Reactivity of group 5 bielement clusters with H2.

Bielement metal clusters composed of the group 5 elements, A(n)B(m) (A, B = V, Nb, Ta), were prepared in the gas phase over the wide n and m ranges. We measured reactivities of the attachment reaction of the neutral clusters to H(2), which were plotted as a function of a map with n and m. The clusters with n + m = 4 and 5 were found to be more reactive than the other clusters. The measurement of the ionization energies reveals that there is no strong correlation between the reactivity and the ionization energy. In addition, the reactivities of the cations, A(n)B(m)(+), were also highest at n + m = 4 and 5. These findings suggest that the electronic structure does not totally determine the reactivity of the neutral clusters. On the other hand, theoretical calculations for A(n)B(m) (n + m = 4) reported by Metha et al. showed that the optimized geometrical structures of the congener group 5 bielement clusters do not change significantly by changing n and m within n + m = 4. Hence, it is highly likely that the pyramidal (n + m = 4) and bipyramidal (n + m = 5) structures of A(n)B(m) determine the high reactivity of the clusters.