As a prototype of both β-diketone and α,β-enone compounds, ethyl acetoacetate (EAC) has been known to exist in multiple structures in solutions. In this paper, femtosecond infrared pump-probe spectroscopy was used to examine the vibrational relaxation processes of the C=O and C=C stretching modes, appeared in the region of 6 μm wavelength (1600～1800 cm^-1 in frequency), of the EAC tautomers in deuterated water (D₂O) and cyclohexane. Steady-state infrared spectroscopy and quantum chemistry computations were used to understand the characteristics of these vibrational bands and their line shapes. The results showed that in water EAC exists in the keto form, while in cyclohexane it exists in both keto and enol forms, and perhaps in two possible keto forms. As a result of structural rigidity, the steady-state infrared absorption peak is narrow for the enol form, suggesting mainly homogenous broadening; while those of the keto form exhibit the feature of both homogenous and heterogeneous broadenings. The apparent infrared line shapes of the keto and enol forms were somewhat different even in the same solvent. Population dynamics and anisotropy dynamics of the C=O and C=C stretching mode were obtained and examined by transient infrared pump-probe measurement. In D₂O, the vibrational energy relaxation rate of the C=O modes was found to be faster than that in cyclohexane, which can be attributed to the formation hydrogen bond between EAC and solvent molecules. The vibrational relaxation process of the enol form was found to be different from that of the keto form, which can be contributed to Fermi resonance interaction between the C=O stretching mode and the COH bending mode. With the aid of the quantum chemistry calculations, it is concluded that there exists a fast vibrational population equilibrium process between the C=O and C=C stretching modes and the COH in-plane bending mode via Fermi resonance. Anisotropy experiments showed a fast orientation dynamics for EAC in cyclohexane than in water, reflecting different solvations. Further, quantum beating was observed in the anisotropy dynamics due to the coherence between the ss- and as-modes, especially for the keto form, suggesting a coherent excitation between the related vibrational modes.

Key words: IR pump-probe, vibrational energy relaxation dynamics, anisotropy dynamics, ethyl acetoacetate