Origin of the unusual properties of Au(n)(BO2) clusters.

We report the discovery of a new class of clusters consisting of Au(n)(BO(2)) that forms during the oxygenation of gold clusters when boron nitride is used as insulation in a pulsed-arc cluster ion source (PACIS). Photoelectron and mass spectroscopy of these clusters further revealed some remarkable properties: instead of the expected Au(n)O(m) peaks, the mass spectra contain intense peaks corresponding to Au(n)(BO(2)) composition. Some of the most predominant features of the electronic structure of the bare Au clusters, namely even-odd alternation in the electron affinity, are preserved in the Au(n)(BO(2)) species. Most importantly, Au(n)(BO(2)) [odd n] clusters possess unusually large electron affinity values for a closed-shell cluster, ranging from 2.8-3.5 eV. The open-shell Au(n)(BO(2)) [even n] clusters on the other hand, possess electron affinities exceeding that of F, the most electronegative element in the periodic table. Using calculations based on density functional theory, we trace the origin of these species to the unusual stability and high electron affinity of the BO(2) moiety. The resulting bond formed between BO(2) and Au(n) clusters preserves the geometric and electronic structure of the bare Au(n) clusters. The large electron affinity of these clusters is due to the delocalization of the extra electron over the Au(n) cluster.