HYDROGEN BOND-MEDIATED PATHWAYS OF ULTRAFAST VIBRATIONAL RELAXATION: CARBONYL STRETCHING OF METHYL ACETATE

Understanding the structure and ultrafast dynamics of hydrogen bonding interactions of solvated molecules is among the urgent spectroscopic challenges. Ultrafast vibrational relaxation of the energy excess deposited in molecules with infrared femtosecond pulses can be strongly altered when H-bonds between the solute and its solvation shell are established. In the model system of the present study, ester carbonyl stretching vibration in methyl acetate, ultrafast vibrational relaxation is up to 15 times faster, when H-bonded complex with molecules of a protic solvent is established. After the excitation, the energy excess dissipates along different relaxation pathways, including the direct relaxation to the solvent, intra-molecular vibrational relaxation, and energy transfer to H-bonded molecules. We use a combination of linear and non-linear infrared spectroscopy (including the ultrafast pump-probe, two-dimensional (2D-IR) and two-color 2D-IR methods) to reveal the role of the specific H-bonding interactions in guiding the energy transport on the molecular level. In particular, with non-linear spectroscopy we spectrally differentiate between the different H-bonding conformations of methyl acetate, namely, the H-bonding to one, two, or none solvent molecules, and follow the energy flow in each of the cases. The experimental observations are interpreted with a novel theoretical approach based on the analysis of normal modes of the H-bonded complex and their anharmonic couplings, which qualitatively reproduces experimental results. Our studies promote the understanding of relaxation and transport phenomena in complexes, where H-bonds play important structural and functional roles.