Mechanistic and energetic details of adduct formation and sigma-bond activation in Zr+(H-2)(n) clusters

The formation of Zr+(H-2)(n) clusters (n = 1-7) has been studied both exper imentally and theoretically. Binding energies were determined via temperatu re-dependent equilibrium measurements, giving -Delta Ho degrees = 14.5 +/- 0.3, 10.7 +/- 0.2, 10.1 +/- 0.3, 9.1 +/- 0.5, 9.2 +/- 0.5, 8.9 +/- 0.6, and 8.5 +/- 0.8 kcal/mol for n = 1-7, respectively. Both DFT(B3LYP) and MP2 ca lculations gave binding energies in excellent agreement with experiment. Zr + appears to insert into the first H-2 to form the dihydride as predicted b y theory. The rate of insertion was observed to have a negative temperature dependence and a positive pressure dependence, suggesting a cluster-assist ed insertion mechanism. Interestingly, both DFT and MP2 calculations sugges t that the dihydride may "uninsert" to form a pure dihydrogen cluster with n between 5 and 7.