Composition-dependent hydrogen-bond network reorganization in formic acid-dimethyl sulfoxide mixtures revealed by Raman spectroscopy, 2D-COS and DFT

Hydrogen-bond competition and composition-dependent solvation restructuring in carboxylic acid-sulfoxide mixtures were investigated using formic acid-dimethyl sulfoxide (FA-DMSO) as a model system. Concentration-dependent Raman spectroscopy, Gaussian band deconvolution, two-dimensional correlation spectroscopy (2D-COS), and density functional theory (DFT) calculations were combined to track changes in the local hydrogen-bonding environment. Raman spectra show that strong OH···OS hydrogen bonds and weak CH···O interactions coexist and evolve with composition. With increasing formic acid fraction, DMSO skeletal vibrations mainly shift to higher wavenumbers, whereas the SO stretching band red-shifts. At V_HCOOH = 0.2, the SO region resolves into OH rocking, SO stretching, and CH out-of-plane rocking components, indicating pronounced heteromolecular interactions. The CH contribution becomes unresolved at V_HCOOH ≥ 0.6, suggesting attenuation of the weak CH···O interaction. In the CO region, the single band of neat formic acid near 1677 cm^-1 evolves into two components near 1707 and 1737 cm^-1, reflecting distinct carbonyl microenvironments. Their intensity ratio exhibits a crossover near V_HCOOH = 0.7, indicating redistribution of local formic-acid environments. 2D-COS further reveals different vibrational response sequences in DMSO-rich and FA-rich regions. Representative DFT cluster calculations qualitatively support the observed frequency shifts, vibrational assignments, and hydrogen-bonding trends. These results provide molecular-vibrational evidence for composition-dependent hydrogen-bond network reorganization in FA-DMSO mixtures.