15min:

DILRUKSHI HEWAGE, RUCHIRA SILVA AND DONG-SHENG YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.

Reactions of acetylene (C₂H₂) with laser-vaporized La atoms produced La(C₆H₄) and La (C₁₀H₈) in supersonic molecular beams. The organic fragments in these complexes were benzyne and naphthalene. The benzyne species was produced by the La-mediated cyclotrimerization of three acetylene molecules, whereas naphthalene was formed likely by the cyclization of the transient benzyne with two additional acetylene molecules. These cyclized products were identified by mass-analyzed threshold ionization mass spectroscopy, which measured adiabatic ionization energies and several vibrational frequencies. The measured ionization energies were 40875 (5) cm^-1 for La(C₆H₄) and 36767 (5) cm^-1 for La(C₁₀H₈). The most active vibrational transitions of both complexes were metal-ligand stretching with 326 cm^-1 for La(C₆H₄) and 286 cm^-1 for La (C₁₀H₈). By combining the spectra with theoretical calculations, the ground electronic states of the neutral complexes were determined to be ²A₁ (C_2v) and ²A' (C_s) and those of the corresponding ions were ¹A₁ (C_2v) and ¹A' (C_s) for La (C₆H₄) and La(C₁₀H₈), respectively.