Microheterogeneity and Decoupled Dynamics in <i>N</i> -Methyl-2-pyrrolidone (NMP)–Water Solutions Probed with Ultrafast Infrared Spectroscopy and Molecular Dynamics Simulations

Understanding amide–water interactions is central to modeling biological hydration and the behavior of complex aqueous solutions. Herein, we employ ultrafast IR spectroscopy and molecular dynamics simulations to resolve the structural controversy in N-methyl-2-pyrrolidone (NMP)–water mixtures. By directly probing the orientational dynamics of the OD stretch probe on the picosecond time scale, our measurements extend beyond the static structural picture of stoichiometric complexes (e.g., NMP·2H2O). Instead, our analysis reveals a dynamic microheterogeneous structure, where water occupies cavities between NMP molecules. Both hydrogen-bond lifetimes and local packing reached a maximum in the intermediate concentration range. This microheterogeneity rationalizes the decoupling between the macroscopic viscosity maximum (at XNMP = 0.3) and the peak in molecular rotational slowdown (at XNMP = 0.5). These results further show that the nonideal properties of the solution arise from a competition between microheterogeneous confinement and local packing effects and the progressive reduction in hydrogen-bond network connectivity.