Interference effects of multiple excited states in the resonance Raman spectroscopy of CpCoCOD

The resonance Raman excitation profiles of cyclopentadienylcobalt cycloocta diene, CpCoCOD, contain pronounced decreases in intensity in the region of the lowest energy ligand field absorption band. This deenhancement is cause d by interference between the ligand field state and nearby metal to ligand charge-transfer states. The changes in the Raman intensities are quantitat ively calculated. The relevant electronic excited states are assigned by us ing single-crystal absorption spectroscopy, and the vibrational modes are a ssigned by deuteration studies. Distortions along the metal-ligand vibratio nal normal coordinates in the interfering states are calculated. The origin of the deenhancement, the relative signs of the bond length changes, and t he relationship of the bond length changes to the molecular orbitals involv ed in the transitions are discussed.