Quantum Mechanical Modeling of Structure Evolution of Transition Metal Clusters and Metallocarbohedrenes

Abstract

Ab initio quantum-mechanical modeling based on density functional theory (DFT) was used to study transition metal clusters and metallo-carbohedrenes (MetCars). Combined with the state-of-the-art spectroscopic experiments, DFT calculations are capable of yielding much insight into the structures, chemical bonding and growth mechanisms of these clusters. Two specific cluster systems were investigated in detail: one involves small chromium clusters and another contains titanium carbides. Exhaustive structural search was performed by fully optimizing a variety of cluster geometries. For the chromium clusters, we found that a tightly-bound Cr2 dimer plays a key role in determining the cluster structures. A dimer growth route is discovered for clusters up to Cr11, at which a structural transition occurs from the dimer growth to a bulk-like body-centered-cubic structure. The uncovered structural evolution is consistent with the currently available experimental observations. For MetCars, we found that three factors, i.e., the C2 dimer, cubic framework and layered structures, play an essential role in determining the structures and chemical bonding of the titanium carbide clusters. A growth pathway from Ti3C8 to Ti13C22 with Ti4C8, Ti6C13, Ti7C13 and Ti9C15 as intermediates is thus proposed. Both theory and experiments suggest that the cubic layered growth with C2 dimers can lead to a new type of highly stable one-dimensional quantum wires.