Transformation of Au-144(SCH2CH2Ph)(60) to Au-133(SPh-tBu)(52) Nanomolecules: Theoretical and Experimental Study

Ultrastable gold nanomolecule Au-144(SCH2CH2Ph)(60) upon etching with excess tert-butylbenzenethiol undergoes a core-size conversion and compositional change to form an entirely new core of Au-133(SPh-tBu)(52). This conversion was studied using high-resolution electrospray mass spectrometry which shows that the core size conversion is initiated after 22 ligand exchanges, suggesting a relatively high stability of the Au-144(SCH2CH2Ph)(38)(SPh-tBu)(22) intermediate. The Au-144 -> Au-133 core size conversion is surprisingly different from the Au-144 -> Au-99 core conversion reported in the case of thiophenol, -SPh. Theoretical analysis and ab initio molecular dynamics simulations show that rigid p-tBu groups play a crucial role by reducing the cluster structural freedom, and protecting the cluster from adsorption of exogenous and reactive species, thus rationalizing the kinetic factors that stabilize the Au-133 core size. This 144-atom to 133-atom nanomolecule's compositional change is reflected in optical spectroscopy and electrochemistry.