Discovery of Isomerization Intermediates in CdS Magic-Size Clusters

Isomerization, the process by which a molecule is coherently transformed into another molecule with the same molecular formula but a different atomic structure, is an important and well-known phenomenon of organic chemistry, but has only recently been observed for inorganic nanoclusters. Previously, CdS nanoclusters were found to isomerize between two end point structures rapidly and reversibly (the α-phase and β-phase), mediated by hydroxyl groups on the surface. This observation raised many significant structural and pathway questions. One critical question is why no intermediate states were observed during the isomerization; it is not obvious why an atomic cluster should only have two stable end points rather than multiple intermediate arrangements. In this study, we report that the use of amide functional groups can stabilize intermediate phases during the transformation of CdS magic-size clusters between the α-phase and the β-phase. When treated with amides in organic solvents, the amides not only facilitate the α-phase to β-phase isomerization but also exhibit three distinct excitonic features, which we call the β₃₄₀-phase, β₃₅₀-phase, and β₃₆₇-phase. Based on pair distribution function analysis, these intermediates strongly resemble the β-phase structure but deviate greatly from the α-phase structure. All phases (β₃₄₀-phase, β₃₅₀-phase, and β₃₆₇-phase) have nearly identical structures to the β-phase, with the β₃₄₀-phase having the largest deviation. Despite these intermediates having similar atomic structures, they have up to a 583 meV difference in band gap compared to the β-phase. Kinetic studies show that the isomers and intermediates follow a traditional progression in the thermodynamic stability of β₃₄₀-phase/β₃₅₀-phase < α-phase < β₃₆₇-phase < β-phase. The solvent identity and polarity play a crucial role in kinetically arresting these intermediates. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy studies paired with simple density functional theory calculations reveal that the likely mechanism is due to the multifunctional nature of the amides that form an amphoteric surface binding bond motif, which promotes a change in the carboxylic acid binding mode. This change from chelating binding modes to bridging binding modes initiates the isomerization. We propose that the carbonyl group is responsible for the direct interaction with the surface, acting as an L-type ligand which then pulls electron density away from the electron-poor nitrogen site, enabling them to interact with the carboxylate ligands and initiate the change in the binding mode. The isomerization of CdS nanoclusters continues to be a topic of interest, giving insight into fundamental nanoscale chemistry and physics.