The Structure and Growth Mechanism of Small Titanium Carbide Clusters: A Competition between C2and C4Carbon Chains

Abstract

DFT calculations have been carried out on the titanium carbide clusters Ti3C8, Ti4C8, Ti6C13, and Ti7C13 and their anions, in relation with the recent experimental and theoretical report by L.-S. Wang and his group on those clusters (Wang, L.-S.; Wang, X.-B.; Wu, H.; Cheng, H. J. Am. Chem. Soc. 1998, 120, 6556). The small clusters Ti3C8 and Ti4C8, characterized by a C/Ti ratio higher than or equal to 2, favor conformations in which the carbon phase is assembled into cis-butadiene-like C4 chains without bond length alternation. The 19- and 20-atom clusters Ti6C13 and Ti7C13 prefer structures which are reminiscent of the very stable metallocarbohedrene (MetCar) Ti8C12 Td structure. An analysis of the structure most probable for Ti7C13- suggests that this conformation cannot be a precursor to multicage clusters. The layer-by-layer growth pathway proposed by Wang et al. therefore appears susceptible to account for the formation and stability of high nuclearity titanium carbide clusters. However, the existence of multicage structures cannot be ruled out with other metals. The surprising absence of the MetCar species Ti8C12- and Zr8C12- from the mass spectrum of anionic clusters, as observed by Wang under specific experimental conditions, is tentatively explained by the formation of unstable molecular Rydberg states.