Two-dimensional infrared spectroscopy of a structured liquid: neat formamide

Vibrational dynamics of liquid formamide is studied in the spectral region of the amide I mode by means of linear and two-dimensional infrared spectroscopies. The two-dimensional spectrum has a complex structure to be connected to the partially excitonic nature of the vibrational states. The measurements performed on a 1:10 (12)C:(13)C formamide isotopic mixture allow separating the broadening contribution due to the inhomogeneous frequency distribution of the local oscillators from that of excitonic origin. A model based on the Kubo picture of the line broadening is used, together with the dynamic information obtained from a molecular dynamics simulation, to fit the spectra of the (12)C formamide impurity in the isotopic mixture. The relevant dynamical information, such as the amplitude of the frequency fluctuations, lifetime of the second vibrational excited state, and anharmonicity, is thus recovered. By appropriately combining the outcomes of experiments and molecular dynamics simulation, we demonstrate that motional narrowing determines the line shape of the amide I resonance to a large extent. The same analysis provides an estimate of the transition dipole moment of formamide, which results in good agreement with an ab initio calculation. The calculated frequency fluctuation correlation time is found to be comparable to the hydrogen-bond lifetime, which defines the basic structural relaxation rate of the networked liquid.