Knowledge Bank

The cyclopentadienyl dialuminum complex, (C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$)Al$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$, is synthesized in a supersonic molecular beam, where aluminum atoms and dimers are produced by laser vaporization, and cyclopentadienyl radicals are generated from photolysis of cyclopentadiene (C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$) molecules. Vibrationally resolved electronic spectra of the complex are measured using pulsed-field ionization zero electron kinetic energy spectroscopy. Three initial conformations are considered for the complex: one with an Al atom binding to each side of the C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$ ring and two others with an Al$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ molecule lying vertically or horizontally on one side of the ligand ring. A structure with the Al$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ molecule lying vertically on the top of the C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$ ring is identified by comparing the spectroscopic measurements to $ab$ $initio$ calculations. The (C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$)Al$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ neutral complex has a $2$A ground electronic state in Jahn-Teller distorted $C$ symmetry and the corresponding ion has a $1$A$1$ ground state in $C$ symmetry. The ionization energy of (C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$)Al$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ is measured to be 40690(5) cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. For the cationic $1$A$1$ state, the frequencies of the metal-ligand stretching, metal-metal stretching, and metal-ligand bending modes are 482, 175, and 50 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, respectively. In addition, a (C$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$)-Al$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ bending mode of the neutral $2$A state is measured to be 26 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$.