15min:

JUNG SUP LEE, SUDESH KUMARI AND DONG-SHENG YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.

Transition metal bis(arene) sandwich complexes may adopt eclipsed or staggered conformations due to the aromatic ring rotations about the metal-arene axis.^, In this study, the group VI (Cr, Mo, and W) metal bis(toluene) complexes are synthesized in a laser-ablation molecular beam source, and their rotational conformers are identified by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy. For Cr-bis(toluene), the ZEKE spectrum shows three distinctive vibrationless (0-0) transitions between the ground electronic states of the neutral and ionic complexes at 42739(5), 42745(5), and 42805(5) cm^-1, corresponding to ionization energies of 180^o, 60^o/120^o, and 0^o rotamers. In addition, the spectrum exhibits metal-toluene bending (164, 180, 196, and 223 cm^-1) and stretching (278 and 291 cm^-1) frequencies of these rotamers. The ground electronic states of the 0^o and 180^o rotamers are ¹A₁ (C_2v) and ¹A_g (C_2h) in the neutral form and ²A₁ (C_2v) and ²A_g (C_2h) in the ionized form, respectively. For the 60^o and 120^o rotamers, the ground states of the neutral molecules are ¹A (C₂), and those of the corresponding ions are ²A (C₂). Through the variation of the molecular beam conditions, the eclipsed conformer (0^o) is determined to be more stable than the staggered ones (180^o, 120^o, and 60^o). Similarly, multiple conformers are identified for the Mo and W complexes.