GAS PHASE ELECTRONIC SPECTROSCOPY OF ARENE CHROMIUM TRICARBONYLS: DIRECT DISSOCIATION VS. PREDISSOCIATION IN THE LOWEST ENERGY EXCITED ELECTRONIC - STATE

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1987

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Ohio State University

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We have measured the first gas phase electronic absorption spectra of ($n_{6}-C_{6}H_{6})Cr(CO)_{3}, (n_{6}-C_{6}H_{5}CH_{3})Cr(CO)_{3}$, and $(n_{6}-C_{6}H_{5}CH_{3})Cr(CO)_{3}$ at low resolution, $200 cm^{-1}$. These spectra show intense (92,000 liter/mole-cm) absorption features extending into the vacuum ultraviolet indicating the existence of at least five different electronic states with energies below $61000 cm^{-1}$. Our low resolution data suggest that only one of those states shifts appreciably with arene substitution. We compare these results with solution phase spectra and other data. At higher resolution, $40 cm^{-1}$, our data suggest that the onset of detectable absorption is lower in energy for $(n_{6}-C_{6}H_{6})Cr(CO)_{3}$ than both $(n_{6}-C_{6}H_{5}Cl)Cr(CO)_{3}$, and $(n_{6}-C_{6}H_{5}CH_{3})Cr(CO)_{3}$. The vibronic envelope of this transition suggests significant structural deformation occurs with the electronic transition suggests significant structural deformation occurs with the electronic transition. The deformation may be correlated with the well known photolability of the CO ligands. We speculate that the lowest energy excited electronic state of the ACl is dissociative with respect to the CO ligands. We also demonstrate rate that there is a significant increase in the number of low frequency vibrations upon mono-substitution of the benzene ring. We discuss these new data in the light of our earlier multiphoton spectroscopy results on these molecules.

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Author Institution: Department of Chemistry, Syracuse University

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