Dipole orientation and surface cluster size effects on chemisorption- induced magnetism: A DFT study of the interaction of gold-thiopolypeptide

A nanosystem formed by a high electric dipole moment thiopolypeptide α-helix, consisting of eight L-glycine units, chemisorbed on the (111) surface of Au₂₃ and Au₅₅ clusters, with the S as the linking atom, was studied using the wave function broken symmetry UDFT method. We have found a strong correlation between the orientation of the electric dipole of the α-helix and charge transfer and the magnetic behavior of the adsorbate-cluster system. Upon chemisorption, dipole moments may be quenched or enhanced, with respect to the gas phase value, with the strongest reduction corresponding to the magnetic state. A reduction of the α-helix's electric dipole with the net charge transfer from the Au surface was obtained for the more stable state. In this state description, it may happen that the calculated spin densities of the chemisorbed α-helix and its free radical form are similar. The magnetic properties are strongly dependent on the size of the Au cluster and on its electronic structure with respect to nuclei positions. In general, the localized spin density per atom increases and the magnetization of the extended system decreases with cluster size, a trend found experimentally for organic monolayers with a similar type of adsorbate we consider here.