preferential occurrence of the intracluster reaction in the size region of n = 11−24 is attributed to a concerted proton-transfer mechanism, in which a chain of at least two water molecules is needed to transfer a proton between two first solvation shell water molecules, leading to formation of an Al(OH)2 +(H2O) n hydrated aluminum dihydroxide cation and molecular hydrogen. The Al+(H2O) n species with n ≥ 13 and all investigated Al(OH)2 +(H2O) m species are able to react with and “dissolve” HCl. The maximum number of HCl molecules in the cluster strongly depends on the number of water molecules available for solvation. The presence of HCl in the cluster removes the upper limit for the intracluster reaction, which leads to the formation of molecular hydrogen, driven by blackbody radiation. This is taken as further evidence for the validity of the proton-transfer mechanism.