Infrared Study of OCS Binding and Size-Selective Reactivity with Gold Clusters, Au (n = 1-10)

OCS binding to and reactivity with isolated gold cluster cations, Au⁺ (n = 1-10), has been studied by infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with quantum chemical calculations. The distribution of complexes AuS(OCS)⁺ formed reflects the relative reactivity of different cluster sizes with OCS, under the multiple collision conditions of our ablation source. The IR-MPD spectra of Au(OCS)⁺ (n = 3-10) clusters are interpreted in terms of either μ¹ or μ² S binding motifs. Analysis of the fragmentation products following infrared excitation of parent Au(OCS)⁺ clusters reveals strongly size-selective (odd-even) branching ratios for OCS and CO loss, respectively. CO loss signifies infrared-driven OCS decomposition on the cluster surface and is observed to occur predominantly on even n clusters (i.e., those with odd electron counts). The experimental data, including fragmentation branching ratios, are consistent with calculated potential energy landscapes, in which the initial species trapped are molecularly bound entrance channel complexes, rather than global minimum inserted structures. Attempts to generate Rh(OCS)⁺ and Pt(OCS)⁺ equivalents failed; only sulfide reaction products were observed in the mass spectrum, even after cooling the cluster source to -100 °C.