Effect of total charge on the electronic structure of thiolate-protected X@Ag 12 superatoms (X = Ag, Au).

Electronic structures of chemically synthesized silver-based clusters [XAg 16 (TBBT) 12 ] 3- (X = Ag or Au; TBBT = 4- tert -butylbenzenethiolate) having an icosahedral X@Ag 12 superatomic core were studied by gas-phase photoelectron spectroscopy and density functional theory calculations. The electron binding energy of the highest occupied molecular orbital (HOMO) with a 1P superatomic nature was determined to be 0.23 and 0.29 eV for X = Ag or Au, respectively. Resonant tunnelling electron emission through the repulsive Coulomb barrier (RCB) was observed. From the kinetic energy of the tunnelling electrons, it was estimated that the lowest unoccupied molecular orbital (LUMO) was supported at 1.51 and 1.62 eV above the vacuum level by the RCB for X = Ag or Au, respectively. The HOMO of [XAg 16 (TBBT) 12 ] 3- (X = Ag or Au) was destabilized by 3.74 and 3.71 eV, respectively, compared with those of [XAg 24 (DMBT) 18 ] - (DMBT = 2,4-dimethylbenzenethiolate) having the icosahedral X@Ag 12 core due to the larger negative charge imparted by the ligand layers.