Decoupling of Structural and Dynamical Heterogeneity in Hydrogen-Bonded <i>N</i> -Methylformamide–Water Mixtures: A Kerr Effect Study

We investigate composition-dependent orientational dynamics in hydrogen-bonded <i>N</i>-methylformamide (NMF)-water mixtures using time-resolved optical Kerr effect (OKE) spectroscopy, molecular dynamics (MD) simulations, and nuclear magnetic resonance (NMR) spectroscopy. Fits of the derivative of the stretched exponential to the OKE response yield average relaxation time, τ_β^OKE, that rises sharply at a low NMF content and plateaus near equimolar compositions, while the stretch parameter, β, displays a clear maximum at <i>x</i>_NMF of ∼0.5, indicating the most dynamically homogeneous regime. In contrast, Voronoi tessellation of MD configurations shows maximal local-density fluctuations near <i>x</i>_NMF of ∼0.15, identifying the most structurally heterogeneous regime at a much lower NMF content. This offset establishes a robust decoupling between structural and dynamical heterogeneity. NMR chemical shifts and diffusion coefficients exhibit non-monotonic trends consistent with composition-dependent hydrogen-bond reorganization, while axis-resolved MD reorientation times corroborate the OKE trends. Our results demonstrate that orientational relaxation in NMF-water is governed not by viscosity or static structure alone but by a cooperative hydrogen-bond network whose dynamics evolve non-trivially with composition.