Distinct Hydrogen Bonding Dynamics Underlies the Microheterogeneity in DMF–Water Mixtures

Abstract

The hydrogen bonding interaction between the amide functional group and water is fundamental to understanding the liquid–liquid heterogeneity in biological systems. Herein, the structure and dynamics of the N,N-dimethyl­formamide (DMF)–water mixtures have been investigated by linear and nonlinear IR spectroscopies, using the hydroxyl stretch and extrinsic probe of thiocyanate as local vibrational reporters. According to vibrational relaxation dynamics measurements, the orientational dynamics of water is not directly tied to those of DMF molecules. Wobbling-in-a-cone analysis demonstrates that the water molecules have varying degrees of angular restriction depending on their composition due to the formation of specific water–DMF networks. Because of the preferential solvation by DMF molecules, the rotational dynamics of the extrinsic probe is slowed significantly, and its rotational time constants are correlated to the change of solution viscosity. The unique structural dynamics observed in the DMF–water mixtures is expected to provide important insights into the underlying mechanism of microscopic heterogeneity in binary mixtures.

Dynamics of Formamide–Water Mixtures Investigated by Linear and Nonlinear Infrared Spectroscopy

“…In contrast to prior investigations on DMF–water and DMSO–water binary solutions, , where the vibrational lifetime of the anionic probe exhibited a consistent increase with the rise in the solute’s mole fraction, the results shown in Figure B reveal a distinct trend. It is evident that the vibrational population decay of the SCN – anionic probe proves highly sensitive to the local environment, attributed to structural alterations resulting from hydrogen bonding exchanges.…”

“…Notably, all OD stretch vibrational relaxation curves within the presence of FA in the solution align with a single exponential decay function�a significant departure from the behavior observed in DMF water solutions. 25 These fitting results, plotted against FA concentrations in Figure 2B, reveal an average vibrational lifetime of around 1.6 ps as X FA varies from 0.1 to 0.8. The vibrational population relaxation primarily correlates with the disruption of the hydrogen bond networks.…”

“…This stands in contrast to DMF water mixtures, where the aprotic DMF disrupts the hydrogen bond networks. 25 Prior studies have highlighted the presence of three-dimensional hydrogenbonded networks in liquid FA, akin to those observed in liquid water. 1,8 To investigate the structural dynamics of water molecules within these binary solutions, we utilized OD as a local vibrational probe to avoid resonant energy transfer.…”

“…In our previous studies, we have shown that the concentration dependent reorientational dynamics of extrinsic probe of SCN − in DMSO and DMF water mixtures generally follow the trend of the viscosity change. 25,62 However, the discrepancy can be observed in the intermediate concentration region (X solute = 0.3−0.4), where the reorientational time constant is significantly higher than the normalized viscosity value. In the case of FA−water mixtures, the result shown in Figure 5B is quite unique, revealing different structural dynamics in the binary mixture solutions.…”

“…23,24 In our previous work, nonlinear vibrational spectroscopy provided evidence of microscopic heterogeneity in dimethylformamide (DMF) aqueous solutions. 25 Initial beliefs surrounding FA−water mixture solutions suggested the formation of an ideal solution with water. 26,27 Thermodynamic analyses during the mixing process with water, along with XRD and neutron diffraction data, initially supported this notion.…”