Discovery\nof Isomerization Intermediates in CdS Magic-Size\nClusters

Isomerization,\nthe process by which a molecule is coherently transformed\ninto another molecule with the same molecular formula but a different\natomic structure, is an important and well-known phenomenon of organic\nchemistry, but has only recently been observed for inorganic nanoclusters.\nPreviously, CdS nanoclusters were found to isomerize between two end\npoint structures rapidly and reversibly (the α-phase and β-phase),\nmediated by hydroxyl groups on the surface. This observation raised\nmany significant structural and pathway questions. One critical question\nis why no intermediate states were observed during the isomerization;\nit is not obvious why an atomic cluster should only have two stable\nend points rather than multiple intermediate arrangements. In this\nstudy, we report that the use of amide functional groups can stabilize\nintermediate phases during the transformation of CdS magic-size clusters\nbetween the α-phase and the β-phase. When treated with\namides in organic solvents, the amides not only facilitate the α-phase\nto β-phase isomerization but also exhibit three distinct excitonic\nfeatures, which we call the β<sub>340</sub>-phase, β<sub>350</sub>-phase, and β<sub>367</sub>-phase. Based on pair distribution\nfunction analysis, these intermediates strongly resemble the β-phase\nstructure but deviate greatly from the α-phase structure. All\nphases (β<sub>340</sub>-phase, β<sub>350</sub>-phase,\nand β<sub>367</sub>-phase) have nearly identical structures\nto the β-phase, with the β<sub>340</sub>-phase having\nthe largest deviation. Despite these intermediates having similar\natomic structures, they have up to a 583 meV difference in band gap\ncompared to the β-phase. Kinetic studies show that the isomers\nand intermediates follow a traditional progression in the thermodynamic\nstability of β<sub>340</sub>-phase/β<sub>350</sub>-phase\n< α-phase < β<sub>367</sub>-phase < β-phase.\nThe solvent identity and polarity play a crucial role in kinetically\narresting these intermediates. Fourier transform infrared spectroscopy\nand X-ray photoelectron spectroscopy studies paired with simple density\nfunctional theory calculations reveal that the likely mechanism is\ndue to the multifunctional nature of the amides that form an amphoteric\nsurface binding bond motif, which promotes a change in the carboxylic\nacid binding mode. This change from chelating binding modes to bridging\nbinding modes initiates the isomerization. We propose that the carbonyl\ngroup is responsible for the direct interaction with the surface,\nacting as an L-type ligand which then pulls electron density away\nfrom the electron-poor nitrogen site, enabling them to interact with\nthe carboxylate ligands and initiate the change in the binding mode.\nThe isomerization of CdS nanoclusters continues to be a topic of interest,\ngiving insight into fundamental nanoscale chemistry and physics.