Size, Adsorption Site,and Spin Effects in the Reactionof Al Clusters with Water Molecules: Al17and Al28as Examples.

The first step of the reaction of two relatively largeAlmclusters (m= 17,28) with a fewwater molecules has been studied by electronic structure methods.The complexes Alm·(H2O)n(n= 1–2) have beencharacterized, and the saddle points corresponding to the first stepin the reaction, namely, formation of HAlmOH·(H2O)n−1systems,have been located. The Al28cluster is special in the senseit has two electronic states, singlet and triplet, which are veryclose 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 clusterdistortion being moderately significant in the stability of the complexes.The interaction with water does not appear, in general, to bring thetriplet state of the Al28·(H2O)2adducts below the singlet; not even the corresponding saddle pointsappear to be lower in energy. The rate coefficients, tunneling transmissionfactors, and activation free energies have been computed and comparedwith those of the Al13and Al3clusters, evenwith those of the Al atom. It turns out the rates are quite closeto those of Al3and much larger than those of Al and Al13. There is no dramatic difference between the reactivityof the singlet and triplet state of Al28; however, thereare very significant differences between different sites. Finally,we studied the interaction between the lowest-lying singlet and tripletstates of Al28through multireference configuration interaction(MRCI) spin–orbit computations. The vertical excitation energiescorresponding to a number of low-lying singlet and triplet statesare also determined by MRCI computations. It turns out that the spin–orbitinteraction is very weak, which suggests that both states, the lowest-lyingsinglet and triplet, could evolve somehow independently, at leastwhen interacting with closed-shell molecules. It is suggested thatthe situation could be quite different in a reaction with molecularradicals or if external fields are applied. [ABSTRACT FROM AUTHOR]