Electron Binding in a Superatom with a Repulsive Coulomb Barrier: The Case of [Ag44(SC6H3F2)30]4- in the Gas Phase

The electron binding mechanism in [Ag-44(SC6H3F2)(30)](4-) (SC6H3F2 = 3,4-difluorobenzenethiolate) tetra-anion was studied by photoelectron spectroscopy (PES), collision-induced dissociation mass spectrometry (CID-MS), and density functional theory (DFT) computations. PES showed that [Ag-44(SC6H3F2)(30)](4-) is energetically metastable with respect to electron autodetachment {[Ag-44(SC6H3F2)(30)](3-) + e(-)} and features a repulsive Coulomb barrier (RCB) with a height of 2.7 eV. However, CID-MS revealed that [Ag-44(SC6H3F2)(30)](4-) does not release an electron upon collisional excitation but undergoes dissociation. DFT computations performed on the known structure of [Ag-44(SC6H3F2)(30)](4-) confirmed the negative adiabatic electron affinity of [Ag-44(SC6H3F2)(30)](3-) and interpreted the experimental PE spectrum by taking into account tunneling electron photodetachment through the RCB.