Ultrafast NH and CH Vibrational Dynamics in Hydrogen-Bonded 2‑Pyrrolidinone

Over the past two and a half decades, two-dimensional infrared (2DIR) spectroscopy has undergone significant methodological and technological advancements, enabling the exploration of numerous physical and chemical processes in complex molecules, particularly biological ones. Among these, interactions between vibrational modes, such as NH and CH stretches, are of special interest in biological contexts. However, these interactions have not been thoroughly investigated until now. In this study, broadband 2DIR spectroscopy, spanning a spectral range of 2750 cm^-1 to 3350 cm^-1, is employed to examine the molecular structure and dynamics of 2-pyrrolidinone. This approach provides time-resolved insights into the coupling of NH and CH stretch vibrations. Distinct signatures of chemical exchange, as well as coherent and incoherent couplings, are observed. The chemical exchange arises from the pseudorotation of the five-membered ring, transitioning between axial and equatorial conformers. Coherent coupling is identified between symmetric and asymmetric CH stretch vibrations, while incoherent coupling is observed between asymmetric CH stretch and NH stretch vibrations. Furthermore, intermolecular hydrogen bonding in 2-pyrrolidinone enables the detailed study of hydrogen bond making and breaking processes, which are fundamental to the behavior of biological molecules. This investigation sheds light on the intricate vibrational dynamics and interactions in 2-pyrrolidinone, offering valuable insights into processes critical to biological systems.