Rydberg transitions in bis(η6-benzene)chromium; a study of isotopic effects and vibronic structures

Gas-phase electronic absorption spectra of (η 6 -C 6 H 6 ) 2 Cr 1 and (η 6 -C 6 D 6 ) 2 Cr 2 have been measured with 30 cm -1 resolution. Both spectra show 3d(a 1g )→Rnp(e 1u ) (n=4–12) and 3d(a 1g )→Rnp(a 2u ) (n=5–8) series. The first ionization potentials, determined as a convergence limit of the Rnp(e 1u ) series, are 5.459±0.004 and 5.446±0.004 eV for 1 and 2, respectively. The Rydberg np 0 0 0 bands display a ca. 100 cm -1 shift on going from 1 to 2. The decreases in ionization potential and Rydberg frequencies have vibrational nature and indicate lower zero-level vibrational energy for bis(η 6 -benzene)chromium in the ground electronic state in comparison with that of the molecule in the Rydberg np states and the ground-state cation. A complete assignment of the vibronic components of the Rydberg transitions in 1 and 2 has been made on the basis of comparison of the spectra. In addition to vibrational progressions on the totally symmetric metal–ring stretching mode, the R4p(e 1u ) and R5p(e 1u ) transitions show components arising from excitation of CH vibrations. The corresponding absorption features are shifted towards the 0 0 0 bands on going from 1 to 2.