Interference Effects of Multiple Excited States in the Resonance Raman Spectroscopy of CpCoCOD

The resonance Raman excitation profiles of cyclopentadienylcobalt cyclooctadiene, CpCoCOD, contain pronounced decreases in intensity in the region of the lowest energy ligand field absorption band. This deenhancement is caused by interference between the ligand field state and nearby metal to ligand charge-transfer states. The changes in the Raman intensities are quantitatively calculated. The relevant electronic excited states are assigned by using single-crystal absorption spectroscopy, and the vibrational modes are assigned by deuteration studies. Distortions along the metal−ligand vibrational normal coordinates in the interfering states are calculated. The origin of the deenhancement, the relative signs of the bond length changes, and the relationship of the bond length changes to the molecular orbitals involved in the transitions are discussed.