| Résumé | We report computational studies on Al+(H2O)n, n = 6-9, and HAlOH+(H2O)n-1, n = 6-14, by the density functional theory based ab initio molecular dynamics method, employing a planewave basis set with pseudopotentials, and also by conventional methods with Gaussian basis sets. The mechanism for the intracluster H2 elimination reaction is explored. First, a new size-dependent insertion reaction for the transformation of Al+(H2O)n into HAlOH+(H2O)n-1 is discovered for n 8. This is because of the presence of a fairly stable six-water-ring structure in Al+(H2O)n with 12 members, including the Al+. This structure promotes acidic dissociation and, for n 8, leads to the insertion reaction. Gaussian based BPW91 and MP2 calculations with 6-31G* and 6-31G** basis sets confirmed the existence of such structures and located the transition structures for the insertion reaction. The calculated transition barrier is 10.0 kcal/mol for n = 9 and 7.1 kcal/mol for n = 8 at the MP2/6-31G** level, with zero-point energy corrections. Second, the experimentally observed size-dependent H2 elimination reaction is related to the conformation of HAlOH+(H2O)n-1, instead of Al+(H2O)n. As n increases from 6 to 14, the structure of the HAlOH+(H2O)n-1 cluster changes into a caged structure, with the Al-H bond buried inside, and protons produced in acidic dissociation could then travel through the H2O network to the vicinity of the Al-H bond and react with the hydride H to produce H2. The structural transformation is completed at n = 13, coincident approximately with the onset of the H2 elimination reaction. From constrained ab initio MD simulations, we estimated the free energy barrier for the H2 elimination reaction to be 0.7 eV (16 kcal/mol) at n = 13, 1.5 eV (35 kcal/mol) at n = 12, and 4.5 eV (100 kcal/mol) at n = 8. The existence of transition structures for the H2 elimination has also been verified by ab initio calculations at the MP2/6-31G** level. Finally, the switch-off of the H2 elimination for n > 24 is explored and attributed to the diffusion of protons through enlarged hydrogen bonded H2O networks, which reduces the probability of finding a proton near the Al-H bond. |
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