Adsorption of Multiple NO Molecules on Anionic Gold Clusters: Electron Transfer and Disproportionation Enhanced by Molecular Aggregation

The reactions of Aun- clusters with multiple nitric oxide (NO) molecules are explored at 150 K by utilizing a mini-flow-tube reactor and a time-of-flight mass spectrometer. Adsorption of multiple NO molecules is observed on most Aun-, while disproportionation reactions only occur on even-sized Aun- with n = 4, 6, 8, 20 and odd-sized ones with n = 5 and 7. Theoretical calculations reveal the geometric structures and electronic states of the products containing bimolecular and trimolecular NO units, where two NO molecules typically form dimers. Different from NO monomers that weakly interact with odd-sized Aun- and form electron-sharing covalent bonds with Au10-(D3h) and Au16-, NO dimers can extract significant charge from parent Aun-. Regarding the three NO molecules, a predilection toward condensation into trimers on even-sized Aun- is observed, while the tendency is more toward an adsorption pattern of a dimer plus a monomer on odd-sized Aun-. The NO trimers register even higher charge gain from Aun- as compared with the NO dimers, which leads to an elevated degree of activation and induces the progression of disproportionation reactions. Therefore, when considering the reaction between NO and Aun-, it appears that NO has a propensity to form dimers or trimers on Aun-. This behavior of aggregate formation substantially enhances the ability of NO to absorb negative charges from Aun- although the occurrence of disproportionate dissociation reactions is initiated only for specific sizes.