The structure and growth mechanism of small titanium carbide clusters: A competition between C-2 and C-4 carbon chains

DFT calculations have been carried out on the titanium carbide clusters Ti3 C8, 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 c lusters (Wang, L.-S.; Wang, X.-B.; Wu, H.; Cheng, H. J. Am. Chern. Sec. 199 8, 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 ph ase is assembled into cis-butadiene-like C-4 chains without bond length alt ernation. The 19- and 20-atom clusters Ti6C13 and Ti7C13 prefer structures which are reminiscent of the very stable metallocarbohedrene (MetCar) Ti8C1 2 T-d structure. An analysis of the structure most probable for Ti7C13-sugg ests that this conformation cannot be a precursor to multicage clusters. Th e layer-by-layer growth pathway proposed by Wang et al. therefore appears s usceptible to account for the formation and stability of high nuclearity ti tanium carbide clusters. However, the existence of multicage structures can not be ruled out with other metals. The surprising absence of the MetCar sp ecies Ti8C12- and Zr8C12- from the mass spectrum of anionic clusters, as ob served by Wang under specific experimental conditions, is tentatively expla ined by the formation of unstable molecular Rydberg states.