Unusual Intensities in the Resonance Raman Spectra and Excitation Profiles of an Intervalence Metal-to-Metal Charge Transfer Complex

Abstract

The resonance Raman spectra of the metal-to-metal charge transfer (MMCT) mixed-valence complex [(OC) 5 Cr 0 -CN-Os III (NH 3 ) 5 ](CF 3 SO 3 ) 2 are measured and analyzed. The excitation profiles (the Raman intensities as a function of excitation wavelength) are obtained using the SO 3 stretch of the triflate ion as an internal standard. The ratios of the Raman intensities are not constant with different excitation wavelengths. This unusual observation is attributed to interference between two overlapping electronic states of the molecule. The electronic states are observed in the low-temperature polarized single crystal absorption spectrum. The resonance Raman intensities and excitation profiles are calculated with the time dependent theory of spectroscopy and include the interference between the electronic states. The origin of the unusual intensities is quantitatively explained in terms of the real and imaginary parts of the Raman cross sections from the two states.

Resonance Raman Spectroscopy with Overtones Involving Metal‐Ligand and Ligand‐Centered Modes in (o‐Benzoquinonediimine)ruthenium(II) Complexes

“…Calculated Raman spectra obtained from DFT are in good agreement with the experimental spectra, as shown by the dotted traces in Figure 2. In particular, the calculated spectra do not show any bands where overtones and combination bands are observed, and they indicate a larger number of bands in the 1200 to 1500 cm -1 region for [Ru(BQDI)(acac) 2 ] than for [Ru(BQDI)(NH 3 ) 2 Cl 2 ], providing a qualitative rationale for the large, poorly resolved feature observed for the former complex in this wavenumber range. The Raman peaks in the off-resonance spectrum of [Ru(BQDI)(acac) 2 ] are weak and broad, especially in the 1200 to 1500 cm -1 region, complicating the identification of fundamental bands.…”

“…The analysis of the Raman spectra of [Ru(BQDI)(NH 3 ) 2 Cl 2 ] is more straightforward because the identification of the fundamental frequencies in the off-resonance Raman spectrum is obvious. In both systems, it appears clearly that the peaks labeled II and III in Figure 1 observed at Raman shifts of ap- 2 ]. The index k refers to the numerical labels shown in Figure 2 and numbers the normal coordinates in Equation (3); the parameters E 00 and Γ refer to Equation (2).…”

“…[1][2][3][4][5] Charge-transfer processes in transition-metal complexes, in particular those of ruthenium(II), are of particular interest for spectroscopic investigations, as detailed spectra combined with theoretical models provide quantitative insight on the structural and dynamics aspects of these important excited states. [6,7] In the vast majority of reported resonance Raman studies on ruthenium(II) complexes only fundamental bands are observed.…”

“…The covalent interactions between the metal and the ligand can be separated into ligand-to-metal donation and metal-toligand back-donation. [12][13][14][15][16][17][18] Complexes with these ligands, in particular [Ru(BQDI)(acac) 2 lengths near the lowest-energy intense absorption maximum at ca. 20000 cm -1 , and are shown in Figure 1.…”

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

“…This loss of intensity at an excitation energy where a maximum is expected is called "resonance deenhancement". [7][8][9][10][11][12] Theoretical treatments that use the Kramer-Heisenberg-Dirac (sum-over-states) method 11 or the time-dependent theory of resonance Raman spectroscopy 12,13 have been published. During the course of our studies of cyclopentadienyl(1,5-cyclooctadiene)cobalt, CpCoCOD, we discovered that metalligand stretching normal modes exhibit pronounced resonance deenhancements in the region of the lowest energy ligand field absorption band.…”