Mass Spectrometrically Detected Statistical Aspects of Ligand Populations in Mixed Monolayer Au25L18 Nanoparticles

Ligand exchange reactions of Au-25(SCH2CH2Ph)(18) with hexanethiol (HSC6) and thiophenol (HSPh) as incoming ligands, and Brust reaction nanoparticle syntheses using mixtures of thiols (HSCH2CH2Ph and HSC6), produce nanoparticles having different, ideally statistically determined, relative populations of the two thiolate ligands (X and Y), i.e., Au-25(X)(m)(Y)(m)', where m and m' vary but always sum to 18. By choice of reactant concentrations, the exchange reaction can reach an equilibrium state or a near-complete exchange of ligands or be at a kinetically determined (nonequilibrium) mixed population, at the time of reaction quenching and subsequent matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometric examination. With the assumption that the reactivities of the 18 ligand sites are identical and independent, the equilibrium distributions of ligand populations of the mixed monolayer exchange products should adhere to a binominal distribution. A simulated kinetic model for ligand exchange shows that mixed ligand distributions in nanoparticles not at exchange equilibrium also conform to the binominal distribution. The theory successfully describes MALDI mass spectrometrically determined experimental ligand populations produced in the ligand exchange reaction between Au-25(SCH2CH2Pb)(18) and hexanethiol, while that between Au-25(SCH2CH2Ph)(18) and thiophenol yields a more narrow distribution than predicted by random exchanges and no interactions between ligands. Previous nanoparticle ligand analyses by methods such as nuclear magnetic resonance, gas-liquid chromatography, and infrared spectroscopies yield average ligand populations in mixed monolayers and are incapable of detecting such nonrandom ligand distributions.