Engineering magic number Au19 and Au20 cage structures using electron withdrawing atoms

Abstract

Gold cages are a subset of gold nanoparticles and these structures are of major interest due to their favourable physiochemical properties. In order for these structures to be useful in applications, they must be chemically stable. The objective of this research is to transform non-magic number cage structures into magic number cage structures by the addition of electron-withdrawing groups on the cages. The electronic properties for Au₁₉X and Au₂₀X₂ (X = F, Cl, Br, I) are calculated and observed. It is expected that the electron-withdrawing groups will remove the electron density from the gold cages and leave them positively charged. We first optimize the geometries of the initial gold cages and verify the structures are a local minima. From there, we attach our halogens to the gold cages and optimize the structures to determine the NICS values and HOMO–LUMO gaps. NICS values were found to be more negative when a more electronegative halogen was used. Calculations have found that Au₁₉F and Au₂₀F₂ have the most negative NICS values, indicating greater spherical aromaticity. Iodine, being the least electronegative atom, had the most positive NICS value and smallest HOMO–LUMO gap. All calculations are compared to the magic cluster Au₁₈ which satisfies Hirsh's 2(N + 1)² rule for n = 2.