Electron paramagnetic resonance in positively charged Au25 molecular nanoclusters

In this study, we investigated the unpaired electrons and singly occupied molecular orbitals (SOMO) of positively charged Au-25 molecular clusters using solid-state electron paramagnetic resonance (EPR). The EPR powder spectra of the positively charged (Au-25(+)) and neutral (Au-25(0)) species of Au-25 are discussed and compared. Our study demonstrates that Au-25(+) is paramagnetic with a SOMO that is mostly localized about the central gold atom in the core of the molecule and possesses a strong p-type atomic character. The unpaired electron spin is demonstrated to strongly interact with the nuclear spins from other Au-197 nuclei in the core of Au-25(+) molecules and the hyperfine tensor describing such interaction was extracted from the comparison of the EPR spectra with quantum mechanical simulations assuming an anisotropic structure of the core. Our simulations suggest that the core of Au-25(+) molecular clusters is more distorted than in the corresponding neutral counterpart. They also confirm previous hypotheses suggesting that the icosahedral core of Au-25(+) experiences contraction with decreasing temperature. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773061]