Knowledge Bank

Unlike benzene, 1,3,5,7-cyclooctatetraene (COT) is not an aromatic molecule and has a tub-like structure in its ground electronic state. However, coordination of Sc or Ti converts the tub-like structure into a planar ring structure. The M-COT (M = Sc, Ti) complexes formed by interactions of laser-vaporized metal atoms and the organic ligand are studied by pulsed field ionization-zero electron kinetic energy photoelectron spectroscopy and density functional theory calculations. The theoretical calculations predict that both Sc/Sc$+$ and Ti/Ti$+$ prefer the $\eta 8$ binding with slightly different M-C distances. The resulting Sc/Sc$+$ and Ti$+$ complexes have C molecular symmetry, whereas Ti-COT has C symmetry, due to Jahn-Teller distortion. By comparing the experimental measurements and theoretical calculations, the photoelectron spectrum of the Sc complex is assigned to the $1A1\leftarrow 2A1$ transition with a strong 0-0 transition at 42261(5) cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. On the other hand, the spectrum of the Ti complex is due to the transitions from two neutral states with similar energies, $3$B$1$ and $3$B$2$, to ionic $2$A$1$ electronic state. These transitions display strong 0-0 transitions at 40887(5) and 40863(5) cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, respectively. In addition, a major vibrational progression (338 and 330 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ for the Sc$+$- and Ti$+$-COT stretch frequencies, respectively) and a number of vibrational hot bands are observed for the two complexes.