Hydrogen interactions with a Pd4 cluster:: Triplet and singlet states and transition probability

The DFT method is used to analyze the sin-let and triplet PES cross-sections for it number of reaction pathways of the interaction between a hydrogen molecule and a palladium tetramer. Stationary points characterizing stable singlet and triplet complexes and transition states are determined, Predissociated Pd4H2 complexes with binding, energies of 5.0-6.4 kcal/mol are formed without an activation barrier at any initial orientation of reactants in the ground triplet state. The dissociated triplet complexes with adsorption energy of 4.7-8.4 kcal/mol are separated from the predissociated structure.,, bo, the barrier of I I kcal/mol. The dissociated singlet structures with hydrogen atoms located in two 3-fold position,, and in the bridge positions on nonintersecting Pd-Pd bonds have the same binding energy of 22.1 kcal/mol and correspond to the ground state of the Pd4H2 system. Several more local minima corresponding to dissociated and nondissociated H-H bond,., are found on the sin-let PES. In contrast to the low-index bulk palladium surfaces, no spontaneous pathway for hydrogen dissociation on the cluster is found. Activated H-2 dissociation on the palladium tetramer includes the triplet-singlet transition induced by the spin-orbit interaction iv, a key step, Numerical estimation of the matrix spin-orbit coupling element and the corresponding transmission coefficient kappa is performed within nonadiabatic theory for two reaction pathways, which are expected to have the maximal reaction probability. The estimated kappa values of similar to0.1 and similar to0.4 an corresponding activation barriers of similar to 25 and similar to 20 kcal/mol are found for the reaction pathways leading to the ground-state three-coordinated dissociated complex and to the saddle point heading toward the ground-state bridge structure, respectively.