Infrared spectroscopy of the hydrated clusters of cationic pentane, which are generated through the vacuum ultraviolet photoionization in the gas phase, is carried out to probe the acidic properties of their CH bonds. The monohydrated pentane cation forms the proton-shared structure, in which the proton of CH in cationic pentane is shared between the pentyl radical and water molecule. In the di- and trihydrated clusters, the proton of CH is completely transferred to the water moiety so that the clusters are composed of the pentyl radical and protonated water cluster. These results indicate that two water molecules are enough to cause the proton transfer from CH of cationic pentane, and thus its acidity is highly enhanced with the ionization.

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