Magnetism in thiolated gold model junctions

Three stable, neutral diradical model molecules with an even number of electrons, based on pairs of small thiolated Au clusters connected via Au-Au bonding, are studied within a broken symmetry unrestricted density functional theory involving a set of exchange-correlation functionals, including PBE, M06L, TPSS, B3LYP, M06, and TPSSh. The models, mostly with an antiferromagnetic ground state within the theory used, are analyzed in terms of the vertical spin-flip energy splitting, total spin expectation values, Heisenberg exchange coupling constant, magnetic orbitals and their overlap, diradical character, binding energies, natural bond-order analysis, and bond index of the central Au-Au bond. The spin-symmetry breaking is attributed to the open-shell nature of the dimerized monomer constituents and the structural feature of the facing S-Au-S edges, in combination with the attractive unsupported metallophilic d-d interaction of the incident Au atoms, allowing a weak coupling of the spins localized sideways. The modeling provides insight to Au ^I-Au ^I interactions, potentially useful in design of novel gold-based magnetic nanoscopic assemblies.