Abstract: The structural and vibrational dynamics of the non-bridged C≡O stretching vibrations of two different tautomers of dimeric π-cyclopentadienyldicarbonyliron [CpFe(CO)₂]₂ in CH₂Cl₂ were examined using steady-state and femtosecond infrared pump-probe methods at 5-μm wavelength. The two main species in [CpFe(CO)₂]₂ had a cis:trans molar ratio of 1.7, and showed different vibrational and rotational relaxation dynamics. Both species showed biexponential decay in their two C≡O stretching vibrational excited-state populations, with a fast component (<1 ps) and a slow component (20 ps). The former was believed to be related to the rapid dephasing processes of the coherent state caused by broadband excitation, while the latter was the typical lifetime for the C≡O stretching vibrational excited state. Having a significant permanent dipole, the cis structure could interact strongly with solvent, resulting in relatively slower rotational dynamics. Our work demonstrated that the frequency and vibrational-rotational dynamics of the non-bridged C≡O stretching vibrations were very sensitive to both molecular structures and the solvent.

Key words: Dimeric π-cyclopentadienyldicarbonyliron, Femtosecond infrared pump-probe spectroscopy, Vibrational relaxation dynamics, Molecular structural dynamics