Dynamic reconstruction of hydrogen-bond networks in N,N-dimethylformamide–water system revealed by 2D correlation Raman spectroscopy

Abstract

N,N-dimethylformamide (DMF)–H2O binary solutions exhibit complex hydrogen bond (HB) network dynamics due to the dual hydrophilic–hydrophobic properties of DMF, with their underlying molecular mechanisms remaining unclear. Herein, we systematically investigated the HB network reconstruction mechanism by combining two-dimensional correlation Raman spectroscopy with difference Raman spectroscopy, complemented by density functional theory (DFT) calculations and reduced density gradient (RDG) analysis. The results show that the system exhibits three critical transition points during HB network evolution (xDMF = 0.4, 0.6, 0.8), confirming the phased variation of OH vibrational characteristics with concentration. It further displays a “both-ends-first” response order: symmetric and free OH stretching vibrations respond earlier to concentration perturbations than antisymmetric stretching vibrations, clarifying the intrinsic dynamic pathway of HB network reconstruction. DFT calculations and RDG visualization verify the correlation between Raman shifts and HB evolution: DMF enhances local HBs at low concentrations, leading to OH redshift; HBs weaken gradually at intermediate concentrations, resulting in a redshift-to-blueshift transition of OH vibrations (xDMF = 0.6 for structural fine-tuning); and HBs are significantly weakened at high concentrations, yielding nearly free OH bonds with prominent blueshift, and the system interactions transition from strong HBs to weak van der Waals forces. In addition, synergistic regulatory effects exist between C–H⋯O and O–H⋯O HBs. This study establishes a combined spectroscopic–theoretical framework for resolving the microscopic interaction mechanisms in complex HB liquids.