Fast time-resolved infrared spectroscopy (TRIR) is used to probe the v(CO) spectrum of ClRe(CO)3(4,4'-bipyridyl)2 in the lowest MLCT (metal-to-ligand charge transfer) excited state. The three ground-state v(CO) bands all shift up in frequency, but not by the same amount. The energy-factored force field (EFFF) of the ground state is accurately solved by employing ,3CO-enrichment. Approximate methods yield information on the force field of the excited state; the principal EFFF force constants (¿co(ami) and fcco(equatoriai)) change by the same amount on excitation. This leads to a new interpretation of the resonance Raman spectrum of the compound. has been time-resolved resonance Raman (TR3) spectroscopy. Classic experiments have probed the structure of the excited states of a range of ruthenium complexes, and have shown, for instance, that in the lowest excited state of [Ru(bpy)3]2+ there is charge transfer from Ru to one of the bpy groups.7 Detailed isotopic studies8 have unravelled the details of the excited state structure. There is, however, a limitation of resonance Raman spectroscopy: only certain of the vibrational modes in the excited state will be resonance enhanced, partly because of the nature of the
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