Experimental and Computational Study of Hydration Reactions of Aluminum Oxide Anion Clusters

Hydration of aluminum oxide anion clusters was studied in the gas phase using an ion trap secondary ion mass spectrometer. Hydration of both AlO 2 - and Al 2 O 4 H - occurred by the consecutive addition of two H 2 O molecules. For hydration of AlO 2 -, the rate constants for addition of the first and second water molecules are 4 × 10 -11 and 4 × 10 -10 cm 3 molecule -1 s -1, respectively. The first and second hydration rate constants for Al 2 O 4 H - are 2 × 10 -9 and 8 × 10 -10 cm 3 molecule -1 s -1, respectively. A comparison of the experimental rate constants to the theoretical rate constants reveals that addition of the first H 2 O to AlO 2 - is only 2% efficient, whereas addition of the first H 2 O to Al 2 O 4 H - is 100% efficient. Ab initio calculations were performed to assist in the interpretation of the kinetic results. Reaction mechanisms and energetics for the hydration of the AlO 2 - system were calculated using the HF/6-311+G(d(Al),p), B3LYP/6-31+G(d), B3LYP/6-311+G(2d,p), B3LYP/6-311+G(3d2f,2p), and MP2/6-311+G(2d,p) levels of theory. Calculations on the hydration of the Al 2 O 4 H - system were performed using the B3LYP/6-311+G(2d,p) level of theory. Ab initio results revealed that the addition of the first and second waters, for both the AlO 2 - and Al 2 O 4 H - systems, results in the formation of four-membered transition states, with simultaneous Al−O bond formation and proton transfer. However, a significant later transition state is observed, with respect to the Al−O and H−O bond lengths, for the addition of the second water molecule in the Al 2 O 4 H - system. A comparison of the reaction mechanisms and energetics was not sufficient to account for the 2 orders of magnitude difference in rate constants; however, the reactivity differences do correlate with the dipole moment of the aluminum oxide anions, which may serve to preorient the incoming water molecule, thus enhancing the reaction rate.