IR Spectroscopic Characterization of H(2) Adsorption on Cationic Cu(n)(+) (n = 4–7) Clusters

[Image: see text] IR spectra of cationic copper clusters Cu(n)(+) (n = 4–7) complexed with hydrogen molecules are recorded via IR multiple-photon dissociation (IRMPD) spectroscopy. To this end, the copper clusters are generated via laser ablation and reacted with H(2) and D(2) in a flow-tube-type reaction channel. The complexes formed are irradiated using IR light provided by the free-electron laser for intracavity experiments (FELICE). The spectra are interpreted by making use of isotope-induced shifts of the vibrational bands and by comparing them to density functional theory calculated spectra for candidate structures. The structural candidates have been obtained from global sampling with the minima hopping method, and spectra are calculated at the semilocal (PBE) and hybrid (PBE0) functional level. The highest-quality spectra have been recorded for [5Cu, 2H/2D](+), and we find that the semilocal functional provides better agreement for the lowest-energy isomers. The interaction of hydrogen with the copper clusters strongly depends on their size. Binding energies are largest for Cu(5)(+), which goes hand in hand with the observed predominantly dissociative adsorption. Due to smaller binding energies for dissociated H(2) and D(2) for Cu(4)(+), also a significant amount of molecular adsorption is observed as to be expected according to the Evans–Polanyi principle. This is confirmed by transition-state calculations for Cu(4)(+) and Cu(5)(+), which show that hydrogen dissociation is not hindered by an endothermic reaction barrier for Cu(5)(+) and by a slightly endothermic barrier for Cu(4)(+). For Cu(6)(+) and Cu(7)(+), it was difficult to draw clear conclusions because the IR spectra could not be unambiguously assigned to structures.