Two-dimensional ultrafast infrared vibrational echo studies of solute-solvent interactions and dynamics

Two-dimensional spectrally resolved ultrafast infrared vibrational echo experiments were used to investigate the nature of solute-solvent interactions in solution. The experiments were performed on (acetylacetonato)dicarbonylrhodium(I) in dibutylphthalate at 150 K. The 2D spectra display features that reflect the 0-1 and 1-2 transitions and the combination band transition of the symmetric (S) and antisymmetric (A) CO stretching modes. Three oscillations in the data arise from the frequency difference between the S and A modes (quantum beats) and the S and A anharmonicities. The novel mechanism that gives rise to the anharmonic oscillations, which is distinct from that of a conventional quantum beat, is described. The frequency of the S/A mode-splitting quantum beats varies for different observation wavelengths across the 0-1 inhomogeneous lines. For either the S or A lines, as the wavelength of observation of the vibrational echo is moved to higher energy, the quantum beat frequency decreases. The change in frequency is related to the nature of the solute-solvent interactions (inhomogeneous broadening) of the S and A transitions. A simple analytical approach is used to determine how a solute vibrational oscillator is influenced by the solvent. Three models of solute-solvent interactions are considered in terms of CO local mode energies and coupling. In one, the transition energies in the S and A lines are anticorrelated either because the inhomogeneous broadening arises from variations in the local mode coupling or the local mode energies are anticorrelated. In the other two, the local mode energies are either correlated or uncorrelated. The results of the model calculations indicate that interactions with the solvent result in local mode frequencies that are strongly correlated. (C) 2001 American Institute of Physics.