Excited‐State Raman Spectroscopy of Inorganic Compounds

Abstract

Abstract— Raman spectra of inorganic complexes in excited electronic states are discussed. A brief overview of the field of transient Raman spectroscopy and experimental considerations are presented. Two examples from the author's laboratory are used to illustrate the type of information that can be obtained. The first example, an excited‐state Raman spectroscopic study of K3[Mn(CN)5NO], is chosen because it illustrates the connections between excited‐state molecular structure and vibrational properties. The pump pulse causes a change from a linear sp‐hybridized NO containing a triple bond to a bent sp2‐hybridized NO containing a double bond. Both the NO stretch and normal modes involving other ligands are measured and interpreted. The second example is chosen to illustrate the vibrational consequences of photoinduced electron transfer. The Raman spectra of W(CO)4(diimine) complexes (diimine = 2,2'‐bipyridine, 4,4'‐dimethyl‐2,2'‐bipyridine, and isopropyl‐pyridine‐2‐carbaldehyde imine) in the lowest tungsten to diimine charge transfer excited state are discussed. The excited‐state peaks are assigned to ligand ring deformation modes and to carbonyl stretching modes. The totally symmetric cis‐carbonyl stretching mode in the charge transfer excited state is about 50 cm' higher in energy than that of the molecule in the ground electronic state. The increase is interpreted in terms of loss of metal‐car‐bonyl back‐bonding in the charge transfer excited state. Finally, a summary of the field's strengths and difficulties and a brief discussion of the future perspectives are presented.