Size, adsorption site, and spin effects in the reaction of Al clusters with water molecules: Al17 and Al28 as...

The first step of the reaction of two relatively large Alm clusters (m = 17, 28) with a few water molecules has been studied by electronic structure methods. The complexes Alm·(H2O)n (n = 1-2) have been characterized, and the saddle points corresponding to the first step in the reaction, namely, formation of HAlmOH·(H2O)n-1 systems, have been located. The Al28 cluster is special in the sense it has two electronic states, singlet and triplet, which are very close in energy and also have quite similar equilibrium structures. The preferred adsorption and reaction sites have been determined. We find quite clear preferences toward some sites, the effect of cluster distortion being moderately significant in the stability of the complexes. The interaction with water does not appear, in general, to bring the triplet state of the Al28·(H2O)2 adducts below the singlet; not even the corresponding saddle points appear to be lower in energy. The rate coefficients, tunneling transmission factors, and activation free energies have been computed and compared with those of the Al13 and Al3 clusters, even with those of the Al atom. It turns out the rates are quite close to those of Al3 and much larger than those of Al and Al13. There is no dramatic difference between the reactivity of the singlet and triplet state of Al28; however, there are very significant differences between different sites. Finally, we studied the interaction between the lowest-lying singlet and triplet states of Al28 through multireference configuration interaction (MRCI) spin-orbit computations. The vertical excitation energies corresponding to a number of low-lying singlet and triplet states are also determined by MRCI computations. It turns out that the spin-orbit interaction is very weak, which suggests that both states, the lowest-lying singlet and triplet, could evolve somehow independently, at least when interacting with closed-shell molecules. It is suggested that the situation could be quite different in a reaction with molecular radicals or if external fields are applied.