OCS binding to and reactivity with isolated gold cluster cations, Aun + (n = 1-10) has been studied by infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with quantum chemical calculations. The distribution of complexes AunSx(OCS)m + 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 Aun(OCS) + (n = 3-10) clusters are interpreted in terms of either µ 1 or µ 2 S-binding motifs. Analysis of the fragmentation products following infrared excitation of parent Aun(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 molecularlybound entrance channel complexes, rather than global minimum inserted structures. Attempts to generate Rhn(OCS) + and Ptn(OCS) + equivalents failed; only sulfide reaction products were observed in the mass spectrum, even after cooling the cluster source to -100℃. gold clusters have revealed optimum activity when 60% of gold is positively charged, [28][29][30] and trace water levels can perturb the electron density on a gold catalyst particle enhancing its reactivity. 12 Gas-phase cluster studies can achieve exquisite control of both cluster size and charge state. Mass spectrometry techniques permit the study of trends in reactivity and charge characteristics, which may guide target properties for deposited particles. [31][32][33] Such fundamental studies, using a range of techniques, [34][35][36] including ion mobility, [37][38][39] and visible photodissociation spectroscopy, 40 continue to reveal unique physico-chemical features in gold clusters, Aun +/0/-, such as the transformation from planar to three-dimensional ground state structures at surprisingly large cluster sizes (n = 8,11,12 for cations, neutrals, and anions, respectively)another result arising from relativistic effects. These and other investigations of Aun +/0/clusters often reveal oscillating trends with increasing cluster size, n, in properties including cluster stability, dissociation energy, ionisation potential, electron affinity, fragmentation pathways and HOMO-LUMO gap. 20,[41][42][43][44][45][46][47][48] Due to the electronic structure of gold, the cluster size, n, and charge effects are very closely linked, 49 and many observations have been interpreted in terms of electron counting models in which each gold atom, with the [Xe]4f 14 5d 10 6s 1 configuration, is considered as monovalent, similar to alkali metals. 20,50 Within such jellium models, 51 gold clusters exhibit electronic shell structures with magic numbers corresponding to shell closures at 8, 18, 20, etc.. [52][53] Similarly, oscillating ...
