Reactions of hydrated aluminum ions with methanol and formic acid

The reactions of hydrated aluminum ions Al(H₂O)_n⁺, 20<n<60, with methanol and formic acid were investigated using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Besides black body radiation and collisionally induced fragmentation, also efficient ligand transfer processes as well as an intracluster oxidation of the aluminum take place. While in pure Al(H₂O)_n⁺ ions this reaction, where the Al⁺ is oxidized to Al³⁺ and water reduced to yield molecular H₂, proceeds only in a limited size range of 11<n<24. Addition of formic acid, similar to the previously studied addition of HCl, removes the upper limit, but the reactions proceed more slowly. With methanol, the upper limit remains, but it is shifted to higher values of n. These observations are consistent with our previously proposed "acid catalyzed", concerted proton transfer mechanism, with the efficiency of the reaction being highest in the presence of HCl, and decreasing with the decreasing acidities of formic acid and methanol.